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

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

.equ STRINGSIZE,          500

/* Initialized data */
.data
szMessString:            .asciz "String :\n"
szMessEncrip:            .asciz "\nEncrypted :\n"
szMessDecrip:            .asciz "\nDecrypted :\n"
szString1:               .asciz "Why study medicine because there is internet ?"

szCarriageReturn:       .asciz "\n"

/* UnInitialized data */
.bss
szString2:                .skip  STRINGSIZE
szString3:                .skip  STRINGSIZE
/*  code section */
.text
.global main
main:

    ldr r0,iAdrszMessString                     @ display message
    bl affichageMess
    ldr r0,iAdrszString1                        @ display string
    bl affichageMess
    ldr r0,iAdrszString1
    ldr r1,iAdrszString2
    mov r2,#20                                  @ key
    bl encrypt
    ldr r0,iAdrszMessEncrip
    bl affichageMess
    ldr r0,iAdrszString2                        @ display string
    bl affichageMess
    ldr r0,iAdrszString2
    ldr r1,iAdrszString3
    mov r2,#20                                  @ key
    bl decrypt
    ldr r0,iAdrszMessDecrip
    bl affichageMess
    ldr r0,iAdrszString3                        @ 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
iAdrszMessDecrip:         .int szMessDecrip
iAdrszMessEncrip:         .int szMessEncrip
iAdrszString1:            .int szString1
iAdrszString2:            .int szString2
iAdrszString3:            .int szString2
iAdrszCarriageReturn:     .int szCarriageReturn
/******************************************************************/
/*     encrypt strings                         */
/******************************************************************/
/* r0 contains the address of the string1 */
/* r1 contains the address of the encrypted string */
/* r2 contains the key (1-25)                     */
encrypt:
    push {r3,r4,lr}           @ save  registers
    mov r3,#0                 @ counter byte string 1
1:
    ldrb r4,[r0,r3]           @ load byte string 1
    cmp r4,#0                 @ zero final ?
    streqb r4,[r1,r3]
    moveq r0,r3
    beq 100f
    cmp r4,#65                @ < A ?
    strltb r4,[r1,r3]
    addlt r3,#1
    blt 1b
    cmp r4,#90                @ > Z
    bgt 2f
    add r4,r2                 @ add key
    cmp r4,#90                @ > Z
    subgt r4,#26
    strb r4,[r1,r3]
    add r3,#1
    b 1b
2:
    cmp r4,#97                @ < a ?
    strltb r4,[r1,r3]
    addlt r3,#1
    blt 1b
    cmp r4,#122               @> z
    strgtb r4,[r1,r3]
    addgt r3,#1
    bgt 1b
    add r4,r2
    cmp r4,#122
    subgt r4,#26
    strb r4,[r1,r3]
    add r3,#1
    b 1b

100:
    pop {r3,r4,lr}            @ restaur registers
    bx lr                     @ return
/******************************************************************/
/*     decrypt strings                                           */
/******************************************************************/
/* r0 contains the address of the encrypted string1 */
/* r1 contains the address of the decrypted string */
/* r2 contains the key (1-25)                     */
decrypt:
    push {r3,r4,lr}          @ save  registers
    mov r3,#0                @ counter byte string 1
1:
    ldrb r4,[r0,r3]          @ load byte string 1
    cmp r4,#0                @ zero final ?
    streqb r4,[r1,r3]
    moveq r0,r3
    beq 100f
    cmp r4,#65               @ < A ?
    strltb r4,[r1,r3]
    addlt r3,#1
    blt 1b
    cmp r4,#90               @ > Z
    bgt 2f
    sub r4,r2                @ substract key
    cmp r4,#65               @ < A
    addlt r4,#26
    strb r4,[r1,r3]
    add r3,#1
    b 1b
2:
    cmp r4,#97               @ < a ?
    strltb r4,[r1,r3]
    addlt r3,#1
    blt 1b
    cmp r4,#122              @ > z
    strgtb r4,[r1,r3]
    addgt r3,#1
    bgt 1b
    sub r4,r2                @ substract key
    cmp r4,#97               @ < a
    addlt r4,#26
    strb r4,[r1,r3]
    add r3,#1
    b 1b

100:
    pop {r3,r4,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