8-bit computing

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. The 1976 Zilog Z80, one of the most popular 8-bit CPUs (though with 4-bit ALU, at least in the original), was discontinued in 2024 (its product line Z84C00), with Last Time Buy (LTB) orders by 14 June 2024. == Details ==

An 8-bit register can store 28 different values. The range of integer values that can be stored in 8 bits depends on the integer representation used. With the two most common representations, the range is 0 through 255 for representation as an (unsigned) binary number, and −128 through 127 for representation as two's complement. 8-bit CPUs use an 8-bit data bus and can therefore access 8 bits of data in a single machine instruction. The address bus is typically a double octet (16 bits) wide, due to practical and economical considerations. This implies a direct address space of 64 KB (65,536 bytes) on most 8-bit processors. Most home computers from the 8-bit era fully exploited the address space, such as the BBC Micro (Model B) with 32 KB of RAM plus 32 KB of ROM. Others like the very popular Commodore 64 had full 64 KB RAM, plus 20 KB ROM, meaning with 16-bit addressing not all of the RAM could be used by default (e.g. from the included BASIC language interpreter in ROM); without exploiting bank switching, which allows for breaking the 64 KB (RAM) limit in some systems. Other computers would have as low as 1 KB (plus 4 KB ROM), such as the Sinclair ZX80 (while the later very popular ZX Spectrum had more memory), or even only 128 bytes of RAM (plus storage from a ROM cartridge), as in an early game console Atari 2600 and thus 8-bit addressing would have been enough for the RAM, if it would not have needed to cover ROM too). The Commodore 128, and other 8-bit systems, meaning still with 16-bit addressing, could use more than 64 KB, i.e. 128 KB RAM, also the BBC Master with it expandable to 512 KB of RAM. While in general 8-bit CPUs have 16-bit addressing, in some architectures both are available, such as in the MOS Technology 6502 CPU, where the zero page is used extensively, saving one byte in the instructions accessing that page, and also having 16-bit addressing instructions that take 2 bytes for the address plus 1 for the opcode. Some index registers, such as the two in the 6502, are 8-bit. This limits the size of the arrays addressed using indexed addressing instructions to objects of up to 256 bytes without requiring more complicated code. Other 8-bit CPUs, such as the Motorola 6800 and Intel 8080, have 16-bit index registers. == Notable 8-bit CPUs ==

The first commercial 8-bit processor was the Intel 8008 (1972) which was originally intended for the Datapoint 2200 intelligent terminal. Most competitors to Intel started off with such character oriented 8-bit microprocessors. Modernized variants of these 8-bit machines are still one of the most common types of processor in embedded systems. The MOS Technology 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. == Use for training, prototyping, and general hardware education ==

8-bit processors continue to be designed for general education about computer hardware, as well as for hobbyists' interests. One such CPU was designed and implemented using 7400-series integrated circuits on a breadboard. Designing 8-bit CPUs and their respective assemblers is a common training exercise for engineering students, engineers, and hobbyists. FPGAs are used for this purpose. == See also ==