A Brief History of Home Computer CPUs


If you're utterly bored if I mention the words Zilog Z80 and MOS 65C02, please hit the 'Back' button just as soon as you can. If not, then read on (I hope this article will be interesting, no, really, I do!).

I decided to throw together a brief summary of the different CPUs (Central Processing Units) used in our beloved 8-bit and 16-bit computers of olde. They often shared the same ones, making them somewhat compatible with each other, although the different methods the computer manufacturers used to display information to the screen and interface with peripherals effectively removed this cross-compatibility.

I promise to not go into Op Codes and Operands here (that's for another day), just a summary here of the brief history and the computers that made use of them. Interestingly, it is a combination of stories that involve some rather clever fellows looking into the future and, not being content with the status quo, marching out and doing what they wanted - classic stuff of legend!


A MOS 6510 pulled from a Commodore 64MOS Technology 65xx (6502, 65C02, 6510, etc)

The 6502 was designed by Chuck Peddle and Bill Mensch for MOS Technology in 1975, the same team that developed the earlier Motorola 6800. The design team for the 6800 started work on a cheaper alternative to the expensive ($300) 6800, but with persistent management sceptiticism due to good sales of the 6800, many who held the belief and vision of their prototype for a cost-effective IC, along with Peddle and Mensch, left and joined MOS Technology who at the time were a small outfit fabricating chips for third parties as a second source. And so the 6501 was born - pin-compatible with the Motorola 6800 but with a different instruction set. Motorola sued immediately, and it was back to the drawing board for MOS, who soon came up with the 6502 which was unable to work on a 6800 motherboard to keep the lawsuits away, but was otherwise still the 6501 underneath.

At its introduction at the Wescon show in September 1975 it was the cheapest full-featured microprocessor on the market by a considerable margin, costing less than 1/6th of an Intel or Motorola equivalent at $25 (the Intel 8080 and Motorola 6800 were selling for $179 at that same show). The 6502 design features approx. 4,000 transistors, and was also produced under license by Rockwell and Synertek.

The 6510, a later version of the 6502, included a digital I/O port and a tri-state address bus (allowing multiple circuits to share the same bus)

This microprocessor IC family was the core of the following home computers and consoles:

If you're wanting a little more techie info, you can view the entire instruction set for the 65xx family of CPUs here or alternatively visit the 6502 microprocessor resource website at http://www.6502.org.

Here are a few more files that may be of interest:

Assembly in one Step

65C02 Reference Manual

6502 Undocumented Opcodes


Zilog Z80

The Zilog Z80 is an 8-bit microprocessor designed and sold by Zilog from July 1976 onwards. It was designed to be 100% binary compatible with Intel's 8080 microprocessor, as the inventor of the Z80 was an ex-Intel employee who worked on the 8080 and left to found Zilog. It was originally sold in an NMOS package with a maximum clock frequency of 2.5 MHz, although faster versions soon followed, including the Z80A (4 MHz), Z80B (6 MHz), and Z80H (8 MHz). A CMOS version was also developed with clock frequencies ranging from 4 MHz up to 20 MHz.

The Z80's success was huge in the 1970s and 1980s home computer market where it dominated along with the MOS 6502 family. It was succeeded with the Z800 and Z280, but these never shared the same degree of popularity. The Z80 was also used in home and office appliances including fax machines, photocopiers, hard disks, modems, and washing machines!

The Zilog Z80 was used in the following home computers and consoles:

The Z80 was manufactured under license by other companies including NEC, who branded theirs as the NEC 780C-1.
More helpful technical data can be found in these downloads, or at the Z80 home website at http://z80.info.

Z80 Family CPU User Manual (PDF format)

Z80 Undocumented


Motorola 68000 (also referred to simple as the 68K)

Motorola's 68xxx CPU was used in these home computers of the 1980s and 1990s:

  • Atari Jaguar
  • Commodore Amiga
  • Sega Megadrive / Genesis
  • Sega Saturn (as a sound generator, not main CPU)

Some further links for you:

68k Instruction Set

680x0 Programmer's Reference Manual (PDF format)

68k Undocumented Behaviour Notes



An article such as this wouldn't be complete without a comparison chart:-

  MOS 6502 Zilog Z80 Motorola 68000
Core Frequency
1 MHz
4 MHz
8 MHz
Board Frequency
1 MHz
4 MHz
8 MHz
Data bus
Address bus
Number of transistors
Package types