String Comparison

Comparing Strings

Comparing two strings byte-by-byte is one of the most common operations in programming. In assembly, you implement it manually -- there is no built-in strcmp.

The Algorithm

Compare corresponding bytes from two strings until:

1. You find two bytes that differ → strings are not equal
2. Both bytes are null terminators → strings are equal

LDR X0, =str1
LDR X1, =str2

cmp_loop:
    LDRB W2, [X0], #1     // Load byte from str1, advance
    LDRB W3, [X1], #1     // Load byte from str2, advance
    CMP W2, W3
    B.NE not_equal         // Bytes differ
    CBZ W2, equal          // Both are null → match!
    B cmp_loop

equal:
    // Strings are equal
not_equal:
    // Strings are different

Post-Increment for Iteration

Notice [X0], #1 -- this is the post-index addressing mode. It loads the byte at [X0], then adds 1 to X0. Perfect for walking through strings.

Why Check Both Conditions?

• CMP W2, W3 / B.NE catches mismatched bytes at any position
• CBZ W2 catches the end of both strings (if we reach here, W2 == W3, so if W2 is 0 then both strings ended at the same point)

Your Task

Compare the strings "hello" and "hello". If they are equal, print EQ followed by a newline. If not, print NE followed by a newline.