Intel 8080

The Intel 8080 was Intel's second 8-bit microprocessor. It came out in April 1974. It improved on the earlier Intel 8008 and was much more powerful and easier to use.

The 8080 was first made for jobs like calculators, cash registers, and computer terminals. But it soon became popular for many other things, helping start the microcomputer industry.

Several smart design choices made the 8080 very successful. It had a simpler 40-pin package, worked faster, and could handle more memory than the chip before it. These changes came from ideas and feedback from people who used the earlier chip.

The 8080 was used in early personal computers like the Altair 8800 and helped create the x86 architecture that many computers still use today. It could run at 2 MHz and do many tasks quickly, making it a key part of the first home computers.

History

The Intel 8080 was made to fix problems with an earlier chip called the 8008. People wanted a better chip, so Intel began working on the 8080 in 1971. The designers wanted to make a chip that could do more things and work faster.

The 8080 was finished in 1973 and officially released in April 1974. It became popular because it was better and faster than older chips. This chip helped start the era of small computers that many people could use.

Description

Programming model

The Intel 8080 is the successor to the Intel 8008. It uses the same basic instruction set and register model as the 8008, but it is not source code compatible or binary code compatible with it. The 8080 adds 16-bit operations to its instruction set. It can access all of its 16-bit memory space directly. The 8080's 40-pin DIP packaging has a 16-bit address bus and an 8-bit data bus that can reach 64 KiB (2¹⁶ bytes) of memory.

Registers

The processor has seven 8-bit registers (A, B, C, D, E, H, and L). A is the main 8-bit accumulator. The other six registers can be used alone or paired as three 16-bit registers (BC, DE, and HL). The 8080 has a 16-bit stack pointer and a 16-bit program counter.

Flags

The processor keeps internal flag bits (a status register) to show the results of arithmetic and logic instructions. The flags are:

• Sign (S), set if the result is negative.
• Zero (Z), set if the result is zero.
• Parity (P), set if the number of 1 bits in the result is even.
• Carry (C), set if the last addition had a carry or the last subtraction needed a borrow.
• Auxiliary carry (AC or H), used for binary-coded decimal arithmetic (BCD).

Commands, instructions

Instructions are stored in one byte, with some followed by one or two more bytes of data. It has special instructions for calling and returning from programs and for saving and loading 16-bit register pairs. There are eight special instructions (RST) for small programs at fixed addresses. The slowest instruction is XTHL, which swaps the HL register pair with the top of the stack.

8-bit instructions

All 8-bit math operations with two numbers can only use the 8-bit accumulator (the A register). You can increase or decrease any 8-bit register or a byte in memory pointed to by HL. Copying between any two 8-bit registers or between a register and a byte in memory pointed to by HL is supported.

16-bit operations

Though the 8080 is mainly an 8-bit processor, it can do some 16-bit operations. You can load any of the three 16-bit register pairs (BC, DE, or HL) or SP with a 16-bit number, increase or decrease it, or add it to HL. The only 16-bit instruction that changes a flag is DAD, which sets the CY (carry) flag. You can save and load BC, DE, HL, or PSW to and from the stack using PUSH and POP.

Instruction set

See also: Intel 8008 § Instruction set, and Intel 8085 § Instruction set

Input/output scheme

Input output port space

The 8080 has 256 input/output (I/O) ports. These are used with special instructions that take port addresses. This lets the processor use its limited address space more freely. Many CPUs use memory-mapped I/O (MMIO), which uses the same address space for both memory and devices.

Separate stack space

One bit in the processor's state shows when it is using the stack. This feature is rarely used.

Status word

For more advanced systems, at the start of each operation, the processor puts an 8-bit status word on the data bus. This byte has flags that decide whether the processor uses memory or an I/O port.

Interrupts

Hardware interrupts start when the interrupt request (INT) pin is activated. You can turn interrupts on and off with EI and DI instructions. Interrupts are turned off after an INTA; you must turn them back on with code. The 8080 does not support interrupts that cannot be turned off.

Example code

The following 8080/8085 assembler code shows a program named memcpy that copies a group of bytes from one place to another.

Pin use

The address bus uses 16 pins, and the data bus uses 8 pins. The processor needs three power sources (−5, +5, and +12 V) and two timing signals.

Support chips

The Intel 8080 microprocessor was popular because it had many supporting chips to help it work. These chips helped with tasks like sending information, keeping time, controlling devices, and managing work.

The supporting chips included:

• 8214 - Priority Interrupt Control Unit
• 8224 – Clock generator
• 8228/8238 – System controller and bus driver
• (/wiki/Intel_8251) – Communication controller
• (/wiki/Intel_8253) – Programmable interval timer
• (/wiki/Intel_8255) – Programmable peripheral interface
• (/wiki/Intel_8257) – DMA controller
• (/wiki/Intel_8259) – Programmable interrupt controller

Physical implementation

The Intel 8080 microprocessor used a special design called NMOS. This design needed extra power levels, including +12 V and −5 V, besides the usual +5 V used in many electronic parts.

It was made using a process called silicon gate. The chip had very tiny details measuring just 6 micrometers. The chip had around 4,500 tiny parts called transistors connected by metal layers. The chip itself was about 20 mm² in size.

Commercial impact

Applications and successors

The 8080 was used in many early computers, like the MITS Altair 8800, Processor Technology SOL-20, and IMSAI 8080. These computers often used the CP/M operating system. Later, the Zilog Z80 processor became very popular along with CP/M from about 1976 to 1983.

Even after newer processors like the Z80 and 8085 were made, the 8080 stayed popular. In 1979, five companies sold about 500,000 units each month for around $3 to $4 each. The 8080 was also used in early single-board computers like MYCRO-1 and in systems for managing money on buses and trains around the world. It even powered early arcade games, including Gun Fight and Space Invaders.

The 8080 influenced many later processors. Intel made the 8085 after it, and later the 16-bit Intel 8086 and Intel 8088, which IBM used in its first PC. The ideas from the 8080 are still used in computers today.

Industry change

The 8080 changed how computers were made. Before it, big companies like Digital Equipment Corporation, Hewlett-Packard, and IBM built whole computers themselves. The 8080 made it easier for anyone to create computer systems. For example, Hewlett-Packard used it in smart terminals that could run BASIC, and Microsoft’s first product, Microsoft BASIC, was made for the 8080.

The 8080 helped create the x86 family of processors, which are still used in many computers today. Many of the 8080’s basic ideas and instructions are still part of these modern chips.

US Patent

US patent 4010449, Federico Faggin, Masatoshi Shima, Stanley Mazor, "MOS computer employing a plurality of separate chips", issued March 1, 1977. This patent includes important ideas for early computer design.

Cultural impact

The Intel 8080 chip helped change computers forever, and its influence went beyond technology. An asteroid in space was named 8080 Intel to honor the chip’s big role in how computers developed. Some phone numbers, like Microsoft’s 425-882-8080, were chosen because they included the numbers from this important chip. Many of Intel’s own phone numbers also used the ending 8080 to remember this key part of their history.