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How the assembler command 'ldrb' works

Il Han
Note: This was originally posted on 9th September 2011 at http://forums.arm.com

Hi,

arch/arm/boot/compressed/head.S (Linux Kernel 3.0.x source)

                /*
              * The kernel build system appends the size of the
              * decompressed kernel at the end of the compressed data
              * in little-endian form.
              */
                ldrb    r9, [r10, #0]
                ldrb    lr, [r10, #1]
                orr  r9, r9, lr, lsl #8
                ldrb    lr, [r10, #2]
                ldrb    r10, [r10, #3]
                orr  r9, r9, lr, lsl #16
                orr  r9, r9, r10, lsl #24

Because the size of decompressed kernel is appended at the end of the compressed data in little-endian form,
the above codes change its byte order to load it into a register.

If a value in the memory is 0x12345678,
it's stored like [78|56|34|12] in the ARM architecture memory space. (in little-endian mode)
And, after the above codes are excuted, it's loaded like [12345678] in the r9 register.

But why is the command 'ldrb' used in the above codes?
Even if just the command 'ldr' is used, the result is same.
And besides, it's far much more simple.


ldr r9 [r10]

The above code also changes its byte order.
It's also loaded like [12345678] in the r9 register.

In big-endian mode of ARM, the result is different??
Parents
  • Arm Employee Badge
    Note: This was originally posted on 9th September 2011 at http://forums.arm.com

    Storing a sequence of (unique) bytes and then loading them as a word will return different values between big and little-endian.



      MOV  r0,#0x00
      MOV  r1,#0x11
      MOV  r2,#0x22
      MOV  r3,#0x33

      STRB r0,[r4,#0]
      STRB r1,[r4,#1]
      STRB r2,[r4,#2]
      STRB r3,[r4,#3]

      LDR  r5,[r4] // Little-endian r5=0x33221100, big-endian r5=0x00112233


    hth
    s.
Reply
  • Arm Employee Badge
    Note: This was originally posted on 9th September 2011 at http://forums.arm.com

    Storing a sequence of (unique) bytes and then loading them as a word will return different values between big and little-endian.



      MOV  r0,#0x00
      MOV  r1,#0x11
      MOV  r2,#0x22
      MOV  r3,#0x33

      STRB r0,[r4,#0]
      STRB r1,[r4,#1]
      STRB r2,[r4,#2]
      STRB r3,[r4,#3]

      LDR  r5,[r4] // Little-endian r5=0x33221100, big-endian r5=0x00112233


    hth
    s.
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