Introduction to 8-Bit Microprocessors
8-bit microprocessors were the first widely used microprocessors in the computing industry, marking a major shift from mainframes and minicomputers to smaller, more affordable systems. The introduction of 8-bit processors in the 1970s enabled the production of personal computers, leading to the popularization of computing and setting the foundation for the modern computing landscape.
8008
The chip, limited by its 18-pin DIP, has a single 8-bit bus working triple duty to transfer 8 data bits, 14 address bits, and two status bits. The small package requires about 30 TTL support chips to interface to memory. For example, the 14-bit address, which can access "16 K × 8 bits of memory", needs to be latched by some of this logic into an external memory address register (MAR). The 8008 can access 8 input ports and 24 output ports.
For controller and CRT terminal use, this is an acceptable design, but it is rather cumbersome to use for most other tasks, at least compared to the next generations of microprocessors. A few early computer designs were based on it, but most would use the later and greatly improved Intel 8080 instead.
8080
6800
The Motorola 6800, also introduced in 1974, was another influential design, showcasing innovation and contributing to the democratization of computing. Built from scratch, unlike the 8008 and 8080, it took a different approach with fewer registers and a more regular instruction set. It was used in "grown-up" devices like cash machines. This was used in the SWTPC 6800 Computer System, or SWTPC 6800, an early microcomputer developed by the Southwest Technical Products Corporation and introduced in 1975. The SWTPC 6800 was one of the first microcomputers based around the Motorola 6800.
6502
The MOS Technology 6502 was released in 1975. When it was introduced, the 6502 was the least expensive microprocessor on the market by a considerable margin. It initially sold for less than one-sixth the cost of competing designs from larger companies, such as the 6800 or Intel 8080. The 6502 and variants of it, were used in personal computers, such as the Apple I, Apple II, Atari 8-bit computers, BBC Micro, PET, VIC-20, and in home video game consoles such as the Atari 2600 and the Nintendo Entertainment System.
Z-80
The Zilog Z-80 released in 1976, was developed by Zilog as an enhanced version of the 8080, offering improvements in interrupt handling, instruction set, and memory management. Ziolog was founded in 1974 by Federico Faggin and Ralph Ungermann, who were soon joined by Masatoshi Shima. All three had left Intel after working on the 4004 and 8080 microprocessors.
Other Chips
The Intel 8085, released in 1976, was an enhanced version of the 8080, aimed at systems requiring fewer chips.
There are other 8-bit microprocessors including offerings by: Fairchild, Microchip, RCA, and Signetics, but we will not be covering them here today.
Each of the processor families had its own instruction set and assembly language. The Z-80 was compatible with the 8080 in that it could run the native 8080 code.
Going forward we are only going to be looking at the 8080, and the Z-80, as this is what CP/M was developed to run on in the beginning.
References:
This article uses material from the Wikipedia articles: Microprocessor, 8 Bit Computing, MOS Technology 6502, SWTPC-6800, Zilog, Zilog Z-80. which is released under the Creative Commons Attribution-Share-Alike License 4.0,
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