A 16-bit two's complement negative one:
FF FF = 1111 1111 1111 1111
Sign-extended:
FF FF FF FF = 1111 1111 1111 1111 1111 1111 1111 1111
The sign-extended version is a 32-bit negative one.
addiu Instruction
addiu d,s,const # d ← s + const
# Const is 16-bit two's comp. sign-extended to 32 bits
# when the addition is done. No overflow trap.
The addiu
instruction
includes a 16-bit immediate operand.
When the ALU executes the instruction,
the immediate operand is sign-extended to 32 bits.
If two's complement overflow occurs during the addition, it is ignored.
addi d,s,const # d ← s + const
# Const is 16-bit two's comp. sign-extended to 32 bits
# when the addition is done. Two's comp. overflow trap.
The addi
instruction does trap
if overflow is detected during addition.
We won't use it.
Here is the previous program, that added +146 with -82.
Rewrite it using the addiu instruction.
Put the result in $10.
ori $7, $0, 146 # put +146 into $7
ori $8, $0, 82 # put 82 into $8
nor $8, $8, $0 # reflect
ori $9, $9, 1 #
addu $8, $8, $9 # add 1: $8 = -82
addu $10, $7, $8 # (+146) + (-82)