Assembler for Custom 8-bit TTL CPU

Overview

This project is a two-pass assembler written in C.
It translates my own assembly language into machine code that runs on a modified Ben Eater–style TTL CPU.
The assembler handles opcodes, directives, and labels, and outputs a 16-byte machine code image.

Features

• Supports labels, opcodes, and directives (.org, .word).
• Two-pass design:
  • First pass resolves labels and addresses.
  • Second pass generates final machine code.
• Error reporting shows the filename, line number, and offending source line.
• Produces a 16-byte machine code image (addresses 0x0–0xF).
• Written in portable C (no dependencies beyond standard library).

Usage

Compile

gcc main.c -o assembler

Run

./assembler input.asm

Arguments

• input.asm → The assembly source file to compile.

Example

Assembly Input (program.asm)

    .org 0xf
    .word 0x0 ; i
    .org 0x0

add_loop:

    lda 0xf
    out
    add #0x1
    jc sub_loop
    sta 0xf
    jmp add_loop

sub_loop:

    lda 0xf
    out 
    sub #0x1
    jz add_loop
    sta 0xf
    jmp sub_loop

Run

./assembler program.asm

Output

Successfully compiled!

0000    00011111
0001    11100000
0010    01100001
...

Instruction Set

Mnemonic	Binary Opcode	Notes
lda	0001	Load memory to A register
ldi	0010	Load immediate
sta	0011	Store A register to memory
add	0100	Add memory to A register
sub	0101	Subtract memory from A register
adi	0110	Add immediate
sbi	0111	Subtract immediate
jnz	1010	Jump if not zero
jmp	1011	Unconditional jump
jz	1100	Jump if zero
jc	1101	Jump if carry
out	1110	Output A register
hlt	1111	Halt (no operand)

Assembly Language Notes

• .org <hex> → Sets the starting address (0–15).
• .word <hex> → Inserts a raw 8-bit value (0–255).
• Immediate values use # (e.g., lda #5).
• Labels must end with : when defined.
• Opcodes without operands: hlt, out.

Limitations

• Memory is limited to 16 instructions.
• Immediate/address operands must fit within 0xF.
• Only one input file per run.

Author

Made by Gerald Negvesky
For custom Ben Eater TTL CPU build.