Encoding:

POOL32A

000000

rt

rs

rd

0

SUB

0110010000

6

5

5

5

1

10

Format: 

SUB rd, rs, rt

microMIPS

Subtract Word

Purpose:

Subtract Word

To subtract 32-bit integers. If overflow occurs, then trap.

Description: 

 GPR[rd] = GPR[rs] - GPR[rt]

The 32-bit word value in GPR rt is subtracted from the 32-bit value in GPR rs to produce a 32-bit result. If the subtraction results in 32-bit 2's complement arithmetic overflow, then the destination register is not modified and an Integer Overflow exception occurs. If it does not overflow, the 32-bit result is sign-extended and placed into GPR rd.

Restrictions:

On 64-bit processors, if either GPR rt or GPR rs does not contain sign-extended 32-bit values (bits 63..31 equal), then the result of the operation is UNPREDICTABLE.

Operation: 

if NotWordValue(GPR[rs]) or NotWordValue(GPR[rt]) then
   UNPREDICTABLE
endif
temp = (GPR[rs]31||GPR[rs]31..0) - (GPR[rt]31||GPR[rt]31..0)
if temp32 != temp31 then
   SignalException(IntegerOverflow)
else
   GPR[rd] = sign_extend(temp31..0)
endif

Exceptions:

Integer Overflow

Programming Notes:

SUBU performs the same arithmetic operation but does not trap on overflow.