Translate a statement with array variables

  • Translate the following program statement: 

    int A[10];
short B[20];
char  k;

int main( )
{
    B[k] = A[4] + 7;
}

    Assembler code:

Translate a statement with array variables

  • (1) Translate the variable definitions: 

    int A[10];
short B[20];
char  k;

int main( )
{
    B[k] = A[4] + 7;
}

    Assembler code: 

    
 .data

 A: .skip 40
 B: .skip 40
 k: .skip 1

Translate a statement with array variables

  • (2) Translate the statement:   load A[4] into a register (for computation) 

    int A[10];
short B[20];
char  k;

int main( )
{
    B[k] = A[4] + 7;
}

    Assembler code: 

     .text        // Get A[4] (in r0)

 movw r0, #:lower16:A
 movt r0, #:upper16:A
 ldr  r0, [r0, #16]    // r0 = A[4]

Translate a statement with array variables

  • (2) Translate the statement:   add 7 

    int A[10];
short B[20];
char  k;

int main( )
{
    B[k] = A[4] + 7;
}

    Assembler code: 

     .text        // Get A[4] (in r0)

 movw r0, #:lower16:A
 movt r0, #:upper16:A
 ldr  r0, [r0, #16]

 mov  r1, #7
 add  r0, r0, r1       // r0 = A[4]+7

Translate a statement with array variables

  • (2) Translate the statement:   get base address and offset of B[k] 

    int A[10];
short B[20];
char  k;

int main( )
{
    B[k] = A[4] + 7;
}

    Assembler code: 

     .text        // Get A[4] (in r0)

 movw r0, #:lower16:A      movw  r2, #:lower16:k
 movt r0, #:upper16:A      movt  r2, #:upper16:k
 ldr  r0, [r0, #16]	   ldrsb r2, [r2]         // r2 = k
                           add   r2, r2, r2       // r2 = 2k
 mov  r1, #7                                      //    = offset of B[k]
 add  r0, r0, r1   

 movw r1, #:lower16:B
 movt r1, #:upper16:B  // r1 = base addr of B[k]

Translate a statement with array variables

  • (2) Translate the statement:   store the LHS (r0) in B[k]: 

    int A[10];
short B[20];
char  k;

int main( )
{
    B[k] = A[4] + 7;
}

    Assembler code: 

     .text        // Get A[4] (in r0)

 movw r0, #:lower16:A      movw  r2, #:lower16:k
 movt r0, #:upper16:A      movt  r2, #:upper16:k
 ldr  r0, [r0, #16]	   ldrsb r2, [r2]         // r2 = k
                           add   r2, r2, r2       // r2 = 2k
 mov  r1, #7                                      //    = offset of B[k]
 add  r0, r0, r1           strh  r0, [r1, r2]

 movw r1, #:lower16:B
 movt r1, #:upper16:B  // r1 = base addr of B[k]

DEMO: demo/asm/3-array/array-example.s