zdocs: rewrite OpenFirmware description.
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Maxim Poliakovski
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OpenFirmware is a standard to defined a firmware's interfaces. It primarily uses the Forth programming language. It's available on most PCI Power Macs and all New World Power Macs by holding Command, Option, O, and F together during bootup.
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# OpenFirmware in Power Macintosh
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The bootpath for Old World Macs is usually set to "/AAPL,ROM". New World Macs specify "\\:tbxi" as the boot path instead.
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*compiled from various sources by Max Poliakovski.*
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When using an Old World PCI Power Mac, it gets relocated from 0xFF800000 to 0x00400000 during boot-up, then instruction and data translations are enabled.
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[Open Firmware](https://en.wikipedia.org/wiki/Open_Firmware) is a platform-independent
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boot firmware architecture covered by an IEEE standard.
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There is also a section of NVRAM reserved for boot-up code called "NVRAMRC", ran each time the system is booted up.
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All PowerMacintosh computers run OpenFirmware except the very first generation
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that uses [Nubus](https://en.wikipedia.org/wiki/NuBus) instead of
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[PCI](https://en.wikipedia.org/wiki/Peripheral_Component_Interconnect).
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It gets compiled to native PowerPC code during boot-up.
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OpenFirmware is used to perform hardware identification and initialization during
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the booting process after power-on. It also provides a platform-independent
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description of the attached devices available for operating systems.
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In this respect, OpenFirmware can be compared with [BIOS](https://en.wikipedia.org/wiki/BIOS),
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widely used in the PC world.
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Being based upon the [Forth programming language](https://en.wikipedia.org/wiki/Forth_(programming_language)),
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OpenFirmware offers an operating system, an interactive environment as well as a
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programming language in one package. Its shell can be used as well by users for
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controlling the boot enviroment as by developers for developing and debugging device
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drivers.
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This document focuses on various aspects of Apple's OpenFirmware implementation
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as found in various PowerMacintosh models.
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## OpenFirmware Versions
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### Old World Macs
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| ROM dump/Machine | BootROM codename | OF version | OF image offset |
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|:----------------------------------- :|:----------------:|:-------------:|:---------------:|
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| Power Macintosh 7300/7600/8600/9600 | TNT | 1.0.5 | 0x330000 |
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| Bandai Pippin | Pip | 1.0.5 | 0x330000 |
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| Power Macintosh/Performa 6400 | Alchemy | 2.0 | 0x330000 |
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| Power Macintosh G3 desktop | Gossamer | 2.0f1 | 0x320000 |
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| PowerBook G3 Wallstreet v2 | GRX | 2.0.1 | 0x330000 |
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| Power Macintosh 4400/7220 | Zanzibar | 2.0.2 | 0x330000 |
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| Power Macintosh 6500 | Gazelle | 2.0.3 | 0x330000 |
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| Power Macintosh G3 v3 | Gossamer | 2.4 | 0x320000 |
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### NewWorld Macs
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*TBD*
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## OpenFirmware image
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### Old World Macs
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OpenFirmware in OldWorld Macs is stored in the monolithic 4MB ROM. Its hibernated
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image is located at offset `0x320000` or `0x330000` from beginning of the ROM.
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That corresponds to the physical address `0xFFF20000` or `0xFFF30000`, respectively.
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The size of the OpenFirmware image varies from 98KB (v1.0.5) to 172KB (v2.4).
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Apple's OpenFirmware image has the following structure:
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| Section type | Architecture | Relative Size (v1.0.5) | Relative Size (v2.4) |
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|:------------------:|:------------:|:----------------------:|:--------------------:|
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| OF kernel | PowerPC | 26% | 18% |
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| OF main code | FCode | 62% | 51% |
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| OF device packages | FCode | 12% | 31% |
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OF image is wrapped in the COFF container. At the start of the image, a COFF file
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header and a section header are located. The following example parses the COFF
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headers of the OF image from the Gossamer ROM v3:
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```
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org 0xFFF20000
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; OF COFF file header
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dc.w 0x1DF ; COFF magic
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dc.w 1 ; number of sections
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dc.l 'Gary' ; Gary Davidian's signature replaces time and date
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dc.l 0 ; ptr to the symbol table
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dc.l 0 ; number of entries in the symbol table
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dc.w 0 ; size of the optional header
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dc.w 0 ; flags
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; OF COFF section header
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dc.b '.text', 0, 0, 0 ; section name
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dc.l 0 ; physical address
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dc.l 0 ; virtual address
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dc.l 0x2ADB0 ; section size in bytes (= 171KB)
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dc.l 0x3C ; file offset to section data
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dc.l 0 ; file offset to section relocations
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dc.l 0 ; file offset to line number entries
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dc.w 0 ; number of relocation entries
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dc.w 0 ; number of line number entries
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dc.l 0x20 ; section flags (this section contains executable code)
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```
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Right after the COFF section header, a vital OF kernel data structure called `StartVec`
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is located. It contains among others several important offsets into the OF image
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for finding all required OF parts. That's also the location HWInit passes control
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to when invoking OF:
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```
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org 0xFFF2003C
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; OF execution starts here
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mflr r11 ; save return address to HWInit in R11
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bl OF_kernel_start ; pass control to OF kernel
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; LR will contain physical address of StartVec
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; Begin of StartVec structure
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; All offsets are from the beginning of the .text section (0xFFF2003C)
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FFF20044 dc.l 0x7918 ; offset to the FCode stream of the OF main package
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...
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FFF2007C dc.l 0x28C48 ; offset to the last driver in the device packages
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...
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FFF20084 dc.l 0x7720 ; offset to the last Forth word descriptor of the kernel
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```
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### OpenFirmware internals
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Apple's OpenFirmware contains a small kernel implemented in the native PowerPC code. This kernel performs the following actions:
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* set up memory translation for OF execution
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* relocate itself from ROM to RAM
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* initialize Forth runtime environment
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* recompile OF main image from FCode to native PPC code
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* pass control to recompiled OF that starts building the device tree
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* process low-level exceptions
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## OpenFirmware and the Macintosh boot process
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### Old World Macs
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1. In response to power coming on, HWInit code in the Power Macintosh ROM performs initialization of the memory controller and the basic I/O facilities as well as some self-testing. After that, the startup chime is played.
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2. HWInit passes control to OpenFirmware kernel that prepares OF execution from RAM. OF builds the **device tree** - a platform-independent description of the attached HW.
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3. OF returns control to HWInit that initializes several low-level data structures required by the Nanokernel.
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4. HWInit passes control to the Nanokernel that initializes the low-level virtual machine and the 68k emulator.
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5. 68k emulator executes the start-up code in the Macintosh ROM that initializes various managers.
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6. The device tree generated by the OpenFirmware in step 2 is imported by the Expansion Bus Manager initialization and stored in the **NameRegistry**.
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7. An operating system is located and loaded.
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### New World Macs
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*TBD*
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## OpenFirmware bugs
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Apple OF is known to contain numerous bugs. The following table lists some recently discrovered bugs, not mentioned elsewhere.
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| OF version affected | Bug description |
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|:-------------------:|-----------------|
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| 2.0f1, 2.4 | A numerical overflow in `um/mod used by get-usecs-60x` causes the OF console to become unresponsible after approx. 71 minutes. You have to restart your computer once the bug is triggered. |
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