Caesar decoder in ARM Assembly - c

Question

As it said in the title, my objective is to write a program in C and ARM assembly that decodes a string by shifting the byte value of each character by a certain amount. The "space" character isn't shifted, instead just copied over to the resulting string. The process ends when the null-terminator is located.
Here is my C code:
#include <stdio.h>
extern void init(char * encrypt);
extern char * decrypt(char * encrypt, int shift);
int main(int argc, char * argv[])
{
char * result;
char encrypt[] = "GSRKVEXYPEXMSRW CSY LEZI JSYRH XLI WLMJX ZEPYI";
int i;
init(encrypt);
for (i = 1; i < 5; i++) {
result = decrypt(encrypt, i);
printf("Possible decrypt with shift %d: %s\n", i, result);
}
}
Here is my ARM code (this is all in one file called decrypt.s):
# init: reserves space for the decryption
.global init
.text
init: stmfd sp!, {v1-v6, lr}
mov v1, a1
bl strlen
bl malloc
bufferAddr: .fill 4, 1, 0
str a1, bufferAddr
#mov a2, v1
#bl strcpy
ldmfd sp!, {v1-v6, pc}
# decrypt: performs shifting of letters to decrypt string
.global decrypt
.text
decrypt: stmfd sp!, {v1-v6, lr}
mov v1, a1 # v1 is the pointer to encrypt (string)
ldr v2, =bufferAddr # v2 is the pointer to result (string)
loop:
ldrb v3, [v1], #1 # v3 is the current char (8-bit number)
cmp v3, #0
streqb v3, [v2], #1
beq endLabel
cmp v3, #32 # check if v3 == "space"
streqb v3, [v2], #1 # if true, store space in result, increment
beq loop # if true, proceed to next char
sub v3, v3, a2 # shift v3 by shift-value
cmp v3, #65 # check if v3 >= 65 (A)
strgeb v3, [v2], #1 # if true, store char in result, increment
bge loop
# if less than A
add v3, v3, #26 # add 26, wrap between A-Z
strb v3, [v2], #1 # if true, store char in result, increment
b loop
endLabel:
ldr a1, =bufferAddr
ldmfd sp!, {v1-v6, pc}
.end
The problem is, it only goes through the loop once, then it gets stuck. Sometimes it prints an error along the lines of "sim: unknown SWI..." or "unknown v6 isbn...". Or worst of all, it just prints nothing (infinite loop?)
I'm not sure what the problem is, my logic seems to make sense.
I read the next byte:
If it's null-terminator, I put it in the resulting string as well, and end the loop.
If it's space, I put the space in the resulting string, and continue the loop.
If it's >= 65 (which it SHOULD be, unless Space/null-terminator), I subtract the shift-value. If the resulting value is >= 65, I put it in the resulting string. If it's < 65, I add 26, then put it in the resulting string, therefore successfully wrapping to the capital letters of the alphabet.
There is (should be) no other case.
Yet I still get errors, or possibly an infinite loop. Any ideas?

Answer 1

str a1, bufferAddr stores the value from the register at the memory location pointed to by bufferAddr. Since this is in the middle of the init function it works first time only.
However ldr v2, =bufferAddr loads the value of bufferAddr, this results in in the result overwriting decrypt.
You should use ldr v2, bufferAddr.