Lab 9

3/23 COMP 411

Shifting in MIPS

Connor & Michael

In this lab we will:

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Understand how to combine shifts with AND

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Review XOR

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Quiz

Shift operation

• Used to hone in on a window of bits that you care about
• Usually the window you care about is the right most bit
• Ex (assume 4 bit registers, in MIPS they are 32 bit):
• 12 = 1100
• 12 <<= 1
• Result: 1000
• 12 >>= 1
• Result: 0110

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AND operation

• ANDing with 0 -> make all bits 0 / “masking”
• ANDing with 1 -> Used to copy over the value of a bit into another variable or buffer
• Ex:

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int bit = 1;

int myVar = bit & 1;

(value of myVar is 1)

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int bit = 0;

int myVar = bit & 1;

(value of myVar is 0)

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Exercise 1: using Shift with AND

• In this lab, integers are ALREADY represented as binary
• You just have to tease the binary out using these two operations
• Think of shift and AND as your tools:
• Shift changes the bit you’re focused on
• AND copies the value of a bit into another variable
• In this lab, the challenge is to use these 2 tools to obtain the binary representation of an integer in C (part 1)

Exercise 2: using XOR and shift

• Carefully understand how the algorithm works at each step
• B = R4 XOR R2
• Understand how XOR works
• You will need to use your knowledge of exercise 1, and also append bits while shifting
• Make sure to clearly understand C code, understand implementation

Other cool tricks w/ AND

• Pulling out the bits you want:
• Given 2 bytes: 11111111 10101010
• If you just want the second byte, use (AND + a mask)
• In this case, mask = 00000000 11111111
• 11111111 10101010 AND mask = 00000000 10101010
• Remainder from power of 2
• Using (AND + shift), but with division
• A way to copy the remaining bit after division

• Remainder from power of 2
• Using (AND + shift), but with division
• How division is actually implemented in processor: