3.3: Subtraction in MIPS Assembly

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Subtraction in MIPS assembly is similar to addition with one exception. The sub, subu and subui behave like the add, addu, and addui operators. The only major difference with subtraction is that the subi is not a real instruction. It is implemented as a pseudo instruction, with the value to subtract loaded into the $at register, and then the R instruction sub operator is used. This is the only difference between addition and subtraction.

sub operator, which takes the value of the R_s and R_t registers containing integer numbers, adds the numbers, and stores the value back to the R_d register. The format and meaning are:

format:	`sub R_d, R_s, R_t`
meaning:	`R_d <- R_s - R_t`

sub pseudo operator, which takes the value of R_s, subtracts the 16 bit immediate value in the instruction, and stores the result back in R_t. The format, meaning, and translation are:

format:

subi R_t, R_s, Immediate

meaning:

R_t <- R_s - Immediate

translation:

addi $at, $zero, Immediate

sub R_t, R_s, $at

subi pseudo operator, which takes the value of R_s, subtracts the 16 bit immediate value in the instruction, and stores the result back in R_t. The format, meaning, and translation are:

format:

subi R_t, R_s, Immediate

meaning:

R_t <- R_s - Immediate

translation:

addi $at, $zero, Immediate

sub R_t, R_s, $at

subu operator, which is the same as the add operator, except that the values in the registers are assumed to be unsigned, or whole, binary numbers. There are no negative values, so the values run from 0..2³²-1. The format and the meaning are the same as the add operator above:

format:	`subu R_d, R_s, R_t`
meaning:	`R_d <- R_s + R_t`

subiu pseudo operator, which is the same as the addi operator, but again the numbers are assumed to be unsigned:

format:

subiu R_t, R_s, Immediate

meaning:

R_t <- R_s + Immediate

translation:

addi $at, $zero, Immediate

subu R_t, R_s, $at

In addition to the real operators, there are a number of pseudo sub operators, which use 32-bit immediate values. The 32-bit values are handled exactly as with the add instructions, with a sign extension out to 32 bits.