Bitwise Operations

Bitwise Instructions

Bitwise operations work on individual bits of a value. They are fundamental to low-level programming -- used for hardware registers, network protocols, file permissions, graphics, cryptography, and countless other domains.

"You will be assimilated. Resistance is futile." -- The Borg see everything as ones and zeros, and so do bitwise operations. Welcome to their world.

Quick Reference

AND -- Masking Bits

AND keeps only the bits that are 1 in both operands. It is used to extract or isolate specific bits:

AND X0, X1, #0xFF   // Extract the low byte (bits 0-7)
AND X0, X1, #0x0F   // Extract the low nibble (bits 0-3)

ORR -- Setting Bits

ORR turns bits ON. Any bit that is 1 in either operand will be 1 in the result:

ORR X0, X1, #0x80   // Set bit 7 (turn it on)
ORR X0, X1, X2      // Combine flags from both registers

EOR -- Toggling Bits

EOR flips bits. A bit that differs between the two operands becomes 1:

EOR X0, X1, #0xFF   // Toggle all bits in the low byte
EOR X0, X0, X0      // Clear a register (x XOR x = 0)

Shift Instructions

Shifts move bits left or right, filling the vacated positions with zeros (or sign bits for ASR):

LSL X0, X1, #4      // Shift left by 4: multiply by 16
LSR X0, X1, #4      // Shift right by 4: unsigned divide by 16
ASR X0, X1, #4      // Arithmetic shift right: signed divide by 16

LSL by N multiplies by 2^N. LSR by N divides by 2^N. These are much faster than MUL/UDIV for powers of 2.

Practical Example: Extracting Nibbles

A nibble is 4 bits (half a byte). Each nibble maps to one hex digit. To split a byte into its two hex digits:

LSR X2, X0, #4      // High nibble: shift right by 4
AND X3, X0, #0x0F   // Low nibble: mask bottom 4 bits

Your Task

Write a program that uses bitwise operations to:

1. Start with the value 0xCA (202)
2. Shift right by 4 to get the high nibble (0x0C = 12 = 'C')
3. AND with 0x0F to extract the low nibble (0x0A = 10 = 'A')
4. Print both as hex digits: CA\n

Hint: To convert 0-15 to a hex digit character, add 48 for 0-9 and add 55 for A-F (10 + 55 = 65 = 'A').