如何“不”在mips程序集中的寄存器

Question

既然mips汇编中没有NOT逻辑运算符，那么一个寄存器的内容怎么可能“不”呢？

Answer 1

You can use nor and the zero -register to fit everything into one instruction:

nor $<dest_reg>, $<in_reg>, $zero

The equivalent logical expressions would be:

a nor 0 = not (a or 0) = not a

Another approach would be using xor and -1 as this is encoded as 1111...1111 in 2s-component.

li $<help_reg>, -1                        # load -1 into help-register
xor $<dest_reg>, $<in_reg>, $<help_reg>   # actual not-operation

Equivalent logical expressions would be:

a xor 1 = (not a) and 1 = not a

Answer 2

Load -1 into another register, then XOR the registers.

https://www.cs.umd.edu/class/sum2003/cmsc311/Notes/Mips/bitwise.html