8-Bit Computer

Custom 8-Bit computer emulator enspired by the 6502 written in C++. With a custom Assembler that converts the custom Assembly into custom machine code.

Machine Description

8-bit Data width 16-bit address bus (64 KB) 7 General purpose registers 16 Instructions 1 Flag register with 5 diffrent flags I/O for program communication using terminal

------Instructions---------
0x0 
0x1 ADD reg,8bit/reg      reg = reg + reg/8bit
0x2 ADC reg,8bit/reg      reg = reg + reg/8bit + c
0x3 AND reg,8bit/reg      reg = reg & reg/8bit
0x4 OR  reg,8bit/reg      reg = reg | reg/8bit
0x5 NOT reg               reg = ~reg
0x6 CMP reg, 8bit/reg     Reg get comapred against reg/8bit and sets flags (l,e,z)
0x7 LR reg, 8bit/addr     Loads value or value at memory address into register
0x8 WR 8bit/reg, addr     Writes register or 8bit value to memory
0x9 JMP addr              sets PC to addr
0xa JMF flag, addr        sets PC to addr if inputed flag is set flag number XXXX0YYY YYY = flag code
0xb SUB reg, 8bit/reg     reg = reg - reg/8bit
0xc PUSH 8bit/reg         push the value in a register or an 8bit value onto the stack
0xd POP reg               pop the top value in the stack and put it in reg
0xe IP reg                Puts input from input device(terminal) into register
0xf OP string/reg         Outputs register or string into output device(terminal)

Machine code layout

XXXXYZZZ AAAAAAAA BBBBBBBB

X: 4 bit instruction code
Y: 0 for first paramter and 1 for second paramater eg 8bit/reg 8bit value = 0, reg = 1
Z: Register/Flag
A: Second paramter register, value or first byte of 16bit address
B: Second byte of 16bit address

Registers

General Purpose
A 0x0
B 0x1
C 0x2
D 0x3
E 0x4
G 0x5
H 0x6

16bit SP (Stack Pointer)
16bit PC (Program Counter)
8bit IR (Insctruction Register/Current Insctruction)

Flags

[l,e,c,v,z,0,0,0]

l 0x0 LESS
e 0x1 EQUALS
c 0x2 CARRY
v 0x3 OVERFLOW
z 0x4 ZERO

Memory layout

0x0000-0x000a Storage For CPU
- 0x0001 current key being pressed
- 0x0002 if the user has request to shut the window
0x000b-0xA567 General Storage
0xA568-dfff Graphics Memory Storage for 150x100 Screen
0xe000-0xfffe Instructions
0xffff The stack starts at 0xffff and goes down from there

Assembler

Converts custom assembly langauge into machine code. Using a custom built lexer and a custom built phaser to analiyze and generate code.

Programing the Computer

The Assembler assembles all the code in the TestCode.txt file but this can be changed by changing what filepath is passed to Assemble()