Advanced

String Processing

Working with strings: length, comparison, and manipulation.

Strings in LC-3

Strings are stored as sequences of ASCII values in consecutive memory locations, terminated by a null character (0x0000). The .STRINGZ directive creates these.

String Length

Walk through the string until you hit the null terminator, counting characters:

.ORIG x3000
LEA R0, STR
PUTS              ; Print the string

; Compute length
LEA R1, STR       ; R1 = pointer
AND R2, R2, #0    ; R2 = length counter

STRLEN_LOOP
LDR R3, R1, #0    ; Load current char
BRz STRLEN_DONE   ; If null, done
ADD R2, R2, #1    ; length++
ADD R1, R1, #1    ; pointer++
BR STRLEN_LOOP

STRLEN_DONE
; R2 now holds the string length
HALT

STR .STRINGZ "Hello"
.END

Character Case Conversion

ASCII uppercase letters are 0x41-0x5A (65-90).
ASCII lowercase letters are 0x61-0x7A (97-122).
The difference is exactly 32 (0x20).

To lowercase: OR with 0x20 (set bit 5) To uppercase: AND with 0xDF (clear bit 5)

Since LC-3 doesn't have OR, we use the De Morgan trick or just add/subtract 32.

.ORIG x3000
; Convert uppercase to lowercase
; 'A' (65) + 32 = 'a' (97)
LEA R1, STR
LOOP
LDR R0, R1, #0
BRz DONE
; Add 32 to make lowercase
ADD R0, R0, #15
ADD R0, R0, #15
ADD R0, R0, #2   ; +32
OUT
ADD R1, R1, #1
BR LOOP

DONE HALT
STR .STRINGZ "HELLO"
.END
Exercise

Write a program that reverses a string. Load the string "ABCDE", reverse it in-place, then print it. Hint: find the length first, then swap characters from the ends toward the middle.

1
R0
x00000
R1
x00000
R2
x00000
R3
x00000
R4
x00000
R5
x00000
R6
x00000
R7
x00000
PCx3000
CC
NZP
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