Editing Macintosh Old World ROM (section)

== Architecture Overview ==

=== ROM Organization and Contents ===
The Old World ROM contains several critical components that define the Macintosh computing experience:

'''Core Components:'''
* '''[[Macintosh Toolbox]]''' routines: QuickDraw graphics engine, Window Manager, Menu Manager, Event Manager, Resource Manager, and other GUI managers
* '''Hardware initialization code''': CPU configuration, memory controller setup, bus initialization
* '''POST diagnostics''': Power-On Self-Test routines for memory, ROM checksum, and basic hardware verification
* '''Boot loader code''': Device enumeration and System Folder location routines
* '''68LC040 emulator''' (PowerPC models): Nanokernel providing transparent 68k compatibility

The ROM size evolved significantly across generations:
* '''64 KB''' - Macintosh 128K/512K (1984)
* '''128 KB''' - Macintosh Plus (1986)
* '''256 KB''' - Macintosh SE, II series (1987)
* '''512 KB''' - Macintosh IIci, IIfx (1989)
* '''1 MB''' - Quadra series (1991)
* '''4 MB''' - Power Macintosh (1994)

=== PowerPC Architecture and 68k Emulation ===
PowerPC-based Old World ROM systems implement a sophisticated dual-architecture environment:

# '''Nanokernel initialization''': The PowerPC processor begins execution in the Mac OS nanokernel stored in ROM
# '''68LC040 emulation layer''': The nanokernel implements a complete Motorola 68LC040 processor emulator
# '''Toolbox execution''': The predominantly 68k-based Toolbox ROM code runs within the emulator
# '''Mixed-mode operation''': Once Mac OS loads, both native PowerPC and emulated 68k applications execute transparently
# '''Performance optimization''': Critical OS routines are progressively replaced with native PowerPC code via the Mixed-Mode Manager

This emulation approach allowed Apple to maintain complete backward compatibility while transitioning to the PowerPC architecture, ensuring all existing Macintosh software continued to function.

=== Open Firmware Implementation ===
PCI-based Old World ROM systems include a limited Open Firmware implementation:

'''Characteristics:'''
* '''Version 1.0.5 - 2.4''': Early, minimal implementations designed primarily to enumerate PCI devices
* '''Limited functionality''': Cannot directly boot from HFS volumes or load Mac OS
* '''Toolbox as OF device''': The Macintosh Toolbox appears as an Open Firmware device that is selected by default
* '''Bug-prone''': Particularly on early machines (Power Macintosh 7200, 7500, 8500, 9500), requiring nvramrc patches or bootloader workarounds
* '''Access method''': Hold {{keypress|Cmd|Option|O|F}} at startup to enter OF prompt

The Open Firmware environment can be modified to boot alternative operating systems, but this requires either:
* Custom nvramrc scripts to load non-Mac OS bootloaders
* Specially-prepared boot floppies that trick the Toolbox into loading alternative kernels
* Bootloaders like BootX that run from within Mac OS