/osx-asm

PowerPC assembler experiments

Primary LanguageAssemblyGNU General Public License v2.0GPL-2.0

Writing PowerPC assembler for OS X

ABI compatibility

Register use

  • parameters are passed in GPR3-10, FPR1-13, V2-13

    • UNLESS return type is composite (non-scalar), in which case GPR3 is a pointer to space for the returned value, and parameters start at GPR4

  • additional parameters are passed in stack frame

  • callee can choose to use the linkage area to preserve LR, CR, SP

  • return value is placed in GPR3 if scalar

  • long long values are returned in GPR3/GPR4

  • floating point values are returned in FPR1

  • X = preserve. If function wants to use this register, store in stack frame (or red zone if leaf), and restore on exit. (Because the highest numbered registers are preserved, the lmw and stmw instructions can be used to store multiple consecutive registers. This also implies that registers should be allocated highest first.)

  • P = parameter. Not preserved. Use freely.

    • = scratch. Use freely.

  • Not preserved: LR, CTR, XER

GPR  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
     -  X  - P*  P  P  P  P  P  P  P X*  -  X  X  X  X  X  X  X  X  X  X  X  X  X  X  X  X  X  X  X
FPR  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
     -  P  P  P  P  P  P  P  P  P  P  P  P  P  X  X  X  X  X  X  X  X  X  X  X  X  X  X  X  X  X  X
V    0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
     -  -  P  P  P  P  P  P  P  P  P  P  P  P  -  -  -  -  -  -  X  X  X  X  X  X  X  X  X  X  X  X
CR   0  1  2  3  4  5  6  7
     -  -  X  X  X  -  -  -

Stack frame

  • Leaf functions (those that make no calls) can use the 224 byte "red zone" under SP for local variables, and to preserve registers
  • If they need more than that, they need a stack frame
  • Ordinary functions must setup a stack frame for callees

Layout

  • OUR Saved regs
  • OUR Local vars
  • Parameter area
    • ... extra parameters ... large enough for all calls made from this function
    • 52(SP) 8th parameter (GPR10)
    • 48(SP) 7th parameter (GPR9)
    • 44(SP) 6th parameter (GPR8)
    • 40(SP) 5th parameter (GPR7)
    • 36(SP) 4th parameter (GPR6)
    • 32(SP) 3rd parameter (GPR5)
    • 28(SP) 2nd parameter (GPR4)
    • 24(SP) 1st parameter (GPR3)
  • Linkage area:
    • ... reserved ...
    • 8(SP) Space for us to save LR
    • 4(SP) Space for us to save CR
    • 0(SP) Space for us to save SP <-- SP after stack frame setup, before call