ALU controlΒΆ
The ALU control component generates the ALU control signal, that control what function the ALU performs. First of all, the instruction opcode is compressed from 6 to 4 bits to know the instruction format. This is done using the bits 6, 5, 4, and 2 of the opcode.
Only the top 5 bits are needed as inputs (opcode[6:2]), as the lowe two (opcode[1:0]) are always set to 1 according to the RISC-V specification.
Opcode |
Compressed opcode |
Instr. type |
Instr. format |
|---|---|---|---|
0110011 |
0110 |
Arithmetic |
R |
0010011 |
0010 |
Arithmetic with immediate |
I |
0000011 |
0000 |
Load |
I |
0100011 |
0100 |
Store |
S |
1100011 |
1100 |
Conditional branch |
SB |
1101111 |
1101 |
Unconditional branch |
UJ |
With this information the ALU control knows how to decode the instruction. Then
the field func3 and the feld func7 (in the case of I instructions) are
used to determine the ALU function. The ALU function is generated using the 3
bits of func3, with the top two bits of func7 (func7[6:5]). In case
of I instructions, the non-existent func7 bits are set to 0.
Instruction |
func3 |
func7 |
ALU function |
|---|---|---|---|
ADD/ADDI |
000 |
0000000 |
00000 |
SUB |
000 |
0100000 |
00001 |
XOR/XORI |
100 |
0000000 |
10000 |
OR/ORI |
110 |
0000000 |
11000 |
AND/ANDI |
111 |
0000000 |
11100 |