Intel 8080

The Intel 8080 is Intel's second 8-bit microprocessor. Introduced in April 1974, the 8080 was an enhanced non-binary compatible successor to the earlier Intel 8008 microprocessor. Originally intended for use in embedded systems such as calculators, cash registers, computer terminals, and industrial robots, its performance soon led to adoption in a broader range of systems, ultimately launching the microcomputer industry.

Several key design choices contributed to the 8080’s success. Its 40‑pin package simplified interfacing compared to the 8008’s 18‑pin design, enabling a more efficient data bus. The transition to NMOS technology provided faster transistor speeds than the 8008's PMOS, also making it TTL compatible. An expanded instruction set and a full 16-bit address bus allowed the 8080 to access up to 64 KB of memory, quadrupling the capacity of its predecessor. A broader selection of support chips further enhanced its functionality. Many of these improvements stemmed from customer feedback, as designer Federico Faggin and others at Intel heard from industry about shortcomings in the 8008 architecture.

The 8080 found its way into early personal computers such as the Altair 8800 and subsequent S-100 bus systems, and it served as the original target CPU for the CP/M operating system. It directly influenced the later x86 architecture, which was designed so that its assembly language closely resembled that of the 8080, permitting many instructions to map directly from one to the other.

Originally operating at a clock rate of 2 MHz, with common instructions taking between 4 and 11 clock cycles, the 8080 was capable of executing several hundred thousand instructions per second. Later, two faster variants, the 8080A-1 and 8080A-2, offered improved clock speeds of 3.125 MHz and 2.63 MHz, respectively. In most applications, the processor was paired with two support chips, the 8224 clock generator/driver and the 8228 bus controller, to manage its timing and data flow.

Microprocessor customers were reluctant to adopt the 8008 because of limitations such as the single addressing mode, low clock speed, low pin count, and small on-chip stack, which restricted the scale and complexity of software. There were several proposed designs for the 8080, ranging from simply adding stack instructions to the 8008 to a complete departure from all previous Intel architectures. The final design was a compromise between the proposals.

The conception of the 8080 began in the summer of 1971, when Intel wrapped up development of the 4004 and were still working on the 8008. After rumors about the "CPU on a chip" came out, Intel started to see interest in the microprocessor from all sorts of customers. At the same time, Federico Faggin – who led the design of the 4004 and became the primary architect of the 8080 – was giving some technical seminars on both of the aforementioned microprocessors and visiting customers. He found that they were complaining about the architecture and performance of said microprocessors, especially the 8008 – as its speed at 0.5 MHz was "not adequate."

Faggin later proposed the chip to Intel's management and pushed for its implementation in the spring of 1972, as development of the 8008 was wrapping up. Much to his surprise and frustration, however, Intel didn't approve the project. Faggin says that Intel wanted to see how the market would react to the 4004 and 8008 first, while others noted the problems Intel was having getting its latest generation of memory chips out the door and wanted to focus on that. As a result, Intel didn't approve of the project until fall of that year. Faggin hired Masatoshi Shima, who helped design the logic of the 4004 with him, from Japan in November 1972. Shima did the detailed design under Faggin's direction, using the design methodology for random logic with silicon gate that Faggin had created for the 4000 family and the 8008.

The 8080 was explicitly designed to be a general-purpose microprocessor for a larger number of customers. Much of the development effort was spent trying to integrate the functionalities of the 8008's supplemental chips into one package. It was decided early in development that the 8080 was not to be binary-compatible with the 8008, instead opting for source compatibility once run through a transpiler, to allow new software to not be subject to the same restrictions as the 8008. For the same reason, as well as to expand the capabilities of stack-based routines and interrupts, the stack was moved to external memory.