Implementation of a simple microcode-controlled 16-bit CPU using Logisim-evolution.
The CPU on this branch has been altered to have a screen. It can render arbitrary bitmap graphics (grayscale) and is especially well suited to run Bad Apple. Checkout /bad_apple for program files.
bad_apple2_small.mp4
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 | ld | <addr> | Loads value at <addr> into register R1. |
| 0x1 | st | <addr> | Stores value in register R1 at <addr>. |
| 0x2 | ldr | <addr> | Loads value of address stored at <addr> into register R1. |
| 0x3 | str | <addr> | Stores value of register R1 at address stored at <addr>. |
| 0x4 | add | <addr> | Adds value stored at <addr> to value in register R1. |
| 0x5 | sub | <addr> | Subtracts value stored at <addr> from value in register R1. |
| 0x6 | jmp | <addr> | Jumps unconditionally to <addr>. |
| 0x7 | brz | <addr> | Jumps to <addr> if zero flag is set. |
| 0x8 | brs | <addr> | Jumps to <addr> if signed flag is set. |
| 0x9 | mao | Moves value of R1 to output register. |
Program that counts in alternation from 0 to 10 and then restarts.
start:
ld ten
mao
loop:
sub two
add one
mao
brz exit
jmp loop
exit:
ld iter
add one
st iter
jmp start
ten:
10
one:
1
two:
2
iter:
0
Cpu-1.mp4
- 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