This is a list of home computers, sorted alphanumerically, which lists all relevant details of their video hardware. Home computers are the second generation of desktop computers, entering the market in 1977 and becoming common during the 1980s. A decade later they were generally replaced by IBM PC compatible "PCs", although technically home computers are also classified as personal computers. Examples of early home computers are the TRS-80, Atari 8-bit computers, BBC Micro, ZX Spectrum, MSX, Amstrad CPC 464, and Commodore 64. Examples of late home computers are MSX 2 systems, and the Amiga and Atari ST systems. Note: in cases of manufacturers who have made both home and personal computers, only machines fitting into the home computer category are listed. Systems in the personal computer category, except for early Macintosh PCs, are generally based on the VGA standard and use a video chip known as a Graphics Processing Unit. Very early PCs used one of the much simpler (even compared to most home computer video hardware) video display controller cards, using parts like the MDA, the Hercules Graphics Card, the CGA and the EGA standard). Only after the introduction of the VGA standard could PCs really compete with the home computers of the same era, such as the Amiga and Atari ST, or even with the MSX-2. Also, not listed are systems that are typically only gaming systems, like the Atari 2600 and the Bally Astrocade, even though these systems could sometimes be upgraded to resemble a home computer.
The importance of having capable video hardware Early home computers all used similar hardware and software, mostly using the 6502, the Z80, or in a few cases the 6809 microprocessor. They could have as little as 1 KB of RAM or as much as 128K, and software-wise, they could use a small 4K BASIC interpreter, or an extended 12K or more BASIC. The basic systems were quite similar with the exception of the video display hardware. As a result, the success of a system proved to primarily rely on the performance of the video display hardware, since this had a direct implication on the kind of games that could be played on the system. The most important aspect of a home computer was how far programmers could push the hardware to create games. A case in point is the Commodore 64. Its microprocessor lacked advanced math functions and was relatively slow. In addition, the built-in BASIC interpreter lacked any sort of graphics commands, as it was the same version that was developed for the older Commodore PET (a computer without any high-resolution graphics capabilities at all). However, these drawbacks were of little consequence, because the C64 had the VIC-II chip. When accessed by machine language programs, the graphic capabilities of this chip made it practical to develop arcade-style games on a home system. Additionally, specific machine language code exploiting quirks of the VIC-II chip allowed for special tricks to draw even better pictures out of the VIC-II chip. The comparatively large memory and the audio capabilities of the C64 also lent themselves well toward the production of larger games. An example of the opposite is the Aquarius by Mattel which had such incredibly limited video hardware that it was retracted from the market after only four months due to poor sales.
Video arbitration logic One major problem that early computer video hardware had to overcome was the video bus arbitration problem. The problem was determining a way to give both the video hardware (VDU) and the CPU continuous read access to the video RAM. The obvious solution, using interleaving time slots for the VDU and RAM was hard to implement because the logic circuits and video memory chips of the time did not have the switching speed necessary to do so. For higher resolutions, the logic and the memory chips were barely fast enough to support reading the display data, let alone for dedicating half the available time for the slow 8-bit CPU. That being said, one system, the Apple II, was one of the first to use a feature of the data-bus logic of the 6502 processor to implement a very early interleaving time slot mechanism to eliminate this problem. The BBC Micro used 4 MHz RAM with a 2 MHz 6502 in order to interleave video accesses with CPU accesses. Most other systems used a much simpler approach, and the TRS-80's video logic was so primitive that it simply did not have any bus arbitration at all. The CPU had access to the video memory at all times. Writing to the video RAM simply disabled the video display logic. The result was that the screen often displayed random horizontal black stripes on screen when there was heavy access to the video RAM, like during a video game. Most systems avoided the problem by having a status register that the CPU could read, and which showed when the CPU could safely write to the video memory. That was possible because a composite video signal blanks the video output signal during the "blanking periods" of the horizontal and especially the long vertical video sync pulses. So by simply waiting for the next blanking period, the stripes were avoidable. This approach did have one disadvantage, it relied on the software not to write to the screen during the non-blanking periods. If the software ignored the status register the stripes would re-appear. Another approach, used by most other machines of the time, was to temporarily stop the CPU using the "WAIT/BUSRQ" (Z80) "WAIT" (6809) or "SYNC" (6502) control signal whenever the CPU tried to write to the screen during a non-blanking period. Yet another, more advanced, the solution was to add a hardware FIFO so that the CPU could write to the FIFO instead of directly to the RAM chips, which were updated from the FIFO during a blanking interval by special logic circuitry. Some later systems started using special "two-port" video memory, called VRAM, that had independent data output pins for the CPU interface and the video logic.
The main classes of video hardware There are two main categories of solutions for a home computer to generate a video signal:
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