zero out memory in assembly

Question

I'll start with the code first:

.equ SWI_Open, 0x66        @ open a file
.equ SWI_Close, 0x68       @ close a file
.equ SWI_PrStr, 0x69       @ Write a null-ending string
.equ SWI_RdStr, 0x6a       @ Read a string
.equ Stdout,  1            @ Set output target to be Stdout
.equ SWI_Exit, 0x11        @ Stop execution
.global _start
.text

_start:
    ldr r0,=InFileName
    mov r1,#0
    swi SWI_Open
    bcs InFileError
    ldr r1,=InFileHandle
    str r0,[r1]
    mov r8, r0      @ r8 will hold the address of the file handle
ReadLoop:
    mov r0, r8
    ldr r1, =LineArray
    mov r2, #256
    swi SWI_RdStr
    bcs EndReached
    @ r1 now has address of the read line
    mov r9, r1      @ r9 will hold the memory address of the read line
    mov r4, #0      @ r4 = START
    mov r5, #1      @ r5 = END 
    bl PerLineFunc      @ r0/r1 args passed to PerLineFunc
    @ ShuffleWord within PerLineFunc should change the words around, so now we just need to print the new line of words
    mov r0, #Stdout
    mov r1, r9
    swi SWI_PrStr
    ldr r1, =NL
    swi SWI_PrStr
    bal ReadLoop

SWI_RdStr reads a line from the given file and stores it into memory with the address location stored into r1. My issue is that I don't know how to zero out the memory where the line was read into. If the first line is 20 characters, and the next line is 15, then the last 5 characters of the first line are still in memory after the 2nd line is read.

I was thinking it would be something like:

ldr r1, =LineArray
str #0, [r1]

but that returns a syntax error... I'm just trying to reset the memory before running the ReadLoop again.

Edit: Posting answer in OP. This first part needs to be added to the end of ReadLoop

mov r0, #0
ldr r1, =LineArray
mov r2, #0
bl EraseMemory  @ r0, r1, and r2 passed into EraseMemory

Here's the EraseMemory function

EraseMemory:
    str r0, [r1]
    add r1, r1, #4
    add r2, r2, #1
    cmp r2, #64         @ 256 / 4 = 64
    bxeq lr             @ This loop should run 64 times to erase all 256 bytes of memory that were used
    bal EraseMemory

question from:https://stackoverflow.com/questions/65909705/put-zero-into-address-using-arm-assembly

Answer 1

You first need to write the immediate value to a register:

ldr r1, =LineArray
mov r0, #0
str r0, [r1]