CPUdin 😈

Overview

An 8-bit minimalistic cpu implemented in Odin programming language. It has 4 registers, the flag reister and the program counter registers and can address 256 bytes of memory. Following flags are supported: ZF (zero), OF (overflow), GR (greater). For instruction set see the Encoding section. All programs start with progam counter = 0

Requirements

This project requires editline. It must be able to link with it

Build

To build an executable run:

odin build .

To execute a program you must pass a binary with RAM, containing program and other relevant values. For example (assuming urmom is in the same directory as executable):

./8bit-cpu-odin urmom.bin

Encoding

Each opcode is one byte long. Suppose byte has this structure, where each letter is a bit: AABBCCDD. Then following tables describe different opcodes:

AA	operation type
00	arithmetic
01	move low
10	move high
11	misc

Arithmetic

For an arithmetic operation, CC and DD encode a pair of registers and BB specifies operation (CC op DD):

BB	operation
00	add
01	sub
10	mul
11	div

add sets ZF and OF flags. sub sets ZF, OF and GR flags. mul sets ZF and OF flags. div sets ZF flag

Move

Move operations encode register in BB and CCDD specifies actual bits. move low moves in the least significant bits and move low moves in the most significant bits in order from most to least significant bits

Misc

Misc operation type encodes some more operations based on this table:

BB	misc operation type
00	jmp
01	cmp
10	ldr
11	str

ldr and str interact with memory and exchange values of register encoded by DD with memory cell addressed with content of register specified by CC bits

cmp compares registers, specified by CC and DD bits and sets all flags

jmp encodes following jump variants with CC bits and jumps to address specified by DD register:

CC	jump type
00	unconditional
01	jump if GR
10	jump if OF
11	jump if ZF

REPL

CPUdin supports REPL. It has basic command for printing both CPU state and memory; and stepping the execution. If command can be distinguished uniqely from the beginning substring, it most of the time will be; in other cases the behaviour is unspecified. The previous command will be repeated if you simple hit enter

Print

All examples:

$> print
CPU { pc = 0 (0), Flags { OF = 0, ZF = 0, GR = 0 }, r1 = 0 (0), r2 = 0 (0), r3 = 0 (0), r4 = 0 (0) }
$> p pc
pc = a (10)
$> p fl
Flags { OF = 0, ZF = 0, GR = 0 }
$> p flags
Flags { OF = 1, ZF = 0, GR = 0 }
$> p r4
r4 = 35 (53)
$> p r1
r1 = 20 (32)
$> p mem
[41, 81, 52, 91, f2, f7, b, 40, 82, 5b, 90, d2, c4, 19, c1, 0, 0, 34, 35, 18, fd, 7f, 0, 0, a8, a2, a6, 2f, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, a8, a2, a6, 2f, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, a8, a2, a6, 2f, 0, 0, 0, 0, 20, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, a8, a2, a6, 2f, 0, 0, 0, 0, 20, 0, 0, 0, 0, 0, 0, 0, f0, d7, 40, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 8, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, b1, 5f, 41, 0, 0, 0, 0, 0, e0, 92, a3, 18, fd, 7f, 0, 0, 29, 9c, a3, 18, fd, 7f, 0, 0, 3d, 9c, a3, 18, fd, 7f, 0, 0, 29, 9c, a3, 18, fd, 7f, 0, 0, 29, 9c, a3, 18, fd, 7f, 0, 0, b0, 3d, 41, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 29, 9c, a3, 18, fd, 7f, 0, 0, 29, 9c, a3, 18, fd, 7f, 0, 0, 14, 0, 0, 0, 0, 0, 0, 0, 14, 0, 0, 0, 0, 0, 0, 0]
$> p memory 5
f7 (247)

Quit

To quit simple type something that quit starts with, for example q

Step

To step type something that step starts with, for example s