/* ARM assembly Raspberry PI  */
/*  program appendstr.s   */

/* Constantes    */
.equ STDOUT, 1                           @ Linux output console
.equ EXIT,   1                           @ Linux syscall
.equ WRITE,  4                           @ Linux syscall

.equ BUFFERSIZE,          100

/* Initialized data */
.data
szMessString:            .asciz "String :\n"
szString1:              .asciz "Alphabet : "
sComplement:            .fill BUFFERSIZE,1,0
szString2:              .asciz "abcdefghijklmnopqrstuvwxyz"

szCarriageReturn:       .asciz "\n"

/* UnInitialized data */
.bss

/*  code section */
.text
.global main
main:

    ldr r0,iAdrszMessString                     @ display message
    bl affichageMess
    ldr r0,iAdrszString1                        @ display begin string
    bl affichageMess
    ldr r0,iAdrszCarriageReturn                 @ display line return
    bl affichageMess
    ldr r0,iAdrszString1
    ldr r1,iAdrszString2
    bl append                                   @ append sting2 to string1
    ldr r0,iAdrszMessString
    bl affichageMess
    ldr r0,iAdrszString1                        @ display string
    bl affichageMess
    ldr r0,iAdrszCarriageReturn
    bl affichageMess

100:                                            @ standard end of the program
    mov r0, #0                                  @ return code
    mov r7, #EXIT                               @ request to exit program
    svc 0                                       @ perform system call
iAdrszMessString:         .int szMessString
iAdrszString1:            .int szString1
iAdrszString2:            .int szString2
iAdrszCarriageReturn:     .int szCarriageReturn
/******************************************************************/
/*     append two strings                         */
/******************************************************************/
/* r0 contains the address of the string1 */
/* r1 contains the address of the string2 */
append:
    push {r0,r1,r2,r7,lr}                       @ save  registers
    mov r2,#0                                   @ counter byte string 1
1:
    ldrb r3,[r0,r2]                             @ load byte string 1
    cmp r3,#0                                   @ zero final ?
    addne r2,#1
    bne 1b                                      @ no -> loop
    mov r4,#0                                   @ counter byte string 2
2:
    ldrb r3,[r1,r4]                             @ load byte string 2
    strb r3,[r0,r2]                             @ store byte string 1
    cmp r3,#0                                   @ zero final ?
    addne r2,#1                                 @ no -> increment counter 1
    addne r4,#1                                 @ no -> increment counter 2
    bne 2b                                      @ no -> loop
100:
    pop {r0,r1,r2,r7,lr}                        @ restaur registers
    bx lr                                       @ return

/******************************************************************/
/*     display text with size calculation                         */
/******************************************************************/
/* r0 contains the address of the message */
affichageMess:
    push {r0,r1,r2,r7,lr}                       @ save  registers
    mov r2,#0                                   @ counter length */
1:                                              @ loop length calculation
    ldrb r1,[r0,r2]                             @ read octet start position + index
    cmp r1,#0                                   @ if 0 its over
    addne r2,r2,#1                              @ else add 1 in the length
    bne 1b                                      @ and loop
                                                @ so here r2 contains the length of the message
    mov r1,r0                                   @ address message in r1
    mov r0,#STDOUT                              @ code to write to the standard output Linux
    mov r7, #WRITE                              @ code call system "write"
    svc #0                                      @ call system
    pop {r0,r1,r2,r7,lr}                        @ restaur registers
    bx lr                                       @ return