String Operations

Working with Strings

Strings in assembly are just sequences of bytes in memory. There is no String type, no .length property, no built-in functions -- you process them one byte at a time using LDRB and STRB. This lesson brings together loops, memory operations, comparisons, and bitwise ops.

Processing strings byte by byte is like the universal translator on the Enterprise -- decoding alien languages one character at a time until the message becomes clear.

Null-Terminated Strings

In C and assembly, strings end with a null terminator -- a byte with value 0. The .asciz directive adds this automatically:

msg:  .asciz "hello"   // Stores: 'h','e','l','l','o', 0  (6 bytes)
msg2: .ascii "hello"   // Stores: 'h','e','l','l','o'     (5 bytes, no terminator)

Iterating Over a String

Loop until you hit the null terminator:

LDR X0, =myStr
loop:
    LDRB W1, [X0], #1    // Load byte, advance pointer
    CBZ W1, done          // Stop at null terminator (0)
    // ... process byte in W1 ...
    B loop
done:

String Length

Count characters until the null terminator:

LDR X0, =myStr
MOV X1, #0            // length = 0
len_loop:
    LDRB W2, [X0], #1
    CBZ W2, len_done
    ADD X1, X1, #1
    B len_loop
len_done:
// X1 = length (not counting the null terminator)

ASCII Case Conversion

ASCII was cleverly designed so that uppercase and lowercase letters differ by exactly one bit:

The difference is bit 5 (value 32). To convert case:

• To lowercase: ORR W0, W0, #0x20 (set bit 5)
• To uppercase: SUB W0, W0, #32 or AND W0, W0, #0xDF (clear bit 5)

But you must check the character is actually a letter first:

CMP W1, #97          // 'a'
B.LT skip            // Not a lowercase letter
CMP W1, #122         // 'z'
B.GT skip            // Not a lowercase letter
SUB W1, W1, #32      // Convert to uppercase
skip:

Common mistake: Applying case conversion to spaces, digits, or punctuation. Always check the range first.

Your Task

Write a program that converts the string "hello" to uppercase and prints HELLO\n.

Hint: Iterate over each byte, check if it is a lowercase letter (97-122), convert if so, store the result, then print.