Intel 8080

The Intel 8080 was an early microprocessor designed and manufactured by Intel. The 8-bit CPU was released in April 1974 running at 2 MHz (at up to 500,000 instructions per second), and is generally considered to be the first truly usable microprocessor CPU design. It was implemented in NMOS.

Programming model

The Intel 8080 was the successor to the Intel 8008; this was due to its being assembly language source compatible[vague], since it used the same instruction set developed by Computer Terminal Corporation. The 8080's large 40 pin DIP packaging permitted it to provide a 16-bit address bus and an 8-bit data bus, allowing easy access to 64 kilobytes of memory.

Registers

The processor had seven 8-bit registers, (A, B, C, D, E, H, and L) where A was the 8-bit accumulator and the other six could be used as either byte-registers or as three 16-bit register pairs (BC, DE, HL) depending on the particular instruction. Some instructions also enabled HL to be used as (a limited) 16-bit accumulator. It also had a 16-bit stack pointer to memory (replacing the 8008's internal stack), and a 16-bit program counter.


Commands

Most of the 8-bit operations were possible between the accumulator and either one of the registers or the memory cell, indexed by the 16-bit value of the register pair HL. Moving operations were supported between any two registers, or between any register and the HL-indexed memory cell. Due to the highly regular machine code format for MOV commands, the opcodes included those for moving a byte from a given register into the same register (MOV A,A , for instance). These commands were seldom used, however, unless programmed delays were needed. What would have been instruction to move from the HL-indexed memory cell into the same memory cell (i.e., MOV M,M) instead was HLT (for halt). HLT halted the processor until external reset or interrupt signals were received.

All processor commands were coded by one byte, but some of them were followed by one or two bytes of data, a memory address, or a port number. The register-to-register data-move commands were all coded by one byte, making up about a quarter of the commands in the processor-command system. The processor had eight one-byte commands to call the subroutines located at the fixed addresses 0000h, 0008h, 0010h, ... 0038h (RST). These commands were frequently used in the interrupt-handling or system-library calls.

The most sophisticated command (and the longest to execute) was XTHL, which was used for exchanging the register pair HL with the value stored at the address indicated by the stack pointer.