Re: [avr-gcc-list] Register usage in assembler code

[Bruce D. Lightner]

Dale,

> I thought I could lay my hands on the docs that deal with register usage
> in assembler code that is called from a C program, but I can't.
> 
> Would someone point me in the right direction, please?  I'm sure this
> exists, but for the life of me...
> 
> Thanks,
> 
> Dale Seaburg

Here's what I use as a reference...

--------------------------------------------------------------------------

AVR gcc register usage:

Parameters*    Param 1          Param 2          Param 3          Param 4

f(p8)          R25
f(p8,p8,p8,p8) R25              R24              R23              R22
f(p16,p16)     R25:R24°         R23:R22
f(p16,p16,p16) R25:R24          R23:R22          R21:R20
f(p32,p32)     R25:R24:R23:R22° R21:R20:R19:R18
f(p32,p32,p32) R25:R24:R23:R22  R21:R20:R19:R18  R17:R16:R15:R14
f(p8,p16,p32)  R25              R24:R23          R22:R21:R20:R19

return value
p8              R25:R24°°
p16             R25:R24
p32             R25:R24:R23:R22

°   MSB left, LSB right (r25 - lower, r24 - higher, and so on)
°°  R25 is cleared

  R0   scratch
  R1   0 (assumed to always contain zero!!!)
  R2   preserved by caller
  R3   preserved by caller
  R4   preserved by caller
  R5   preserved by caller
  R6   preserved by caller
  R7   preserved by caller
  R8   preserved by caller
  R9   preserved by caller
  R10  preserved by caller
  R11  preserved by caller
  R12  preserved by caller
  R13  preserved by caller
  R14  preserved by caller
  R15  preserved by caller
  R16  preserved by caller
  R17  preserved by caller
  R18  passed parameter / scratch
  R19  passed parameter / scratch
  R20  passed parameter / scratch
  R21  passed parameter / scratch
  R22  passed parameter / scratch
  R23  passed parameter / scratch
  R24  passed parameter / scratch
  R25  passed parameter / scratch
  R26  scratch
  R27  scratch
  R28  preserved by caller
  R29  preserved by caller
  R30  scratch
  R31  scratch

--------------------------------------------------------------------------

I don't know where I got this originally!

Best regards,

Bruce

-- 
 Bruce D. Lightner
 La Jolla, California
 Email: address@hidden
 URL: http://www.lightner.net/lightner/bruce/
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