4F

An imaginary 16 bit CPU environment

Notes

It's a project intended for me learn how computer works at the lowest level, potentially educational for others.

Project Content

• ISA Spec
• Emulator
• Assembler

ISA Spec

Register

• Register is 3 bit address that hold 16 bit value

ABI Name	Register	Description	Register Address
pc	r0	Program Counter	0x00
sp	r1	Stack Pointer	0x01
x1	r2	General Purpose	0x02
x2	r3	General Purpose	0x03
x3	r4	General Purpose	0x04
mepc	r5	Exception Program Counter	0x05
mcause	r6	Cause of Exception	0x06
mtvec	r7	Trap Handler Address	0x07

Memory

• 64KB address space (16 bit addresses)

Instruction

Encoding:

• Little Endian is used for 16 bit values in memory.

Instruction Set:

• value is two's complement
• word is 16 bit value

Opcode	Machine Code	Mnemonic	Fields Size	Operation
lw	0x01	lw x1, x2, 5	5, 3, 3, 5 (op, dest, base, offset)	x1 = mem[x2 + (offset * 2)]
sw	0x02	sw x1, x2, 5	5, 3, 3, 5 (op, src, base, offset)	mem[x2 + (offset * 2)] = x1
lui	0x03	lui x1, #0xFF	5, 3, 8 (op, dest, val)	x1 = 0xFF << 8
addi	0x04	addi x1, x2, #0x1F	5, 3, 3, 5 (op, dest, src, val)	x1 = x2 + 0x1F
add	0x05	add x1, x2, x3	5, 3, 3, 3, 2 (op, dest, src1, src2, pad)	x1 = x2 + x3
sub	0x06	sub x1, x2, x3	5, 3, 3, 3, 2 (op, dest, src1, src2, pad)	x1 = x2 - x3
and	0x07	and x1, x2, x3	5, 3, 3, 3, 2 (op, dest, src1, src2)	x1 = x2 & x3
not	0x08	not x1, x2	5, 3, 3, 5 (op, dest, src1, pad)	x1 = ~x2
or	0x09	or x1, x2, x3	5, 3, 3, 3, 2 (op, dest, src1, src2)	x1 = x2 OR x3
xor	0x0a	xor x1, x2, x3	5, 3, 3, 3, 2 (op, dest, src1, src2)	x1 = x2 ^ x3
sll	0x0b	sll x1, x2, x3	5, 3, 3, 3, 2 (op, dest, src1, src2)	x1 = x2 << x3
srl	0x0c	srl x1, x2, x3	5, 3, 3, 3, 2 (op, dest, src1, src2)	x1 = x2 >> x3
sra	0x0d	sra x1, x2, x3	5, 3, 3, 3, 2 (op, dest, src1, src2)	x1 = x2 >>> x3
jr	0x0e	jr x1	5, 3, 8 (op, dest, pad)	PC += x1 * 2
beq	0x0f	beq x1, x2, x3	5, 3, 3, 3, 2 (op, dest, src1, src2)	if (x2 == x3) PC += x1 * 2
bne	0x10	bne x1, x2, x3	5, 3, 3, 3, 2 (op, dest, src1, src2)	if (x2 != x3) PC += x1 * 2
halt	0x1F	halt	5, 11 (op, pad)	stop cpu

Footnote

Missing features:

• ISA Spec:
  • Memory layout
• Assembler:
  • Label resolver
  • Expression
• Emulator:
  • Exception handler