Intel 8080 microprocessor architecture

Memory

Program, data and stack memories occupy the same memory space. The total addressable memory size is 64 KB.

Program memory - program can be located anywhere in memory. Jump, branch and call instructions use 16-bit addresses, i.e. they can be used to jump/branch anywhere within 64 KB. All jump/branch instructions use absolute addressing.

Data memory - the processor always uses 16-bit addresses so that data can be placed anywhere.

Stack memory is limited only by the size of memory. Stack grows downward.

First 64 bytes in a zero memory page should be reserved for vectors used by RST instructions.

Interrupts

The processor supports maskable interrupts. When an interrupt occurs the processor fetches from the bus one instruction, usually one of these instructions:

  • One of the 8 RST instructions (RST0 - RST7). The processor saves current program counter into stack and branches to memory location N * 8 (where N is a 3-bit number from 0 to 7 supplied with the RST instruction).
  • CALL instruction (3 byte instruction). The processor calls the subroutine, address of which is specified in the second and third bytes of the instruction.

The interrupts can be enabled or disabled using EI and DI instructions.

I/O ports

256 Input ports
256 Output ports

Registers

Accumulator or A register is an 8-bit register used for arithmetic, logic, I/O and load/store operations.

Flag is an 8-bit register containing 5 1-bit flags:

  • Sign - set if the most significant bit of the result is set.
  • Zero - set if the result is zero.
  • Auxiliary carry - set if there was a carry out from bit 3 to bit 4 of the result.
  • Parity - set if the parity (the number of set bits in the result) is even.
  • Carry - set if there was a carry during addition, or borrow during subtraction/comparison.

General registers:

  • 8-bit B and 8-bit C registers can be used as one 16-bit BC register pair. When used as a pair the C register contains low-order byte. Some instructions may use BC register as a data pointer.
  • 8-bit D and 8-bit E registers can be used as one 16-bit DE register pair. When used as a pair the E register contains low-order byte. Some instructions may use DE register as a data pointer.
  • 8-bit H and 8-bit L registers can be used as one 16-bit HL register pair. When used as a pair the L register contains low-order byte. HL register usually contains a data pointer used to reference memory addresses.

Stack pointer is a 16 bit register. This register is always incremented/decremented by 2.

Program counter is a 16-bit register.

Instruction Set

8080 instruction set consists of the following instructions:

  • Data moving instructions.
  • Arithmetic - add, subtract, increment and decrement.
  • Logic - AND, OR, XOR and rotate.
  • Control transfer - conditional, unconditional, call subroutine, return from subroutine and restarts.
  • Input/Output instructions.
  • Other - setting/clearing flag bits, enabling/disabling interrupts, stack operations, etc.

Addressing modes

Register - references the data in a register or in a register pair.

Register indirect - instruction specifies register pair containing address, where the data is located.

Direct.

Immediate - 8 or 16-bit data.

Comments

Just some friendly corrections

2012-09-07 16:13:28
Posted by: Dan Snyder

I love the I8080 processor quite a lot. I'm not sure why but it was just such an evolutionary step in CPUs! (Not that there weren't many others) But there are two small issues that I believe are in this. To provide some more accuracy:

1) INX/DCX SP (Increment/decrementing the stack pointer) actually only changes the value by 1 not 2. While it seems logical to increment by two (as each value it two bytes) It is the same instruction as any other INX/DCX only using the SP register.

2) For interrupts: CALL instructions could not be issued through an interrupt. Only one 8-bit instruction was fetched and executed, so it was 99% of the time a RST instruction unless it could be handled in one instruction..

I am not 100% certain so here is my source:
www.tramm.li/i8080/Intel%208080-8085%20Assembly%20Language%20Programming%201977%20Intel.pdf

feel free to correct my corrections if I am wrong.

Last modified: 15 Oct 2013
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