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| CALLEE | ||
| REGISTER | USAGE | SAVE |
| Integer Registers | ||
| r01 | Used in prolog/epilog | NO |
| r1 | Stack pointer | YES |
| r2 | TOC pointer (reserved) | YES |
| r3 | 1st para/return | NO |
| r4 | 2nd para/return | NO |
| r5-r10 | 3-8th para | NO |
| r11 | Environment pointer | NO |
| r12 | Used by global linkage | NO |
| r13-31 | Global int registers | YES |
| Floating Point Registers | ||
| f0 | Scratch reg | NO |
| f1-13 | 1-13th fp para | NO |
| f14-f31 | Global fp regs | YES |
| Special Registers | ||
| LR | Link register | YES |
| CTR | Count register | NO |
| XER | Fixed pt exception | NO |
| FPSCR | fp status & ctrl | NO |
| CR0-CR7 | Condition reg fields, each 4 bits wide | 2, 3, 4 : YES |
Note that if an OS X/AIX routine accepts a floating point register, the appropriate number of integer registers are skipped, even though the value is passed in a fp reg. Linux does not waste registers in this way.