Mon Nov 19 10:30:10 EST 2012
GCC AVR ABI
r9 :r8 arg0
r19:r18 arg5 rv call-used
r21:r20 arg6 rv ..
r23:r22 arg7 rv ..
r25:r24 arg8 rv ..
??? Not clear from  or , but easy to test.
rv64 = r19:r18:r21:r20:r23:r22:r25:r24
rv64 = r25:r24:r23:r22:r21:r20:r19:r18
r27:r26 X call-used
r29:r28 Y call-saved, frame pointer
r31:r30 Z call-used
What registers are used by the C compiler?
* char is 8 bits, int is 16 bits, long is 32 bits, long long is 64
bits, float and double are 32 bits (this is the only supported
floating point format), pointers are 16 bits (function pointers
are word addresses, to allow addressing up to 128K program memory
space). There is a -mint8 option (see Options for the C compiler
avr-gcc) to make int 8 bits, but that is not supported by avr-libc
and violates C standards (int must be at least 16 bits). It may be
removed in a future release.
* Call-used registers (r18-r27, r30-r31): May be allocated by gcc
for local data. You may use them freely in assembler
subroutines. Calling C subroutines can clobber any of them - the
caller is responsible for saving and restoring.
* Call-saved registers (r2-r17, r28-r29): May be allocated by gcc
for local data. Calling C subroutines leaves them
unchanged. Assembler subroutines are responsible for saving and
restoring these registers, if changed. r29:r28 (Y pointer) is used
as a frame pointer (points to local data on stack) if
necessary. The requirement for the callee to save/preserve the
contents of these registers even applies in situations where the
compiler assigns them for argument passing.
* Fixed registers (r0, r1): Never allocated by gcc for local data,
but often used for fixed purposes:
r0 - temporary register, can be clobbered by any C code (except
interrupt handlers which save it), may be used to remember
something for a while within one piece of assembler code
r1 - assumed to be always zero in any C code, may be used to
remember something for a while within one piece of assembler code,
but must then be cleared after use (clr r1). This includes any use
of the [f]mul[s[u]] instructions, which return their result in
r1:r0. Interrupt handlers save and clear r1 on entry, and restore
r1 on exit (in case it was non-zero).
Function call conventions:
Arguments - allocated left to right, r25 to r8. All arguments are
aligned to start in even-numbered registers (odd-sized arguments,
including char, have one free register above them). This allows
making better use of the movw instruction on the enhanced core. If
too many, those that don't fit are passed on the stack.
Return values: 8-bit in r24 (not r25!), 16-bit in r25:r24, up to 32
bits in r22-r25, up to 64 bits in r18-r25. 8-bit return values are
zero/sign-extended to 16 bits by the called function (unsigned char
is more efficient than signed char - just clr r25). Arguments to
functions with variable argument lists (printf etc.) are all passed
on stack, and char is extended to int.