Home Computer - Technology


Many home computers were superficially similar. Most had a keyboard integrated into the same case as the motherboard; sometimes a cheap-to-make membrane or chiclet keyboard in the early days, although full-travel keyboards quickly became universal due to overwhelming consumer preference. Most systems could use an RF modulator to display 20–40 column text output on a home television. Indeed, the use of a television set as a display almost defines the pre-PC home computer. Although dedicated composite or "green screen" computer displays were available for this market segment and offered sharper text display and sometimes increased graphics resolution, a monitor was often a later purchase made only after users had bought a floppy disk drive, printer, modem, and the other pieces of a full system. This "peripherals sold separately" approach is another defining characteristic of the home computer era. Many first time computer buyers brought a base C-64 system home and hooked it up to their TV only to find they needed to buy a compatible disk drive (the Commodore 1541 was the only fully compatible model) or Datassette before they could make use of it as anything but a game machine.

In the early part of the 1980s, the dominant microprocessors used in home computers were the 8-bit MOS Technology 6502 (Apple, Commodore, Atari) and Zilog Z80 (TRS-80). A notable exception was the TI-99 series, announced in 1979 with a 16-bit TMS9900 CPU.

Processor clock rates were typically 1–2 MHz for 6502 based CPU's and 2–4 MHz for Z80 based systems (yielding roughly equal performance), but this aspect was not emphasized by users or manufacturers, as the systems' limited RAM capacity, graphics abilities and storage options had more of an effect on performance than CPU speed. Clock rate was considered a technical detail of interest only to users needing accurate timing for their own programs. To economize on component cost, often the same crystal used to produce color television compatible signals was also divided down and used for the processor clock. This meant processors rarely operated at their full rated speed, and had the side-effect that European and North American versions of the same home computer operated at slightly different speeds and different video resolution due to different television standards.

Initially, many home computers used the then-ubiquitous compact audio cassette as a storage mechanism. A rough analogy to how this worked would be to place a recorder on the phone line as a file was uploaded by modem to "save" it, and playing the recording back through the modem to "load". Most cassette implementations were notoriously slow and unreliable, but early 8 inch floppy disk drives as found on more costly business-oriented microcomputers were large and prohibitively expensive. Smaller and cheaper 5.25" form factor drives gained popularity before mass production decreased prices still further, and 5.25" floppy disk drives would remain standard until the end of the 8-bit era. Though external 3.5" drives were made available for home computer systems toward the latter part of the 1980s, most software sold for 8-bit home computers remained on 5.25" disks; 3.5" drives were used for data storage. Standardization of disk formats was not common; sometimes even different models from the same manufacturer used different disk formats. Toward the end of the home computer era, drives for a number of home computer models appeared offering disk-format compatibility with the IBM PC. The disk drives sold with the Commodore 128, Amiga and Atari ST were all able to read and write PC disks, which themselves were undergoing the transition from 5.25" to 3.5" format at the time.

Various copy protection schemes were developed for floppy disks; most were broken in short order. Many users would only tolerate copy protection for games, as wear and tear on disks was a significant issue in an entirely floppy-based system. The ability to make a "working backup" disk of vital application software was seen as important. Copy programs that advertised their ability to copy or even remove common protection schemes were a common category of utility software in this pre-DMCA era.

In contrast to modern computers, home computers most often had their operating system (OS) stored in ROM chips. This made startup times very fast - no more than a few seconds - but made OS upgrades difficult or impossible without buying a new unit. Usually only the most severe bugs were fixed by issuing new ROMs to replace the old ones at the user's cost. In another defining characteristic of the home computer, instead of a command line, the BASIC interpreter served double duty as a user interface. Coupled to a character-based screen or line editor, BASIC's file management commands could be entered in direct mode. The operating systems provided little other support to application programs, but application programs usually accessed hardware directly to perform a specific task anyway, often switching out the ROM based OS completely to free the address space it occupied and maximize RAM capacity. As multitasking was not common on home computers until late in the '80s, this lack of API support wasn't much of a liability.

In an enduring reflection of their early cassette-oriented nature, most home computers loaded their disk operating system (DOS) separately from the main OS. The DOS was only used to send commands to the floppy disk drive and was not loaded to perform other computing functions. One notable exception was Commodore, whose disk drives actually contained a 6502 processor and Commodore DOS in ROM. Many home computers also had a cartridge interface which accepted ROM-based software. This was occasionally used for expansion or upgrades such as fast loaders. Application software on cartridge did exist, and eliminated the need for disk swapping on single drive systems, but the vast majority of cartridges were games.

From about 1985, the high end of the home computer market began to be dominated by "next generation" home computers using the 16-bit Motorola 68000 chip, which enabled the greatly increased abilities of the Amiga and Atari ST series. Graphics resolutions approximately doubled, and color palettes increased from dozens to hundreds or thousands of colors available. Stereo sound became standard for the first time. Clock rates on these systems were approximately 8 MHz with RAM capacities of 256 kB (for the base Amiga 1000 system) up to 1024 kB (1 megabyte, a milestone, first seen on the Atari 1040ST). These systems used 3.5" floppy disks from the beginning but 5.25" drives were made available to facilitate data exchange with IBM PC compatibles. The Amiga and ST both had GUIs inspired by the Apple Macintosh, but at a list price of $2495 (over $5000 in 2007 dollars), the Macintosh itself was too expensive for most households.

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