The ALU conducts calculation and comparison operations for the core. It is purely combinatorial logic.
XLEN=32data width (from rv32)
src1[XLEN-1:0]left-side operandsrc2[XLEN-1:0]right-side operandalu_op[3:0]operation select
result[XLEN-1:0]ALU operation resultzeroresult zero flag
The ALU performs the selected operation on the two input operands and puts the output on result.
The operation selection is decoded using the ALU_* local parameters.
Illegal operation select results in undefined behavior.
The output flags are set based on the result. The carry output is set during add, subtract, or left shift operations. For all other operations the carry output is undefined. The zero output is always set as the bitwise or of all result bits.
Table 1. ALU Op Codes
alu_op |
Name | Operation |
|---|---|---|
| 4'b0000 | ALU_ADD | add |
| 4'b0001 | ALU_SLL | shift left logical |
| 4'b0010 | ALU_SLTU | set if less than unsigned |
| 4'b0011 | ALU_SGEU | set if greater than or equal unsigned |
| 4'b0100 | ALU_XOR | XOR |
| 4'b0101 | ALU_SRL | shift right logical |
| 4'b0110 | ALU_OR | OR |
| 4'b0111 | ALU_AND | AND |
| 4'b1000 | ALU_SUB | subtract |
| 4'b1001 | reserved | |
| 4'b1010 | ALU_SLT | set if less than signed |
| 4'b1011 | ALU_SGE | set if greater than or equal signed |
| 4'b1100 | reserved | |
| 4'b1101 | ALU_SRA | shift right arithmetic |
| 4'b1110 | reserved | |
| 4'b1111 | ALU_NOP | do nothing, pass through src1 to result |
all reserved op codes should act as a NOP