Assembler parameters for routine

[darkfirefighter]

Posted on August 26, 2010 at 14:16

Assembler parameters for routine

[Tesla DeLorean]

Posted on May 17, 2011 at 14:04

Not sure your first example is correct. The value in R0 would be the integer ''i'', it is not a pointer. And loading from an address like ''5'' would cause a bus fault as it is unaligned.

For your second

    LDR r1,[R0, #0] ; ar[0]

    LDR r2,[R0, #4] ; ar[1]

[darkfirefighter]

Posted on May 17, 2011 at 14:04

To my first example:

If I write LDR r1,r0

i get a compiler error

To the second:

Great thanks it works fine :)

Is there a an other way to incremet the pointer? Something like r0++?

Or other said is this LDR r1,[r0,#0] the fastest way to access the array element 0?

Thank your for your great help! :)

[picguy2]

Posted on May 17, 2011 at 14:04

   ldr  r1,[r0]

   ldrh r1,[r0]

   ldrb r1,[r0]

   ldr  r1,[r0],#n

   ldrh r1,[r0],#n

   ldrb r1,[r0],#n

The first 3 instructions are the basic memory loads.  The second 3 add n to the address register after the instruction.  The corresponding stores work the same way.  I don’t know if #-4 would work.  See if the assembler likes it.

I believe the last 3 are 32-bit instructions but don’t know for sure.  My guess is that instructions with the #n take no longer to execute than those without #n.

[damh]

Posted on May 17, 2011 at 14:04

For documentation look at STM32F10xxx_ProgrammingManual.

Fastest way should be:

LDRD r1, r2, [r0]

or

LDMIA r0, {r1,r2}

In both ways r1 is loaded with [r0] and r2 with [r0, 4]; r0 is unchanged.

You can change the ''index'' in many ways (pre, post, up, down) in the same instruction as the load itself.

If you have a loop, you can read i.e. 8 values in one command and increment r0 to the next unread value:

LDMIA r0!, {r1-r8}

LDMIA has better performance in STM32 because the mcu doesn't have a real pipeline infrastructure (the pipeline is blocked in many commands). This command is one of the exceptions that uses the pipeline in fact.

[Tesla DeLorean]

Posted on May 17, 2011 at 14:04

The LOAD/STORE class instructions typically act on memory, long immediate values are usually loaded from a ''literal pool'' in memory. For register to register (CPU) stuff the MOVE class instructions would be more appropriate, but with 3 operand instructions one usually performs some operation on the register as it's being moved to more effectively use the processor.

To step through the array, consider

  LDR R0,[R1,#0]

  ADDS R1, R1, #4 ; ADDS R1,#4