CSE1

A simple custom 8-bit cpu emulator.

I wanted to try out emulating a cpu and originally wanted to start with the 6502 CPU by MOS Technology. After discovering how complex such a task is i decided to try to emulate a very basic cpu first.

This cpu has a very basic, custom instruction set.

Building

Since i did not use a build system you can just open the included Visual Studio 2022 Solution files in 8-bit CPU or 8-bit CPU asm

Structure

The repo is divided into two projects. 8-bit CPU includes the cpu emulation code and 8-bit CPU asm contains the assembler.

The root folder contains the compiled emulator and assembler (both are using a debug build).

Emulator

There are two, 8-bit registers called A and B. All results from arithmetic operations are stored in register A. The cpu has 14 opcodes which are displayed below.

Opcode	Hex	Description	Arg1	Arg2
LDA	0x01	Load a value into register A	Value
LDB	0x02	Load a value into register B	Value
ADD	0x03	Add registers A + B, store in A
SUB	0x04	Subtract B from A, store in A
OUTV	0x05	Output a value from memory	Memory adress
STA	0x06	Store A into memory	Memory adress
LDM	0x07	Load A from memory	Memory adress
JEZ	0x08	Jump if zero flag is set	Memory adress to jump to
JNZ	0x09	Jump if zero flag is not set	Memory adress to jump to
MUL	0x0A	Multiply A * B, store in A
DIV	0x0B	Divide A / B, store in A
INP	0x0C	Take input and store in memory	Memory adress
LD	0x0D	Load a register from memory	Register (0x01 = A, 0x02 = B)	Memory adress
HLT	0xFF	Stop the CPU

You have 256 Bytes of memory available (0x00-0xFF). Obviously memory adresses which contain the program should not be used.

To run a program you need the assembled .bin file that can be generated by the assembler (see below).

Running a program

A program can be run by calling the 8-bit CPU.exe in the cmd using the -I flag to specify the input file. For example 8-bit CPU.exe -I ./add_two_inputs.

Assembler

The assembler can be run using 8-bit CPU asm.exe -I ./add_two_inputs.asm -O ./add_two_inputs.bin where the -I flag is used for the input file and -O for the output file

It has support for creating and using labels. See label_test example Comments using semicolons (;) are also supported.

Example

The following program takes two inputs and adds them together. Note that input1 + input2 > 9 is not handled in this example!

INP 0xF1
LD 0x01 0xF1
LDB 48
SUB
STA 0xF1

INP 0xF2
LD 0x01 0xF2
LDB 48
SUB
STA 0xF2

LD 0x01 0xF1
LD 0x02 0xF2
ADD
STA 0xF3

LD 0x01 0xF3
LDB 48
ADD
STA 0xF3
OUTV 0xF3
HLT

As you can see ASCII conversion has to be done manually.