问题
I'm writing a program and I need to determine if bits 3 and 6 are set. I know that I can rotate a word or left/right shift it.
But how do I access individual bit's state? Do I use a bitwise operator like and/xor?
回答1:
You would do a bitwise and with 0x08 and 0x40 (presuming bit 0 is the lowest order bit). You would use the andi instruction to do this.
If $t0 is the value you want to test:
andi $t1, $t0, 0x08
andi $t2, $t0, 0x40
$t1 will be non-zero if bit 3 is set, $t2 will be non-zero if bit 6 is set.
回答2:
Yes, bitwise operators are what you use. You can AND with a bitmask that has only bits 3 and 6 set. Then do a comparison to zero.
something like (I haven't done assembler in a long time):
and r2, r1, 0x48 # r2 = r1 & 0x48
cmp r2, 0x48
jz zzzzzz #jmp to zzzzz if bits 6 and 3 are set
回答3:
One technique for testing a single bit in MIPS assembly is to shift the desired bit into the most-significant bit position and use bltz/bgez to test the state of the bit. This saves an instruction in cases where the andi instruction can't be used to select the desired bit.
来源：https://stackoverflow.com/questions/750013/how-do-i-access-the-state-of-individual-bits-of-a-word-in-mips