Optional material: advanced address mode for accessing array elements

  • Offset used to access the array element A[i]: 

      offset = 1*i    if A is a  byte typed array

  offset = 2*i    if A is a short typed array

  offset = 4*i    if A is an  int typed array

  • Recall that: 

         x  <<  1   ≡    2 * x 
     x  <<  2   ≡    4 * x

    Examples: 

              x                  x << 1               x << 2 
     ----------------------------------------------------------
      00000011 (= 3)     00000110 (= 2*3)    00001100 (= 4*3)

Optional material: advanced address mode for accessing array elements

  • ARM's advanced addressing mode used to access array elements: 

       ldrXX  rN, [rN, rk, LSL #n] // Advanced load instruction for arrays

   strXX  rN, [rN, rk, LSL #n] // Advanced load instruction for arrays

  • The memory address used by ldrXX rN, [rN, rk, LSL #n] 

            memory address = rN + ( rk << n )

 i.e.:  memory address = rN +   2n*rk

    Examples: 

      ldrsb  rN, [rN, rk, LSL #0]   // Load byte data at array index rk

  ldrsh  rN, [rN, rk, LSL #1]   // Load short data at array index rk

  ldr    rN, [rN, rk, LSL #2]   // Load int data at array index rk

Postscript

  • This advanced addressing mode is not available in EGTAPI

    • EGTAPI is based on an earlier release of ARM

    • This advanced "scale" register index mode is not available

  • Result:

    • You will get compiler error if you try to use it...

DEMO: demo/asm/3-array/array-ldr-advanced.s