This repository contains the implementation of a 16-bit microcode-controlled CPU I developed. Logisim-evolution has to be installed to open the project file.
- 16-bit arithmetic integer operations (addition, subtraction, multiplication, division).
- 32 control lanes driven by a microcode sequencer with support for unconditional and conditional jumps.
- 12-bit memory addresses allow addressing 4096 words (each 16-bit).
- Simple instruction set allows running arbitrary programs (turing-complete).
Instructions are described in a custom-made high level language which can be compiled to a sequence of microcode operations. The source code for the default instruction set can be found in microcode.txt. Before it can be loaded into the sequencer's ROM chip, the source file has to be translated using the aforementioned compiler which can be found here.
Each instruction is 16-bit wide with the four most significant bits being used to store the opcode.
Refer to the assembler I created to translate programs from assembly to machine code. For writing more complex programs, a custom high level language can be used instead. More information about it can be found here.
| OpCode | Mnemonic | Parameter | Description |
|---|---|---|---|
| 0x0 | lda | <addr> | Loads value at <addr> into register A. |
| 0x1 | sta | <addr> | Stores value in register A at <addr>. |
| 0x2 | ldd | <addr> | Loads value of address stored at <addr> into register A. |
| 0x3 | stt | <addr> | Stores value of register A at address stored at <addr>. |
| 0x4 | add | <addr> | Adds value stored at <addr> to value in register A. |
| 0x5 | sub | <addr> | Subtracts value stored at <addr> from value in register A. |
| 0x6 | mul | <addr> | Multiplies value in register A with value stored at <addr>. |
| 0x7 | div | <addr> | Divides value in register A by value stored at <addr>. |
| 0x8 | mao | Moves value of register A into output register. | |
| 0x9 | cmp | <addr> | Compares value stored at <addr> with value in register A. |
| 0xA | jmp | <addr> | Jumps unconditionally to <addr>. |
| 0xB | bre | <addr> | Jumps to <addr> if cmp determined equality (A == B). |
| 0xC | brn | <addr> | Jumps to <addr> if cmp determined inequality (A != B). |
| 0xD | brg | <addr> | Jumps to <addr> if cmp determined A to be greater than B (A > B). |
| 0xE | brl | <addr> | Jumps to <addr> if cmp deternubed A to be less than B (A < B). |
| 0xF | hlt | Halts executon. |
Program that counts in alternation from 0 to 5 and 5 to 0.
// Programs start executing at the top
lda one
mao
inc: // Labels are an abstraction over addresses
add one
mao
cmp five
brz dec
jmp inc
dec:
sub one
mao
cmp zero
brz inc
jmp dec
zero:
0
one:
1
five:
5
- Microcode compiler: https://github.com/oskar2517/microcode-compiler
- Assembler: https://github.com/oskar2517/logisim-assembler
- High level programming language compiler: https://github.com/oskar2517/logisim-compiler