Bitmasking

Abstract

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• A technique used in programming to manipulate individual bits within binary data

AND Bitmasking

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     1 1 1 0 1 1 0 1     input
(&)  0 0 0 0 1 1 1 1      mask
------------------------------
     0 0 0 0 1 1 0 1    output

• The mask position set to 1 extracts the bits of the given input, thus extracting a subnet of bits from the input. In the example above, we are extracting the last 4 bits of the input.

MIPS andi

Perform an AND bitmasking operation between a register and an immediate value.

Given andi $t0, $s0 , 0xF, the mask is 0xF which 1111, so we are getting the last 4 significant bits of $s0.

OR Bitmasking

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     1 1 1 0 1 1 0 1     input
(|)  0 0 0 0 1 1 1 1      mask
------------------------------
     1 1 1 0 1 1 1 1    output

• The mask position set to 1 turns the bits of the given input to 1. In the example above, we are turning the last 4 bits of the input to 1

MIPS ori

Perform an OR bitmasking operation between a register and an immediate value.

Given ori $t0, $s0 , 0xF, the mask is 0xF which 1111, so we are turning the last 4 significant bits of $s0 to 1.

MIPS loading 32-bit constant

Why not just use lw to load the 32-bit constant from memory? Because lw requires us to access memory to fetch data, which is slow.

Why not just use li? It is a pseudo-instruction that gets translated to something similar to what’s shown above.

NOR Bitmasking

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       1 1 1 0 1 1 0 1     input
(nor)  0 0 0 0 0 0 0 0      mask
--------------------------------
       0 0 0 1 0 0 1 0    output

• With 0 mask we are able to use NOR as an NOT to invert the given input

MIPS NOR bitmasking

nor $t0, $t0, $zero is used to to perform NOT operation. MIPS doesn’t have a dedicated NOT instruction to keep the instruction set small.

XOR Bitmasking

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     1 1 1 0 1 1 0 1     input
(^)  0 0 0 0 1 1 1 1      mask
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     1 1 1 0 0 0 1 0    output

• Flip a subset of bits in the given input. The positions in the mask that are set to 1 determine which bits in the input are flipped. The positions in the mask that are set to 0 will not make any changes to the given input

Using XOR as NOT

We can use XOR as NOT by setting the mask to 1.