CODEWITHSUNDEEP
cassemblyarm
Davis
Davis

Reputation: 109

ARM Assembly: Getting a string from STDIN

Im currently in a CS course and we've just started working with ARM Assembly on the Raspberry Pi. It's proving to be decently difficult and was wondering if anyone could help. My current assignment is to take a string from stdin (Using scanf) and count the number of characters in it, then return that number (So basically implement my own strlen). I have the basic idea down with this code: 

.section        .rodata
promptWord:
    .ascii  "Enter a word: \000"

readWord:
    .ascii  "%s\000"

printLength:
    .ascii  "Word length is %d characters.\n\000"

.section .data
    .align 2

    .comm   word,4,4
    .text

addrword:  .word word
addrPromptWord: .word promptWord
addrReadWord: .word readWord
addrPrintLength: .word printLength



    .global  main
                                    /* s: r0 */

main:
    stmfd   sp!, {fp, lr}       /* Save pc, lr, r4*/

    ldr r0, addrPromptWord
    bl  printf

    ldr r0, addrReadWord
    ldr r1, addrword
    bl  scanf

    ldr r0, addrword
    ldr r0, [r0]

    mov r1, #0
skip:
    ldrb    r2,[r0]                 /* r2 <- *a */
    mov     r3,#0
    cmp     r2,r3
    beq     endskip                 /* if (*a == 0) jump endskip */
    mov     r3,#1
    add     r0,r0,r3                /* a++ */
    add     r1, r1, r3              /* len++ */
    bal     skip                    /* go to skip */

endskip:
    mov r0, r1                      /* Return len */

    ldmfd   sp!, {fp, pc}

I'm assuming that the issue is with the .data section part of the code since (I'm assuming) that isnt the proper way to align a string. Any help is much appreciated. Thanks!

Upvotes: 4

Views: 7544

Answers (2)

Ning
Ning

Reputation: 199

Why not write a C program do the same thing, and run

gcc -S file.c

That you will see how the compiler deal with it in file.s(assembly code generated by gcc).Even if you do not understand some lines in file.s, it would lead you to the right place of arm assembly manual.

This is not directly answer of your question. But sad I can not comment your post, otherwise I would do that.

Upvotes: 1

doron
doron

Reputation: 28882

I think you are having problems with the scanf part.

You need to supply scanf (in r1) with the address of where the string input needs to go. The big of memory to obtain will be from the stack. Since the ARM ABI uses a full descending stack, you subtract the number of bytes you need from the current stack pointer, just make sure this is word aligned. You then can copy the new sp to r1 which will then get used in scanf.

You probably do not need the addr stuff in your data section. You might need to specify that your code goes into the text section and there is nothing stopping you putting all your read only data in the text section as well. This will really help if you are using PC relative addresses.

Hope that this helps.

Upvotes: 1

Related Questions