This is a collection of cpu emulators, starting with the very simple SAP-1 computer described in Digital Computer Electronics. The aim is to authentically replicate how a cpu would work if built using TTL logic (and hopefully eventually actually build one).
This is the simplest computer architecture described in Digital Computer Electronics. The cpu only has 16 bytes of memory and only 4 instructions. The only modifications I have made is to add NOP (no operation) and HLT (halt clock) to the instruction set.
This cpu design is very similar to the SAP-1, with a few extra control lines and opcodes. The addition of conditional jump instructions (JC - jump on carry, JZ - jump on zero, JNZ - jump on not zero) make this cpu turing complete, meaning it can theoretically do anything any other cpu can. This is (severely) limited in practice by the fact it can only address 16 bytes of memory.
Extra instructions
| Instruction | Opcode | Description |
|---|---|---|
| STA | 0100 | store accumulator in memory |
| LDI | 0100 | load immediate value into accumulator |
| JMP | 0100 | jump |
| JC | 0100 | jump if carry flag is set |
| JZ | 0100 | jump if zero flag is set |
| JNZ | 0100 | jump if zero flag is not set |
This cpu design is again based on the SAP-1 extended with:
- a full 8-bit address width
- page bit for addressing ROM and RAM
- 8 I/O registers
| Instruction | Opcode | Description | 2 bytes |
|---|---|---|---|
| NOP | 00000 | no operation | |
| HLT | 00001 | halt | |
| LDI 15 | 00010 | load immediate value into accumulator | x |
| LDA [15] | 00011 | load accumulator from memory | x |
| STA 15 | 00100 | store accumulator in memory | x |
| STM [15] | 00101 | Store accumulator to memory address | x |
| ADI 15 | 00110 | add immediate to accumulator | x |
| ADD [15] | 00111 | add memory address to accumulator | x |
| SUI 15 | 01000 | subtract immediate from accumulator | x |
| SUB [15] | 01001 | subtract memory address from accumulator | x |
| CMI 15 | 01010 | compare immediate to accumulator (set flags) | x |
| CMP [15] | 01011 | compare memory address to accumulator (set flags) | x |
| JMP 15 | 10000 | jump | x |
| JC 15 | 10001 | jump if carry flag is set | x |
| JZ 15 | 10010 | jump if zero flag is set | x |
| JNZ 15 | 10011 | jump if zero flag is not set | x |
| PAGE0 15 | 10100 | change page and jump | x |
| PAGE1 15 | 10101 | change page and jump | x |
| RESET | 10110 | reset cpu | |
| OUTA | 11000 | output accumulator to output A | |
| OUTB | 11001 | output accumulator to output B | |
| OUTC | 11010 | output accumulator to output C | |
| OUTD | 11011 | output accumulator to output D | |
| OUTE | 11100 | output accumulator to output E | |
| OUTF | 11101 | output accumulator to output F | |
| INA | 11110 | input A to accumulator | |
| INB | 11111 | input B to accumulator |