The Motorola 6845 , or MC6845 , is a display controller that was widely used in 8-bit computers during the 1980s. Originally intended for designs based on the Motorola 6800 CPU and given a related part number, it was more widely used alongside various other processors, and was most commonly found in machines based on the Zilog Z80 and MOS 6502 .
130-421: The 6845 is not an entire display solution on its own; the chip's main function is to properly time access to the display memory, and to calculate the memory address of the next portion to be drawn. Other circuitry in the machine then uses the address provided by the 6845 to fetch the pattern and then draw it. The implementation of that hardware is entirely up to the designer and varied widely among machines. The 6845
260-437: A CRT is left to other circuits. Because of this, systems using the 6845 may have very different numbers and values of colors, or may not support color at all. Interlaced and non-interlaced output modes are supported, as is a hardware text cursor. The sync generation includes generation of horizontal and vertical video blanking signals, which are used to condition the external pixel generation circuits. Also, an internal latch
390-579: A microcomputer capable of performing various tasks which they could then demonstrate in the TV series The Computer Programme . The list of topics included programming , graphics , sound and music, teletext , controlling external hardware, and artificial intelligence . It developed an ambitious specification for a BBC computer, and discussed the project with several companies including Acorn Computers , Sinclair Research , Newbury Laboratories, Tangerine Computer Systems , and Dragon Data . The introduction of
520-478: A "Bitstik" [1] . The Model A and the Model B were built on the same printed circuit board (PCB), and a Model A can be upgraded to a Model B. Users wishing to operate Model B software need to add the extra RAM and the user/printer MOS Technology 6522 VIA (which many games use for timers) and snip a link, a task that can be achieved without soldering. To do a full upgrade with all the external ports requires soldering
650-559: A "low-cost development" of an existing machine, the Transam Tuscan, which included dual floppy drives and cost £1,700. This proposal was voted down by the ITV companies, citing a possible contravention of the companies' obligations under broadcasting regulations prohibiting sponsorship, along with concerns about a conflict of interest with advertisers of computer products. Despite denials of involvement with ITV from Prism Microproducts,
780-601: A BBC Micro clone called the Dolphin. Unlike the original BBC Micro, the Dolphin featured blue function keys. Production agreements were made with both SCL in India and distributor Harry Mazal in Mexico for the assembly of BBC Micro units from kits of parts, leading to full-scale manufacturing, with SCL also planning to fabricate the 6502 CPU under licence from Rockwell. According to reporting from early 1985, "several thousand Beebs
910-707: A Z80 board and hard disk drive from Torch that allowed the BBC machine to run CP/M programs. Separate pages, each with a codename, are used to control the access to the I/O: The Tube interface allowed Acorn to use BBC Micros with ARM CPUs as software development machines when creating the Acorn Archimedes . This resulted in the ARM development kit for the BBC Micro in 1986, priced at around £4000. From 2006,
1040-598: A broadcast quality signal for use within television programming; it was used on episodes of The Computer Programme and Making the Most of the Micro . The computer included several input/output (I/O) interfaces: serial and parallel printer ports, an 8-bit general purpose digital I/O port, a port offering four analogue inputs, a light pen input, and an expansion connector (the "1 MHz bus") which enabled other hardware to be connected. An Econet network interface and
1170-418: A character code byte and the second byte is a character attribute byte—and the board uses the contents of the bytes, together with the row address, to read font data from ROM and generate the pixels. Different routing of the bits, omitting the character ROM, could be used to emulate a frame buffer. Due to its 128 line limitation, the 6845 is not able to provide linear large linear frame buffers. A solution
1300-404: A character line cache. Using the full address range RA0-RA4:CA0-CA13 the 6845 can address 2 = 524,288 words of memory. A word may be any number of bits chosen by the system designer as the memory width: though the number of unique addresses that the 6845 can address is limited to 524,288 , the amount of memory that the 6845 can address may be significantly larger than might be assumed because
1430-586: A computer retailing strategy. A key feature of the BBC Micro's design is the high-performance random-access memory (RAM) it is equipped with. A common design note in 6502 -based computers of the era was to run the RAM at twice the clock rate as the CPU. This allowed a separate video display controller to access memory while the CPU was busy processing the data just read. In this way, the CPU and graphics driver could share access to RAM through careful timing. This technique
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#17328014444721560-400: A disk drive interface were available as options. All motherboards had space for the electronic components, but Econet was rarely installed. Additionally, an Acorn proprietary interface named the " Tube " allowed a second processor to be added. Three models of second processor were offered by Acorn, based on the 6502 , Z80 and 32016 CPUs. The Tube was used for third-party add-ons, including
1690-563: A few dozen or few hundred bits of such memory could be provided. The first practical form of random-access memory was the Williams tube . It stored data as electrically charged spots on the face of a cathode-ray tube . Since the electron beam of the CRT could read and write the spots on the tube in any order, memory was random access. The capacity of the Williams tube was a few hundred to around
1820-441: A finger on a certain place on the motherboard caused the prototype to work. Acorn put a resistor pack across the data bus, which Furber described as " 'the engineer's finger' and again, we have no idea why it's necessary, and a million and a half machines later it's still working, so nobody asked any questions". The Model A shipped with 16 KB of user RAM, while the Model B had 32 KB. Extra ROMs could be fitted (four on
1950-403: A given price, none of them surpass the BBC ... in terms of versatility and expansion capability". As with Sinclair Research 's ZX Spectrum and Commodore International 's Commodore 64 , both released the next year, in 1982, demand greatly exceeded supply. For some months, there were long delays before customers received the machines they had ordered. Efforts were made to market the machine in
2080-421: A hard drive. This entire pool of memory may be referred to as "RAM" by many developers, even though the various subsystems can have very different access times , violating the original concept behind the random access term in RAM. Even within a hierarchy level such as DRAM, the specific row, column, bank, rank , channel, or interleave organization of the components make the access time variable, although not to
2210-511: A kit with an ARM7TDMI CPU running at 64 MHz, with as much as 64 MB of RAM, was released for the BBC Micro and Master, using the Tube interface to upgrade the 8-bit micros to 32-bit RISC machines. Among the software that operated on the Tube are an enhanced version of the Elite video game and a computer-aided design system that required a second 6502 CPU and a 3-dimensional joystick named
2340-454: A light pen interface, though it was usually an internal connector on the board itself, not on the outside of the computer, and it was usually undocumented in the user manual. Because all aspects of video timing are programmable, a single machine can switch between 50Hz and 60Hz timings in software (or indeed any other refresh rates within the limits of the chip). The 6845 can be used to drive monitors or any other raster display. The chip has
2470-424: A memory capacity that is a power of two. Usually several memory cells share the same address. For example, a 4 bit "wide" RAM chip has four memory cells for each address. Often the width of the memory and that of the microprocessor are different, for a 32 bit microprocessor, eight 4 bit RAM chips would be needed. Often more addresses are needed than can be provided by a device. In that case, external multiplexors to
2600-451: A month" were being produced in India. Meanwhile, the eventual production arrangement in Mexico involved local manufacturer Datum (a company founded by Harry Mazal and others, initially to act as ICL's Mexican distributor ), aiming to assemble 2000 units per month by May 1985, with the initial assembly intended to lead to the manufacture of all aspects of the machines apart from Acorn's proprietary ULA components. Such machines were intended for
2730-404: A portion of a computer's RAM, allowing it to act as a much faster hard drive that is called a RAM disk . A RAM disk loses the stored data when the computer is shut down, unless memory is arranged to have a standby battery source, or changes to the RAM disk are written out to a nonvolatile disk. The RAM disk is reloaded from the physical disk upon RAM disk initialization. Sometimes, the contents of
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#17328014444722860-555: A relatively slow ROM chip are copied to read/write memory to allow for shorter access times. The ROM chip is then disabled while the initialized memory locations are switched in on the same block of addresses (often write-protected). This process, sometimes called shadowing , is fairly common in both computers and embedded systems . As a common example, the BIOS in typical personal computers often has an option called "use shadow BIOS" or similar. When enabled, functions that rely on data from
2990-399: A row of that character's graphic pattern. However, the 6845 left to the designer the freedom to route the bits of the memory and row addresses to video RAM as they saw fit. For this reason the word addressed by the 6845 does not have to equal one pixel or one character. In CGA alphanumeric (text) mode, there are two bytes per character, accessed sequentially by the 6845—the first byte is
3120-542: A single MOS transistor per capacitor. The first commercial DRAM IC chip, the 1K Intel 1103 , was introduced in October 1970. Synchronous dynamic random-access memory (SDRAM) was reintroduced with the Samsung KM48SL2000 chip in 1992. Early computers used relays , mechanical counters or delay lines for main memory functions. Ultrasonic delay lines were serial devices which could only reproduce data in
3250-505: A specific microcomputer to a more general computer literacy initiative was a topic of controversy, however, with criticism aimed at the BBC for promoting a specific commercial product and for going beyond the "traditional BBC pattern" of promoting existing information networks of training and education providers. Accusations were even levelled at the Department of Industry for making the BBC "an arm of Government industrial policy" and using
3380-486: A stop gap", and others criticising the elevated price of £500 (compared to the £400 of the original Model B) in the face of significantly cheaper competition providing as much or even twice as much memory. The extra RAM in the Model B+ is assigned as two blocks, a block of 20 KB dedicated solely for screen display (so-called shadow RAM ) and a block of 12 KB of special sideways RAM . The B+128, introduced towards
3510-491: A switch that lets the control circuitry on the chip read the capacitor's state of charge or change it. As this form of memory is less expensive to produce than static RAM, it is the predominant form of computer memory used in modern computers. Both static and dynamic RAM are considered volatile , as their state is lost or reset when power is removed from the system. By contrast, read-only memory (ROM) stores data by permanently enabling or disabling selected transistors, such that
3640-611: A thousand bits, but it was much smaller, faster, and more power-efficient than using individual vacuum tube latches. Developed at the University of Manchester in England, the Williams tube provided the medium on which the first electronically stored program was implemented in the Manchester Baby computer, which first successfully ran a program on 21 June, 1948. In fact, rather than the Williams tube memory being designed for
3770-454: A total of 18 8-bit registers controlling all aspects of video timings. Only two addresses are exposed to external components - one to select which internal register is to be read or written to and another to access that register. register The 6845 is intended for character based displays. Every address it generates is composed of two parts - a 14 bit character address and a 5 bit row address. The character address increases linearly. When
3900-615: Is a type of flip-flop circuit, usually implemented using FETs . This means that SRAM requires very low power when not being accessed, but it is expensive and has low storage density. A second type, DRAM, is based around a capacitor. Charging and discharging this capacitor can store a "1" or a "0" in the cell. However, the charge in this capacitor slowly leaks away, and must be refreshed periodically. Because of this refresh process, DRAM uses more power, but it can achieve greater storage densities and lower unit costs compared to SRAM. To be useful, memory cells must be readable and writable. Within
4030-579: Is also a long-running problem late in the B/B+'s commercial life infamous amongst B+ owners, when Superior Software released Repton Infinity , which did not run on the B+. A series of unsuccessful replacements were issued before one compatible with both was finally released. During 1986, Acorn followed up with the BBC Master , which offers memory sizes from 128 KB and many other refinements which improved upon
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4160-403: Is far more expensive than the dynamic RAM used for larger memories. Static RAM also consumes far more power. CPU speed improvements slowed significantly partly due to major physical barriers and partly because current CPU designs have already hit the memory wall in some sense. Intel summarized these causes in a 2005 document. First of all, as chip geometries shrink and clock frequencies rise,
4290-406: Is fundamentally incompatible, and the 8271 emulators that existed were necessarily imperfect for all but basic operation. Software that use copy protection techniques involving direct access to the controller do not operate on the new system. Acorn attempted to alleviate this, starting with version 2.20 of the 1770 DFS, via an 8271-backward- compatible Ctrl + Z + Break option. There
4420-725: Is intended for character displays, but could also be used for pixel-based graphics, with some clever programming. Among its better-known uses are the BBC Micro , Amstrad CPC , and Videx VideoTerm display cards for the Apple II . It is also part of many early graphics adapter cards for the IBM PC , including the MDA , Hercules Graphics Card (HGC), Color Graphics Adapter (CGA) and the Plantronics Colorplus . Its functionality
4550-419: Is more expensive to produce, but is generally faster and requires less dynamic power than DRAM. In modern computers, SRAM is often used as cache memory for the CPU . DRAM stores a bit of data using a transistor and capacitor pair (typically a MOSFET and MOS capacitor , respectively), which together comprise a DRAM cell. The capacitor holds a high or low charge (1 or 0, respectively), and the transistor acts as
4680-412: Is provided which when triggered will duplicate and retain a copy of the video address so that it can later be read back by the CPU. This is useful for light pens and light guns which can function by sending a pulse to the 6845 when the electron beam passes, allowing a running program to read back the location that was pointed at. Because of this feature, most computer video adapters using a 6845 included
4810-489: Is reduced by the size of the shadowed ROMs. The ' memory wall is the growing disparity of speed between CPU and the response time of memory (known as memory latency ) outside the CPU chip. An important reason for this disparity is the limited communication bandwidth beyond chip boundaries, which is also referred to as bandwidth wall . From 1986 to 2000, CPU speed improved at an annual rate of 55% while off-chip memory response time only improved at 10%. Given these trends, it
4940-407: Is repeatedly read s times before the next line is read. This means that character displays using the 6845 require high memory bandwidth on the order of the bandwidth required for all-points-addressable graphics displays of the same resolution. A different video display controller that buffers one whole line of character data internally can avoid this repeated reading of each line of characters from
5070-423: Is the processor-memory performance gap, which can be addressed by 3D integrated circuits that reduce the distance between the logic and memory aspects that are further apart in a 2D chip. Memory subsystem design requires a focus on the gap, which is widening over time. The main method of bridging the gap is the use of caches ; small amounts of high-speed memory that houses recent operations and instructions nearby
5200-481: The Acorn Business Computer (ABC)/Acorn Cambridge Workstation range of machines was announced, based primarily on BBC hardware. In mid-1985, Acorn introduced the Model B+ which increased the total RAM to 64 KB. This had a modest market impact and received a rather unsympathetic reception, with one reviewer's assessment being that the machine was "18 months too late" and that it "must be seen as
5330-702: The Apricot PC and Victor 9000 to provide a 800x400 resolution monochrome display. A common clone of this CRT controller is the United Microelectronics Corporation (UMC) UM6845E CRT controller. During the time of cold war technology embargoes , the 6845 was cloned in Bulgaria under the designation CM607 . The 6845 was very similar and related to the later 6545 manufactured by MOS Technology (Commodore Semiconductor Group) and Rockwell (in two versions). The chip generates
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5460-578: The Atanasoff–Berry Computer , the Williams tube and the Selectron tube . In 1966, Robert Dennard invented modern DRAM architecture for which there is a single MOS transistor per capacitor. While examining the characteristics of MOS technology, he found it was capable of building capacitors , and that storing a charge or no charge on the MOS capacitor could represent the 1 and 0 of a bit, while
5590-664: The ITV network to introduce their own initiative and rival computing system, with a CP/M-based system proposed by Transam Computers under consideration for such an initiative by the Independent Television Companies Association at a late 1983 meeting. The proposed machine would have been priced at £399, matching that of the BBC Model B, and was reported as offering 64 KB of RAM, a disc interface, and serial and parallel interfaces, itself being
5720-510: The Rockwell 6545 lack interlaced output support and all 6545s include an optional address skew, which delays display enable for one character cycle if set. This second feature was incorporated into later variations of the Motorola 6845. The 6545 may be set to work in linear 14 bit mode using a status bit. On the 6845 the same thing requires adjustment of the character height. The 6845 reads
5850-490: The Tube interface was incorporated into the design, enabling a Z80 second processor to be added. A new contract between Acorn and BBC Enterprises was agreed in 1984 for another four-year term, with other manufacturers having tendered for the deal. An Acorn representative admitted that the BBC Model B would not be competitive throughout the term of the renewed contract, and that a successor would emerge. The OS ROM v1.0 contains
5980-432: The 1960s with bipolar memory, which used bipolar transistors . Although it was faster, it could not compete with the lower price of magnetic core memory. In 1957, Frosch and Derick manufactured the first silicon dioxide field-effect transistors at Bell Labs, the first transistors in which drain and source were adjacent at the surface. Subsequently, in 1960, a team demonstrated a working MOSFET at Bell Labs. This led to
6110-460: The 1981 original. It has essentially the same 6502-based BBC architecture, with many of the upgrades that the original design intentionally makes possible (extra ROM software, extra paged RAM, second processors) now included on the circuit board as internal plug-in modules. The BBC Micro platform amassed a large software base of both games and educational programs for its two main uses as a home and educational computer. Notable examples of each include
6240-673: The 6502 can translate the request for the local machine or send it across the Tube interface, as direct access is impossible from the coprocessor. Published programs largely conform to the API except for games, which routinely engage with the hardware for greater speed, and require a particular Acorn model. Random-access memory Random-access memory ( RAM ; / r æ m / ) is a form of electronic computer memory that can be read and changed in any order, typically used to store working data and machine code . A random-access memory device allows data items to be read or written in almost
6370-471: The 6845 and 6545. The biggest difference is that the 6545 may be configured so that it has sole access to the address bus for video memory. Two additional registers are included for setting any address the CPU wishes to read and the chip alternates between outputting addresses for display generation and the display set for CPU access. Smaller changes are that the MOS Technology and one variation of
6500-405: The 6845 imposes no limit on the size of each memory location that it addresses. If the word size is one byte, as is often the case, the 6845 can address 512 KiB . If the word size is 32 bits, e.g. for 32-bit color graphics with one pixel per word, then the 6845 can address 2048 KiB; for 64-bit words, it can address twice as much. These limits arise from the combination of the 6845 and the design of
6630-517: The 6845 is configured for a 2-pixel character height and the row address bit RA0 is used for bit 13 of the frame buffer address. This way, 200 vertical pixels can be used despite the 128 line limitation. Graphics mode on other systems used a similar trick: in the Hercules Graphics Card, bits 12-13 of the frame buffer address came from row address bits RA0-RA1, providing a vertical resolution of 348 pixels from 87 four-pixel-high "lines";
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#17328014444726760-462: The Amstrad CPC used a line height of 8 just like in text mode, mapping row address RA0-RA2 to memory address MA11-MA13 and character address CA0-CA10 to memory address MA0-MA10. These modes of operation were possible because the 6845 did not perform any buffering of character data. In the 1970s, 1980s, and to a lesser extent the 1990s, memory was expensive, fast memory was especially so, and this
6890-583: The BASIC III ROM chip, modified to accept the American spelling of COLOR , but the height of the graphics display was reduced to 200 scan lines to suit NTSC TVs, severely affecting applications written for British computers. After the failed US marketing campaign, the unwanted machines were remanufactured for the British market and sold, resulting in a third export variant. In October 1984,
7020-452: The BBC Micro system" for which 200 educational titles were being offered. In October 1984, while preparing a major expansion of its US dealer network, Acorn claimed sales of 85 per cent of the computers in British schools, and delivery of 40,000 machines per month. That December, Acorn stated its intention to become the market leader in US educational computing. The New York Times considered
7150-598: The BBC Models was high compared to competitors such as the ZX Spectrum and the Commodore 64, and from 1983 on, Acorn attempted to counter this by producing a simplified but largely compatible version intended for home use, complementing the use of the BBC Micro in schools: the 32K Acorn Electron . The involvement of the BBC in microcomputing also initiated tentative plans by the independent television companies of
7280-505: The BIOS's ROM instead use DRAM locations (most can also toggle shadowing of video card ROM or other ROM sections). Depending on the system, this may not result in increased performance, and may cause incompatibilities. For example, some hardware may be inaccessible to the operating system if shadow RAM is used. On some systems the benefit may be hypothetical because the BIOS is not used after booting in favor of direct hardware access. Free memory
7410-544: The Baby, the Baby was a testbed to demonstrate the reliability of the memory. Magnetic-core memory was invented in 1947 and developed up until the mid-1970s. It became a widespread form of random-access memory, relying on an array of magnetized rings. By changing the sense of each ring's magnetization, data could be stored with one bit stored per ring. Since every ring had a combination of address wires to select and read or write it, access to any memory location in any sequence
7540-564: The Computer Literacy Project as a way of "funding industry through the back door", obscuring public financial support on behalf of a government that was ostensibly opposed to subsidising industry. The Acorn team had already been working on a successor to their existing Atom microcomputer. Known as the Proton , it included better graphics and a faster 2 MHz MOS Technology 6502 central processing unit . The machine
7670-453: The MOS transistor could control writing the charge to the capacitor. This led to his development of a single-transistor DRAM memory cell. In 1967, Dennard filed a patent under IBM for a single-transistor DRAM memory cell, based on MOS technology. The first commercial DRAM IC chip was the Intel 1103 , which was manufactured on an 8 μm MOS process with a capacity of 1 kbit , and
7800-528: The Mexican and South American markets, potentially also appealing to those south-western states of the US having large Spanish-speaking populations. Ultimately, upon Acorn's withdrawal from the US in 1986, Datum would continue manufacturing at a level of 7000 to 8000 Spanish-language machines per year for the North and South American markets. The initial strategy for the BBC's computer literacy endeavour involved
7930-619: The Most of the Micro , Computers in Control in 1983, and finally Micro Live in 1985. After the Literacy Project's call for bids for a computer to accompany the television programmes and literature, Acorn won the contract with the Proton , a successor of its Atom computer prototyped at short notice. Renamed the BBC Micro, the system was adopted by most schools in the United Kingdom , changing Acorn's fortunes. It
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#17328014444728060-516: The PCB or sixteen with expansion hardware) and accessed via paged memory . The machines included three video ports, one with an RF modulator sending out a signal in the UHF band, another sending composite video suitable for connection to computer monitors , and a separate RGB video port. The separate RGB video out socket was an engineering requirement from the BBC to allow the machine to directly output
8190-476: The Proton was the only machine to match the BBC's specification; it also exceeded the specification in nearly every parameter. Based on the Proton prototype, the BBC signed a contract with Acorn as early as February 1981; by June the BBC Micro's specifications and pricing were decided. As a concession to the BBC's expectation of "industry standard" compatibility with CP/M, apparently under the direction of John Coll,
8320-489: The RAM comes in an easily upgraded form of modules called memory modules or DRAM modules about the size of a few sticks of chewing gum. These can be quickly replaced should they become damaged or when changing needs demand more storage capacity. As suggested above, smaller amounts of RAM (mostly SRAM) are also integrated in the CPU and other ICs on the motherboard , as well as in hard-drives, CD-ROMs , and several other parts of
8450-444: The RAM device, multiplexing and demultiplexing circuitry is used to select memory cells. Typically, a RAM device has a set of address lines A 0 , A 1 , . . . A n {\displaystyle A_{0},A_{1},...A_{n}} , and for each combination of bits that may be applied to these lines, a set of memory cells are activated. Due to this addressing, RAM devices virtually always have
8580-537: The RAM had to allow four million access cycles per second. Hitachi was the only company considering a DRAM which ran at that speed, the HM4816. To equip the prototype machine, the only four 4816s in the country were hand-carried by the Hitachi representative to Acorn. The National Semiconductor 81LS95 multiplexer was needed for the high memory speed. Furber recalled that competitors came to Acorn offering to replace
8710-587: The SP95 memory chip for the System/360 Model 95 . Dynamic random-access memory (DRAM) allowed replacement of a 4 or 6-transistor latch circuit by a single transistor for each memory bit, greatly increasing memory density at the cost of volatility. Data was stored in the tiny capacitance of each transistor, and had to be periodically refreshed every few milliseconds before the charge could leak away. Toshiba 's Toscal BC-1411 electronic calculator , which
8840-576: The United States and West Germany. Acorn's strategy in the US focused on the education market, worth a reported $ 700 million , by offering the BBC Micro in an upgraded form of the Model B with an expanded ROM, speech synthesis hardware, and built-in Econet interface for a price of $ 995, complementing this with the provision of software and materials designed to support teaching and to encourage adoption by teachers "fearful" of computers or skeptical of
8970-432: The chip signals horizontal sync it increases the row address. If the row address does not equal the programmatically set number of rows per character, then the character address is reset to the value it had at the beginning of the scan line that was just completed. Otherwise the row address is reset to zero and the memory address continues increasing linearly. This causes the same sequence of character values to be re-read from
9100-501: The command line. The MOS recognises certain built-in commands, and polls the paged ROMs in descending order for service otherwise; if none of them claims the command, then the OS returns a Bad command error. Suitable ROM (or EPROM) images could be written and provide functions without requiring RAM for the code itself. Not all ROMs offer star commands (ROMs containing data files, for instance), but any ROM can " hook " into vectors to enhance
9230-558: The company had already been pursuing a joint venture with Transam on a product rumoured to be under consideration by the broadcasting group. This product, a business system subsequently known as the Wren, had reportedly been positioned as such an "ITV Micro" towards the end of 1983, also to be offered in a home variant with ORACLE teletext reception capabilities. However, not all ITV franchise holders were equally enthusiastic about scheduling programmes related to microcomputing or about pursuing
9360-529: The competition from IBM, Apple , and Commodore. Another deployment in Phoenix, Arizona valued at $ 174,697 saw 175 BBC Micros installed, with the local Acorn dealer predicting sales worth $ 2 million in the next two years, of which around 85 to 90 percent would be made into education, the remainder going to the small business market. In early 1984, Acorn claimed a US network of more than 1,000 dealers, also reporting "over $ 50 million worth of education orders for
9490-498: The component with their own, but "none of them worked. And we never knew why. Which of course, means we didn't know why the National Semiconductor one did work correctly. And a million and a half BBC Micros later, it was still working, and I still didn't know why". Another mystery was the 6502's data bus . The prototype BBC Micro exceeded the CPU's specifications, causing it to fail. The designers found that putting
9620-405: The computer system. In addition to serving as temporary storage and working space for the operating system and applications, RAM is used in numerous other ways. Most modern operating systems employ a method of extending RAM capacity, known as "virtual memory". A portion of the computer's hard drive is set aside for a paging file or a scratch partition , and the combination of physical RAM and
9750-482: The connectors to the motherboard. The original machines shipped with "OS 0.1", with later updates advertised in magazines, supplied as a clip-in integrated circuit, with the last official version being "OS 1.2". Variations in the Acorn OS exist as a result of home-made projects and modified machines can still be bought on Internet auction sites such as eBay as of 2011. The BBC Model A was phased out of production with
9880-402: The details of the technical changes. Per Watford Electronics comments in their '32K Ram Board Manual': Early issue BBCs (Issue 3 circuit boards and before) are notorious for out of specification timings. If problems occur with this sort of machine, the problem can generally be cured by the use of either a Rockwell 6502A CPU chip, or by replacing IC14 (a 74LS245) with either another 74LS245 or
10010-448: The development of metal–oxide–semiconductor (MOS) memory by John Schmidt at Fairchild Semiconductor in 1964. In addition to higher speeds, MOS semiconductor memory was cheaper and consumed less power than magnetic core memory. The development of silicon-gate MOS integrated circuit (MOS IC) technology by Federico Faggin at Fairchild in 1968 enabled the production of MOS memory chips . MOS memory overtook magnetic core memory as
10140-408: The device are used to activate the correct device that is being accessed. RAM is often byte addressable, although it is also possible to make RAM that is word-addressable. One can read and over-write data in RAM. Many computer systems have a memory hierarchy consisting of processor registers , on- die SRAM caches, external caches , DRAM , paging systems and virtual memory or swap space on
10270-467: The display buffer RAM, reducing the required memory bandwidth and allowing either slower, less expensive memory chips to be used, more time for a system CPU to access the memory, or a combination of both. Adding such a character buffer to the 6845 was deemed to not be cost-effective approach when the chip was introduced; however, only a few years later the VIC-II chip used on the Commodore 64 did include such
10400-437: The dominant memory technology in the early 1970s. Integrated bipolar static random-access memory (SRAM) was invented by Robert H. Norman at Fairchild Semiconductor in 1963. It was followed by the development of MOS SRAM by John Schmidt at Fairchild in 1964. SRAM became an alternative to magnetic-core memory, but required six MOS transistors for each bit of data. Commercial use of SRAM began in 1965, when IBM introduced
10530-557: The early 1980s, the BBC started what became known as the BBC Computer Literacy Project . The project was initiated partly in response to an ITV documentary series The Mighty Micro , in which Christopher Evans of the UK's National Physical Laboratory predicted the coming microcomputer revolution and its effect on the economy, industry, and lifestyle of the United Kingdom. The BBC wanted to base its project on
10660-493: The end of 1985, comes with an additional 64 KB (4 × 16 KB sideways RAM banks) to give a total RAM of 128 KB. The B+ is incapable of operating some original BBC B programs and games, such as the very popular Castle Quest . A particular problem is the replacement of the Intel 8271 floppy-disk controller with the Western Digital 1770 : not only was the new controller mapped to different addresses, it
10790-410: The extent that access time to rotating storage media or a tape is variable. The overall goal of using a memory hierarchy is to obtain the fastest possible average access time while minimizing the total cost of the entire memory system (generally, the memory hierarchy follows the access time with the fast CPU registers at the top and the slow hard drive at the bottom). In many modern personal computers,
10920-402: The external memory connected to it, not from the 6845 alone. As described before, the 6845 cannot ordinarily provide large linear framebuffers . One design could only use 14-bit character addressing and set the number of lines per character to 1 but would be restricted to 128 lines and 16 KB of addressable memory. Although overwhelmingly compatible, a number of small variations exist between
11050-475: The faster 74ALS245. Two export models were developed: one for the US, with Econet and speech hardware as standard; the other for West Germany . Despite concerns of unsuitability for the Australian market, with the design failing at temperatures above 35 °C (95 °F), the machine was still "widely used in Australian schools". Export models were fitted with radio frequency shielding as required by
11180-600: The following ASCII credits string (code here ): Additionally, the last bytes of the BASIC read-only memory (ROM; v2 and v4) include the word "Roger", which is a reference to Sophie Wilson whose name at the time was Roger Wilson. The machine was released as the BBC Microcomputer on 1 December 1981, although production problems pushed delivery of the majority of the initial run into 1982. Nicknamed "the Beeb", it
11310-696: The form of integrated circuit (IC) chips with MOS (metal–oxide–semiconductor) memory cells . RAM is normally associated with volatile types of memory where stored information is lost if power is removed. The two main types of volatile random-access semiconductor memory are static random-access memory (SRAM) and dynamic random-access memory (DRAM). Non-volatile RAM has also been developed and other types of non-volatile memories allow random access for read operations, but either do not allow write operations or have other kinds of limitations. These include most types of ROM and NOR flash memory . The use of semiconductor RAM dates back to 1965 when IBM introduced
11440-412: The fundamental building block of computer memory . The memory cell is an electronic circuit that stores one bit of binary information and it must be set to store a logic 1 (high voltage level) and reset to store a logic 0 (low voltage level). Its value is maintained/stored until it is changed by the set/reset process. The value in the memory cell can be accessed by reading it. In SRAM, the memory cell
11570-693: The inclusion of local area networking to be of prime importance to teachers. The operation resulted in advertisements by at least one dealer in Interface Age magazine, but ultimately the attempt failed. The success of the machine in the UK was due largely to its acceptance as an "educational" computer – UK schools used BBC Micros to teach computer literacy , information technology skills. Acorn became more known for its BBC Model B computer than for its other products. Some Commonwealth countries, including India , started their own computer literacy programmes around 1984. Intending to avoid "re-inventing
11700-654: The inherent performance hit, as was the case for the Amstrad CPC , Atari 8-bit computers , and to a lesser extent the ZX Spectrum . Others, like the MSX systems, used entirely separate pools of memory for the CPU and video, slowing access between the two. Furber believed that the Acorn design should have a flat memory model and allow the CPU and video system to access the bus without interfering with each other. To do so,
11830-535: The introduction of the Acorn Electron , with chairman Chris Curry stating at the time that Acorn "would no longer promote it" (the Model A). Early BBC Micros used linear power supplies at the insistence of the BBC, which, as a broadcaster, was cautious about electromagnetic interference . The supplies were unreliable, and after a few months the BBC allowed switched-mode units. An apparent oversight in
11960-513: The keyboard, with the leftmost socket hard-wired for the OS. The intended purpose for the perforated panel on the left of the keyboard was for a Serial ROM or Speech ROM. The paged ROM system is essentially modular. A language-independent system of star commands , prefixed with an asterisk, provides the ability to select a language (for example *BASIC , *PASCAL ), a filing system ( *TAPE , *DISC ), change settings ( *FX , *OPT ), or carry out ROM-supplied tasks ( *COPY , *BACKUP ) from
12090-461: The manufacturing process resulted in many Model Bs producing a constant buzzing noise from the built-in speaker. This fault can be rectified partly by soldering a resistor across two pads. There are five developments of the main BBC Micro circuit board that addressed various issues through the model's production, from 'Issue 1' through to 'Issue 7' with variants 5 and 6 not being released. The 1985 'BBC Microcomputer Service Manual' from Acorn documents
12220-498: The marketing of the "Acorn Proton-based BBC microcomputer for less than £200". The Model A and the Model B were initially priced at £235 and £335 respectively, but increased almost immediately to £299 and £399 due to higher costs. The Model B price of nearly £400 was roughly £1200 (€1393) in 2011 prices – thirty years after its launch – or around £1900 today. Acorn anticipated total sales to be around 12,000 units, but eventually more than 1.5 million BBC Micros were sold. The cost of
12350-545: The means of producing inductance within solid state devices, resistance-capacitance (RC) delays in signal transmission are growing as feature sizes shrink, imposing an additional bottleneck that frequency increases don't address. The RC delays in signal transmission were also noted in "Clock Rate versus IPC: The End of the Road for Conventional Microarchitectures" which projected a maximum of 12.5% average annual CPU performance improvement between 2000 and 2014. A different concept
12480-436: The memory cannot be altered. Writable variants of ROM (such as EEPROM and NOR flash ) share properties of both ROM and RAM, enabling data to persist without power and to be updated without requiring special equipment. ECC memory (which can be either SRAM or DRAM) includes special circuitry to detect and/or correct random faults (memory errors) in the stored data, using parity bits or error correction codes . In general,
12610-506: The memory for each raster line of each character row, before the 6845 advances the memory address to the next character row and repeats the same pattern. If the character address is used to look up a character reference in RAM and the row address to index a table of character graphics in ROM an ordinary text mode display is constructed. The character reference read from memory must be combined with
12740-467: The monolithic (single-chip) 16-bit SP95 SRAM chip for their System/360 Model 95 computer, and Toshiba used bipolar DRAM memory cells for its 180-bit Toscal BC-1411 electronic calculator , both based on bipolar transistors . While it offered higher speeds than magnetic-core memory , bipolar DRAM could not compete with the lower price of the then-dominant magnetic-core memory. In 1966, Dr. Robert Dennard invented modern DRAM architecture in which there's
12870-443: The order it was written. Drum memory could be expanded at relatively low cost but efficient retrieval of memory items requires knowledge of the physical layout of the drum to optimize speed. Latches built out of triode vacuum tubes , and later, out of discrete transistors , were used for smaller and faster memories such as registers . Such registers were relatively large and too costly to use for large amounts of data; generally only
13000-584: The original release of Elite and Granny's Garden . Programming languages and some applications were supplied on ROM chips to be installed on the motherboard. These load instantly and leave the RAM free for programs or documents. Although appropriate content was little-supported by television broadcasters, telesoftware could be downloaded via the optional Teletext Adapter and the third-party teletext adaptors that emerged. The built-in operating system, Acorn MOS , provides an extensive API to interface with all standard peripherals, ROM-based software, and
13130-451: The other hand, the 6845 put the onus on its users to provide enough memory bandwidth to support rereading data on every line. If a character occupies one "word" in the video buffer, a display of l lines and c columns of characters with s scan lines per character requires l × c words of memory to represent a full screen of characters but takes s times that many memory accesses to complete one refresh cycle: each line of character words
13260-571: The paging file form the system's total memory. (For example, if a computer has 2 GB (1024 B) of RAM and a 1 GB page file, the operating system has 3 GB total memory available to it.) When the system runs low on physical memory, it can " swap " portions of RAM to the paging file to make room for new data, as well as to read previously swapped information back into RAM. Excessive use of this mechanism results in thrashing and generally hampers overall system performance, mainly because hard drives are far slower than RAM. Software can "partition"
13390-442: The play field) and one static (usually a status display). Vertical scrolling appears constrained because only the character start address can be set and the row address is always zeroed at frame start, but by adjusting border times it is possible to shift the position the framebuffer is shown on the raster display for increments in between whole characters. With drawing of blank pixels at the screen edges, this can be made invisible to
13520-577: The processor, speeding up the execution of those operations or instructions in cases where they are called upon frequently. Multiple levels of caching have been developed to deal with the widening gap, and the performance of high-speed modern computers relies on evolving caching techniques. There can be up to a 53% difference between the growth in speed of processor and the lagging speed of main memory access. Solid-state hard drives have continued to increase in speed, from ~400 Mbit/s via SATA3 in 2012 up to ~7 GB/s via NVMe / PCIe in 2024, closing
13650-544: The respective countries. From June 1983 the name was always spelled out completely – "British Broadcasting Corporation Microcomputer System" – to avoid confusion with Brown, Boveri & Cie in international markets, after warnings from the Swiss multinational not to market the computer with the BBC label in West Germany, thus forcing Acorn to relabel "hundreds of machines" to comply with these demands. US models include
13780-598: The role of computers in the curriculum. By October 1983, the US operation reported that American schools had placed orders with it totalling $ 21 million . In one deployment in Lowell, Massachusetts valued at $ 177,000, 138 BBC Micros were installed in eight of the 27 schools in the city, with the computer's networking capabilities, educational credentials, and the availability of software with "high education quality" accompanied by "useful lesson plans and workbooks" all given as reasons for selecting Acorn's machine in preference to
13910-448: The row address to form the address for the character graphics ROM, with the character reference selecting a set of scan line patterns that forms one character and the row address indexing into that set to select one scan line. In other words, the ROM address is split into two parts in order to use the ROM as a two-dimensional array: the first dimension selects a character, and the second selects
14040-455: The same amount of time irrespective of the physical location of data inside the memory, in contrast with other direct-access data storage media (such as hard disks and magnetic tape ), where the time required to read and write data items varies significantly depending on their physical locations on the recording medium, due to mechanical limitations such as media rotation speeds and arm movement. In today's technology, random-access memory takes
14170-435: The same type, simply because it takes longer for signals to traverse a larger circuit. Constructing a memory unit of many gibibytes with a response time of one clock cycle is difficult or impossible. Today's CPUs often still have a mebibyte of 0 wait state cache memory, but it resides on the same chip as the CPU cores due to the bandwidth limitations of chip-to-chip communication. It must also be constructed from static RAM, which
14300-569: The screen. Features specific to some versions of BASIC, like vector graphics , keyboard macros , cursor-based editing, sound queues, and envelopes , are in the MOS ROM and made available to any application. BBC BASIC itself, being in a separate ROM, can be replaced with another language. BASIC, other languages, and utility ROM chips reside in any of four 16 KB paged ROM sockets, with OS support for sixteen sockets via expansion hardware. The five (total) sockets are located partly obscured under
14430-445: The signals necessary to interface with a raster display but does not generate the actual pixels , though it does contribute cursor and video-blanking information to the pixel video (intensity) signals. It is used to produce correctly timed horizontal and vertical sync and provide the address in memory from which the next pixel or set of pixels should be read. The process of reading that value, converting it into pixels, and sending it to
14560-405: The start address for its display once per frame . However, if the internal timing values on the chip are altered at the correct time it can be made to prepare for a new frame without ending the current one - creating a non-continuous break in generated addresses midway through the display. This is commonly used by demos and much more rarely games to provide one moving area of the display (usually
14690-474: The system variables and hardware, favouring official system calls . This was ostensibly to make sure programs keep working when migrated to coprocessors that utilise the Tube interface, but it also makes BBC Micro software more portable across the Acorn range. Whereas untrappable PEEKs and POKEs are used by other computers to reach the system elements, programs in either machine code or BBC BASIC instead pass parameters to an operating system routine. In this way,
14820-685: The system's functionality. Often the ROM is a device driver for mass storage combined with a filing system, starting with Acorn's 1982 Disc Filing System (DFS) which API became the de facto standard for floppy-disc access. The Acorn Graphics Extension ROM (GXR) expands the VDU routines to draw geometric shapes, flood fills, and sprites. During 1985, Micro Power designed and marketed a Basic Extension ROM, introducing statements such as WHILE , ENDWHILE , CASE , WHEN , OTHERWISE , ENDCASE , and direct mode commands including VERIFY . Acorn strongly discouraged programmers from directly accessing
14950-400: The term RAM refers solely to solid-state memory devices (either DRAM or SRAM), and more specifically the main memory in most computers. In optical storage, the term DVD-RAM is somewhat of a misnomer since, it is not random access; it behaves much like a hard disc drive if somewhat slower. Aside, unlike CD-RW or DVD-RW , DVD-RAM does not need to be erased before reuse. The memory cell is
15080-558: The transistor leakage current increases, leading to excess power consumption and heat... Secondly, the advantages of higher clock speeds are in part negated by memory latency, since memory access times have not been able to keep pace with increasing clock frequencies. Third, for certain applications, traditional serial architectures are becoming less efficient as processors get faster (due to the so-called von Neumann bottleneck ), further undercutting any gains that frequency increases might otherwise buy. In addition, partly due to limitations in
15210-570: The user creating just the illusion of a smooth vertical scroll. BBC Micro The BBC Microcomputer System , or BBC Micro , is a series of microcomputers designed and built by Acorn Computers Limited in the 1980s for the Computer Literacy Project of the BBC . The machine was the focus of a number of educational BBC TV programmes on computer literacy, starting with The Computer Programme in 1982, followed by Making
15340-510: The wheel", such efforts adopted the BBC Micro in order to take immediate advantage of the extensive range of software already developed under the United Kingdom's own literacy initiative, proposing that software tailored for local requirements would ultimately also be developed. A clone of the BBC Micro was produced by Semiconductor Complex Limited and named the SCL Unicorn. Another Indian computer manufacturer, Hope Computers Pvt Ltd, made
15470-498: Was Samsung's 64 Mbit DDR SDRAM chip, released in June 1998. GDDR (graphics DDR) is a form of DDR SGRAM (synchronous graphics RAM), which was first released by Samsung as a 16 Mbit memory chip in 1998. The two widely used forms of modern RAM are static RAM (SRAM) and dynamic RAM (DRAM). In SRAM, a bit of data is stored using the state of a six- transistor memory cell , typically using six MOSFETs. This form of RAM
15600-554: Was also successful as a home computer in the UK, despite its high price compared to some other home computers sold in the UK at the time. Acorn later employed the machine to simulate and develop the ARM architecture . While nine models were eventually produced with the BBC brand, the phrase "BBC Micro" is usually used colloquially to refer to the first six (Model A, B, B+64, B+128, Master 128, and Master Compact); subsequent BBC models are considered part of Acorn's Archimedes series. During
15730-412: Was an important concern. In the 1970s and early to mid-1980s, chip circuit densities were not very high either, and putting an 80-byte or larger character buffer into a chip like the 6845 might have enlarged the chip die by 50-100%, in turn making it more expensive by a few times that factor due to the exponential growth of chip defect rates and the consequent decline of production yield with die size. On
15860-564: Was duplicated and extended by custom circuits in the EGA and VGA PC video adapters. Originally designed by Hitachi as the HD46505 , Hitachi-built versions are in a wide variety of Japanese computers, from Sony, Sharp, Panasonic, and Casio. Cloned later as MB89321A by Fujitsu. It is also known as the 6845 CRTC or the CRTC6845 , meaning " cathode-ray tube controller". This version was used on
15990-493: Was expected that memory latency would become an overwhelming bottleneck in computer performance. Another reason for the disparity is the enormous increase in the size of memory since the start of the PC revolution in the 1980s. Originally, PCs contained less than 1 mebibyte of RAM, which often had a response time of 1 CPU clock cycle, meaning that it required 0 wait states. Larger memory units are inherently slower than smaller ones of
16120-410: Was introduced in 1965, used a form of capacitor-bipolar DRAM, storing 180-bit data on discrete memory cells , consisting of germanium bipolar transistors and capacitors. While it offered higher speeds than magnetic-core memory, bipolar DRAM could not compete with the lower price of the then dominant magnetic-core memory. Capacitors had also been used for earlier memory schemes, such as the drum of
16250-475: Was only at the design stage at the time, and the Acorn team, including Steve Furber and Sophie Wilson , had one week to build a working prototype from the sketched designs. The team worked through the night to get a working Proton together to show the BBC. Although the BBC expected a computer with the Zilog Z80 CPU and CP/M operating system, not the Proton's 6502 CPU and proprietary operating system,
16380-480: Was popular in the UK, especially in the educational market; about 80% of British schools had a BBC microcomputer. Byte called the BBC Micro Model B "a no-compromise computer that has many uses beyond self-instruction in computer technology". It called the Tube interface "the most innovative feature" of the computer, and concluded that "although some other British microcomputers offer more features for
16510-460: Was possible. Magnetic core memory was the standard form of computer memory until displaced by semiconductor memory in integrated circuits (ICs) during the early 1970s. Prior to the development of integrated read-only memory (ROM) circuits, permanent (or read-only ) random-access memory was often constructed using diode matrices driven by address decoders , or specially wound core rope memory planes. Semiconductor memory appeared in
16640-433: Was released in 1970. The earliest DRAMs were often synchronized with the CPU clock (clocked) and were used with early microprocessors. In the mid-1970s, DRAMs moved to the asynchronous design, but in the 1990s returned to synchronous operation. In 1992 Samsung released KM48SL2000, which had a capacity of 16 Mbit . and mass-produced in 1993. The first commercial DDR SDRAM ( double data rate SDRAM) memory chip
16770-489: Was to combine row address and character address to provide linear scanlines within a non-linear buffer. This has the advantages of easier programming for non-character display and easy smooth horizontal scrolling but can impede smooth vertical scrolling. For example the IBM CGA graphics mode uses a word size of one byte, and each word represents four or eight pixels (in the medium- or high-resolution graphics modes respectively);
16900-467: Was used, for example, on the Apple II Plus and the early Commodore models. The BBC machine, however, was designed to run at the faster CPU speed, 2 MHz , double that of these earlier machines. In this case, bus contention is normally an issue, as there is not enough time for the CPU to access the memory during the period when the video hardware is idle. Some machines of the era accept
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