Below is the ARM assembly code that multiply two matrices:

.extern printf
.extern scanf
.global main

.text

main:
  push {ip, lr}

  @--read lines and columns of matrix A
  ldr r0, =scanf2
  ldr r1, =linesA
  ldr r2, =columnsA
  bl scanf

  @--read all values of matrix A
  ldr r4, =linesA
  ldr r4, [r4]
  ldr r5, =columnsA
  ldr r5, [r5]
  mul r6, r4, r5
  ldr r7, =matrixA
  loop1:
    cmp r6, #0
    ble endloop1
    ldr r0, =scanf1
    mov r1, r7
    bl scanf
    sub r6, r6, #1
    add r7, r7, #4
    b loop1
  endloop1:

  @--read lines and columns of matrix B
  ldr r0, =scanf2
  ldr r1, =linesB
  ldr r2, =columnsB
  bl scanf

  @--check if matrices are compatible
  ldr r6, = linesB
  ldr r6, [r6]
  ldr r7, = columnsB
  ldr r7, [r7]
  cmp r5, r6
  bne abort
  
  @--read all values of matrix B
  ldr r4, =linesB
  ldr r4, [r4]
  ldr r5, =columnsB
  ldr r5, [r5]
  mul r6, r4, r5
  ldr r7, =matrixB
  loop2:
    cmp r6, #0
    ble endloop2
    ldr r0, =scanf1
    mov r1, r7
    bl scanf
    sub r6, r6, #1
    add r7, r7, #4
    b loop2
  endloop2:
    
  @--load all the necessary numbers on registers r4-r9
  ldr r4, =matrixA
  ldr r5, =linesA
  ldr r5, [r5]
  ldr r6, =columnsA
  ldr r6, [r6]
  ldr r7, =matrixB
  ldr r8, =linesB
  ldr r8, [r8]
  ldr r9, =columnsB
  ldr r9, [r9]

  @--perform calculations and output result
  mov r0, #0
  forLinesA:
    cmp r0, r5
    bge endForLinesA
    mov r1, #0
    forColumnsB:
      cmp r1, r9
      bge endForColumnsB
      mov r2, #0 
      mov r3, #0
      multiply:
        mov r12, #4
        cmp r3, r6
        bge endMultiply
        @--load first element, from matrixA
        mul r10, r0, r6
        add r10, r10, r3
        mul r11, r10, r12
        add r11, r11, r4
        ldr r10, [r11]
        @--load second elements, from matrixB
        mul r11, r9, r3
        add r11, r11, r1
        push {r0}
        mul r0, r11, r12
        add r0, r0, r7
        ldr r11, [r0]
        pop {r0}
        @--multiply and add to result variable
        mul r12, r10, r11
        add r2, r2, r12
        @--loop 
        add r3, r3, #1
        b multiply
      endMultiply:
      @--printf result
      push {r0-r12}
      ldr r0, =printfResult
      mov r1, r2
      bl printf
      pop {r0-r12}
      @--if necessary print white space
      sub r10, r9, #1
      cmp r1, r10
      bge endif
      push {r0-r12}
      ldr r0, =printfSpace
      bl printf
      pop {r0-r12}
      endif:
      @--loop
      add r1, r1, #1
      b forColumnsB
    endForColumnsB:
    @--loop
    sub r10, r5, #1
    cmp r0, r10
    bge endif2
    push {r0-r12}
    ldr r0, =printfNewline
    bl printf
    pop {r0-r12}
    endif2:
    add r0, r0, #1
    b forLinesA
  endForLinesA:        

   b end

  @--abort, imcompatible matrices
  abort:
  ldr r0, =printfError
  bl printf

  @--normal end of program
  end:

  mov r0, #0
  pop {ip, pc}

  
.data
  linesA: .word 0
  columnsA: .word 0
  linesB: .word 0
  columnsB: .word 0
  matrixA: .space 400
  matrixB: .space 400  
  scanf1: .asciz "%d"
  scanf2: .asciz "%d %d"
  printfError: .asciz "Matrizes Incompativeis.n"
  printfResult: .asciz "%05d"
  printfSpace: .asciz " "
  printfNewline: .asciz "n"
  debug: .asciz "elemento = %dn"
  debug2: .asciz "offset = %dn"