diff options
Diffstat (limited to 'rtl/src/alu_op_decode.v')
| -rw-r--r-- | rtl/src/alu_op_decode.v | 13 |
1 files changed, 11 insertions, 2 deletions
diff --git a/rtl/src/alu_op_decode.v b/rtl/src/alu_op_decode.v index 4523255..0e985eb 100644 --- a/rtl/src/alu_op_decode.v +++ b/rtl/src/alu_op_decode.v @@ -1,3 +1,9 @@ +// alu op decode: +// Decodes instruction to corresponding alu_op if alu_ctrl is set to ALU_CTRL_OP +// otherwise, if it is ALU_CTRL_ADD, then its sets alu_op to ALU_OP_ADD. +// This is used if the cpu needs to add two values for example to calculate +// the next address of the pc. + module alu_op_decode ( input [6:0] opcode, input [2:0] funct3, @@ -13,7 +19,7 @@ module alu_op_decode ( always @ (*) begin if (alu_ctrl == ALU_CTRL_ADD) begin alu_op = ALU_OP_ADD; - end else if (opcode == OPCODE_REG || opcode == OPCODE_IMM) begin + end else if (opcode == OPCODE_REG || opcode == OPCODE_IMM) begin // instruction is of r-type or i-type case (funct3) FUNCT3_ALU_ADD_SUB: alu_op = (opcode == OPCODE_REG && funct7 == FUNCT7_ALU_SUB) ? ALU_OP_SUB : ALU_OP_ADD; FUNCT3_ALU_SLL: alu_op = ALU_OP_SLL; @@ -25,12 +31,15 @@ always @ (*) begin FUNCT3_ALU_AND: alu_op = ALU_OP_AND; default: alu_op = ALU_OP_ADD; endcase - end else if (opcode == OPCODE_BRANCH) begin + end else if (opcode == OPCODE_BRANCH) begin // instruction is of b-type case (funct3) + // compare equality FUNCT3_BRANCH_BEQ: alu_op = ALU_OP_SUB; FUNCT3_BRANCH_BNE: alu_op = ALU_OP_SUB; + // compare signed FUNCT3_BRANCH_BLT: alu_op = ALU_OP_SLT; FUNCT3_BRANCH_BGE: alu_op = ALU_OP_SLT; + // compare unsigned FUNCT3_BRANCH_BLTU: alu_op = ALU_OP_SLTU; FUNCT3_BRANCH_BGEU: alu_op = ALU_OP_SLTU; default: alu_op = ALU_OP_ADD; |