Basic MIPS - Problems

City College of San Francisco - CS270
Computer Architecture
Module: MIPS-I

Basic MIPS Problems

Problem One:

The NOT instruction in MIPS is a pseudoinstruction. Normally it is implemented using a NOR instruction. Answer the following questions:

Can you implement the operation not $t0,$t1 where $t0 is set to the bitwise complement of $t1 using the nor instruction? (HINT: you can do this in a single instruction - use $0)
Can you implement not $t0, $t1 using the xor instruction? (HINT: This will take several instructions.)

You can check 1. using the simulator. Simply write a three-line program that defines __start and uses the not instruction. Let's go on to 2:

Let's revisit the truth-table for XOR:

X	Y	X XOR Y
0	0	0
0	1	1
1	0	1
1	1	0

Look at the highlighted cells in this truth-table (the four cells in the lower right corner). This is a complement operation! Examining this, we can see that the complement of any bit of Y is Y XOR 1 ! Thus, we simply need to XOR $t1 with a 32-bit value that is all ones! This sounds easy, but we must create this value in a register in order to perform our operation. Here is one way:

lui $at, 0xffff
sra $at,$at,16

Then the complement of $t1 is simply xor $t0, $t1, $at

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