dingusppc/zdocs/powerpc.md
joevt cd122ce263 Open Firmware is two words
Not OpenFirmware.
Also fixed a spelling mistake and removed some extra spaces.
2022-08-22 16:47:38 -07:00

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The PowerPC is the main processor behind Power Macs.
# General Notes
All instructions are 32 bits wide, regardless of whether the PowerPC is in 32-bit or 64-bit mode.
Code execution generally begins at 0xFFF00100, which the reset exception vector.
# BATs
The 601 BATs are emulated by the Open Firmware.
# TLBs
Up to 128 instruction entries and 128 data entries can be stored at a time.
# Processor Revisions
| Model | PVR Number | Notable Aspects |
| :------------ | :---------- | :-------------------------- |
| 601 | 0x00010001 | Supports POWER instructions |
| 603 | 0x00030001 | Software-controlled TLBs |
| 604 | 0x00040103 | Ability for Multiprocessing |
| 603E | 0x00060101 | |
| 750 (G3) | 0x00080200 | Built-in L2 data cache |
| 7400 (G4) | 0x000C0101 | AltiVec/VMX support added |
# Registers
| Register Type | Number | Purpose |
| :-------------------------------- | :--------------------- | :---------------------------------------------------- |
| General Purpose (GPR) | 32 | Calculate, Store, and Load 32-bit fixed-point numbers |
| Floating Point (FPR) | 32 | Calculate, Store, and Load 32-bit or 64-bit floating-point numbers |
| Special Purpose (SPR) | Up to 1024 (in theory) | Store and load specialized 32-bit fixed-point numbers |
| Segment (SR) | 16 | Calculate, Store, and Load 32-bit fixed-point numbers |
| Time Base Facility (TBR) | 2 | Calculate, Store, and Load 32-bit fixed-point numbers |
| Condition Register | 1 | Stores conditions based on the results of fixed-point operations |
| Floating Point Condition Register | 1 | Stores conditions based on the results of floating-point operations |
| Machine State Register | 1 | |
# HID 0
| Model | Bits Enabled |
| :------------ | :------------------ |
| 601 | (NOT PRESENT) |
| 603 | NHR, DOZE/NAP/SLEEP |
| 604 | NHR |
| 603E | NHR, DOZE/NAP/SLEEP |
| 603EV | NHR, DOZE/NAP/SLEEP |
| 604E | NHR |
| 750 (G3) | NHR, DOZE/NAP/SLEEP |
# Eccentricities
* The HW Init routine used in the ROMs uses the DEC (decrement; SPR 22) register to measure CPU speed. With a PowerPC 601, the DEC register operates on the same frequency as RTC - 7.8125 MHz but uses only 25 most significant bits. In other words, it decrements by 128 at 1/7.8125 MHz.
* Apple's memcpy routine uses double floating-point registers rather than general purpose registers to load and store 2 32-bit values at once. As the PowerPC usually operates on at least a 64-bit bus and floating-point processing comes with the processors by default, this saves some instructions and results in slightly faster speeds.
* As the PowerPC does not have an instruction to load an immediate 32-bit value, it's common to see a lis/ori coding pattern.