mul

Signed multiply

This instruction is defined by:

Encoding

Synopsis

MUL performs an XLEN-bitxXLEN-bit multiplication of rs1 by rs2 and places the lower XLEN bits in the destination register. Any overflow is thrown away.

If both the high and low bits of the same product are required, then the recommended code sequence is: MULH[[S]U] rdh, rs1, rs2; MUL rdl, rs1, rs2 (source register specifiers must be in same order and rdh cannot be the same as rs1 or rs2). Microarchitectures can then fuse these into a single multiply operation instead of performing two separate multiplies.

Access

Always

Decode Variables

Bits<5> rs2 = $encoding[24:20];
Bits<5> rs1 = $encoding[19:15];
Bits<5> rd = $encoding[11:7];

Execution

Pruned, XLEN == 64
Original

XReg src1 = X[rs1];
XReg src2 = X[rs2];
X[rd] = (src1 * src2)[63:0];

if (implemented?(ExtensionName::M) && (CSR[misa].M == 1'b0)) {
  raise(ExceptionCode::IllegalInstruction, mode(), $encoding);
}
XReg src1 = X[rs1];
XReg src2 = X[rs2];
X[rd] = (src1 * src2)[XLEN - 1:0];