8-Bit Instructions

Ben Eater 8-Bit Computer Instructions Overview

Opcode	Dec	Bin	Step 0	Step 1	Step 2	Step 3	Step 4	Usage	Description
NOP	00	0000	CO|MI	RO|II|CE	0	0	0	0000 XXXX	No operation
LDA	01	0001	CO|MI	RO|II|CE	IO|MI	RO|AI	0	0001 AAAA	Load contents of memory address AAAA into A register
ADD	02	0010	CO|MI	RO|II|CE	IO|MI	RO|BI	EO|AI|FI	0010 AAAA	Load memory[AAAA] into B, add A + B, store result in A
SUB	03	0011	CO|MI	RO|II|CE	IO|MI	RO|BI	EO|AI|SU|FI	0011 AAAA	Load memory[AAAA] into B, subtract B from A, store result in A
STA	04	0100	CO|MI	RO|II|CE	IO|MI	AO|RI	0	0100 AAAA	Store contents of A at memory address AAAA
LDI	05	0101	CO|MI	RO|II|CE	IO|AI	0	0	0101 VVVV	Load immediate 4-bit value VVVV into A register
JMP	06	0110	CO|MI	RO|II|CE	IO|J	0	0	0110 AAAA	Unconditional jump to address AAAA
JC	07	0111	CO|MI	RO|II|CE	IO|J	0	0	0111 AAAA	Jump to AAAA if carry flag is set
JZ	08	1000	CO|MI	RO|II|CE	IO|J	0	0	1000 AAAA	Jump to AAAA if zero flag is set
	09	1001	CO|MI	RO|II|CE	0	0	0		—
	10	1010	CO|MI	RO|II|CE	0	0	0		—
	11	1011	CO|MI	RO|II|CE	0	0	0		—
	12	1100	CO|MI	RO|II|CE	0	0	0		—
	13	1101	CO|MI	RO|II|CE	0	0	0		—
OUT	14	1110	CO|MI	RO|II|CE	AO|OI	0	0	1110 XXXX	Output contents of A to 7-segment display
HLT	15	1111	CO|MI	RO|II|CE	HLT	0	0	1111 XXXX	Halt execution

Instruction Set

#	Instruction	Desc
01	HLT	Halt the computer
02	MI	Memory address register in
03	RI	RAM in
04	RO	RAM out
05	II	Instruction register in
06	IO	Instruction register out (not needed with 256B RAM mod)
07	AI	Register A in
08	AO	Register A out
09	EO	ALU sum out
10	SU	Switch ALU to substract (otherwise add)
11	BI	Register B in
12	OI	Output in (display bus value on 7segment display as decimal value)
13	CE	Counter enable (increment program counter PC)
14	CO	Counter out (Put program counter value on bus)
15	J	Jump (Counter in = read bus into program counter)
16	FI	Flags register in (store alu flags in flags register)