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 interrupt 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.
| 8080 Register Set |
|
|
| |
8 Bit
"General" Registers |
|
| B |
|
0 |
|
| C |
|
1 |
|
| D |
|
2 |
|
| E |
|
3 |
|
| H |
|
4 |
High Byte |
| L |
|
5 |
Low Byte |
| A |
|
7 |
Accumulator |
| |
Program Registers |
|
| SP |
|
|
Stack Pointer |
| PC |
|
|
Program Counter |
| PSW |
|
|
Program Status Word |
|
| Memory Space |
|
| 216-1 |
|
65,535 |
| 0 |
|
|