The Instruction Decoder 4 component is a simple 4-bit instruction decoder. It allows you to enter an instruction from a small subset of instructions and configures the outputs appropriately for that instruction.

It is assumed that there are four registers r0-r3 and two buses A and B. The bus output Ae determines what register is enabled to bus A. The bus output Be determines what register is enabled to bus B or if the immediate output is enabled to bus B.

The supported instructions are:

nop - No operation
mov rd, rs - Mov content from register rs to register rd
mov rd, #value - Move immediate value to register rd

There are five outputs:

Ae - What register to enable onto the A bus
Be - What register to enable onto the B bus
ALUe - What register to write from the ALU bus
Im - Any immediate value in the instruction
C - The command to the ALU - what operation to perform

The supported instructions are:

Ins	Usage	Description	C
nop	nop	Does nothing	111
mov	mov rd, rs	rd ← rs	000
mov	mov rd, #value	rd ← value	000
add	add rd, ra, rb	rd ← ra + rb	001
sub	sub rd, ra, rb	rd ← ra - rb	010
and	and rd, ra, rb	rd ← ra & rb	011
or	or rd, ra, rb	rd ← ra \| rb	100
sll	sll rd, ra, rb	rd ← ra << rb (logical)	101
sla	sla rd, ra, rb	rd ← ra << rb (arithmetic)	110

The nop command has no arguments. The mov command has two arguments and all other commands have three arguments. The third argument (second for a mov) can be a register or immediate value (signed decimal, only).

Suppose you enter this instruction: add r1,r2,r3 The output pins for this instruction are set to:

Ae: 10 - This indicates that register r2 should be sent to the A bus
Be: 011 - This indicates that register r3 should be sent to the B bus
ALUe: 001 - This indicates the ALU result should be written to register r1.
Im - There is no immediate value for this instruction so this is ignored.
C: 001 - This is the binary code for ADD.

For this instruction: sub r0,r2,#5, the outputs are set to:

Ae: 10 - This indicates that register r2 should be sent to the A bus
Be: 100 - This indicates that the immediate value (5) should be sent to the B bus
ALUe: 000 - This indicates the ALU result should be written to register r0.
Im: 0101 - This is the immediate value from the instruction.
C: 010 - This is the binary code for SUB.