Editing Macintosh Old World ROM

[[Image:Oldworldromsym.png|thumb|Showing all three icons of the OldWorld ROM (from left to right: Missing OS, Happy Mac (Found OS), and Sad Mac (Macintosh 128k/Plus) logos)]]

'''Old World ROM''' refers to Macintosh systems that rely on a physical ROM chip containing the [[Macintosh Toolbox]] firmware—either socketed or soldered to the motherboard. This includes early Motorola 68k-based Macs, NuBus Power Macs, and PCI-based "Beige" and Platinum Power Mac G3 machines. All such models utilize a unified boot sequence grounded in Toolbox logic. Later ''New World ROM'' systems (iMac, iBook, Blue & White Power Mac G3, PowerBook G3 Lombard, and onward) replaced this approach with disk-loaded Toolbox via flash and enhanced [[Open Firmware]] 3.x.

== Architecture and Boot Process ==

=== Physical Toolbox ROM ===
* Stores the '''Macintosh Toolbox''' routines: QuickDraw, Menu Manager, Event Manager, Window Manager, and other low-level system functions.
* Includes boot code and hardware initialization logic.
* On PowerPC systems, incorporates a '''68k nanokernel emulator''' that allows 68k Toolbox code and legacy applications to run transparently.

=== PowerPC and 68k Emulation ===
* PowerPC-based Old World systems begin execution in the nanokernel.
* The nanokernel emulates a Motorola [[68040#68LC040|68LC040]] CPU, running Toolbox firmware until Mac OS initializes native PowerPC services.
* This dual-architecture environment allows seamless use of both 68k and PPC applications.

=== Role of Open Firmware ===
* Present in PCI-based Old World Macs (e.g. Power Macintosh 7200, 7500, 9500).
* Early Open Firmware revisions were limited: primarily enumerating PCI devices and invoking Toolbox.
* Lacked robust filesystem support (e.g. HFS boot), requiring bootloaders or '''nvramrc''' patches for alternative OS installs.
* Invoked by holding {{keypress|Cmd|Option|O|F}} at startup, although functionality is more restricted compared to New World systems.

== Detailed Boot Sequence ==
The boot process of an Old World ROM Macintosh is rigid and heavily defined by the Toolbox. It consists of a predictable sequence of diagnostic and initialization steps:

=== Stage 1: Power-On and Low-Level Tests ===
* Upon power application or reset, the system begins execution at a fixed ROM vector.
* A minimal '''POST (Power-On Self-Test)''' is executed:
** '''CPU self-test:''' The CPU runs a checksum of its microcode and performs instruction validation loops.
** '''ROM checksum:''' The system verifies the integrity of the ROM contents.
** '''Memory controller test:''' Ensures DRAM controller and refresh logic are functional.
** '''SMU/PMU init (later Macs):''' Initializes power management circuitry.

* Failure at this stage results in no video output or a ''dead'' system.

=== Stage 2: Memory Tests ===
* ROM-based memory tests probe system RAM line by line:
** '''Data bus test:''' Writes alternating patterns (0x55, 0xAA) to detect stuck bits.
** '''Address line test:''' Walking-ones and walking-zeros patterns to check address decoding.
** '''Block pattern tests:''' Large sequential fills verify stability across DRAM banks.
* On early 68k Macs, failing DRAM produces Sad Mac codes and a hex error on-screen.
* On PCI-based Power Macs, DRAM failures trigger the [[Chimes of Death]].

=== Stage 3: Video and Display Init ===
* If onboard video is present, the ROM initializes the video frame buffer, sets a default resolution, and clears the screen to gray.
* If no onboard video exists, the Toolbox scans NuBus or PCI slots for a card containing a valid Option ROM with Apple video driver code.
* Successful video init allows icons (Happy Mac, Sad Mac, question mark) to be displayed.

=== Stage 4: Device Enumeration ===
* Toolbox scans for bootable devices in a fixed order:
** Internal floppy drive (if present).
** Internal SCSI devices, lowest ID first (typically ID 0).
** External SCSI devices.
** Expansion ROMs (NuBus/PCI option cards with boot code).
* Startup disk preference is read from [[parameter RAM]] (PRAM); if not set, the default hierarchy applies.

=== Stage 5: System Folder Verification ===
* If a floppy or SCSI disk is found, the ROM checks the disk for:
** A valid HFS or MFS filesystem.
** A blessed ''System Folder'' (a folder marked as bootable by Mac OS).
* If valid, the '''Happy Mac''' icon is displayed, and control is handed to the System file.
* If no valid boot disk is found, the blinking disk/question mark icon is shown.

=== Stage 6: Error Handling ===
* '''Sad Mac''' displayed for hardware-level or early boot failures, accompanied by a hex error code.
* '''Chimes of Death''' sound emitted on later Power Macs for fatal errors that prevent Toolbox from running.
* Certain codes indicate ROM checksum failure, CPU test failure, or RAM controller issues.

== Developer Tools and ROM Manipulation ==
* Some development models used rewritable ROM SIMMs with VPP and write-enable lines for Apple-internal debugging.
* Engineers could flash new Toolbox builds directly into system ROMs.
* Public Macs were not designed for user-level ROM modification, though third-party ROM SIMMs later enabled custom firmware.

== Alternative Boot Methods ==
* Because Toolbox is always executed first, alternative OS installs required workarounds:
** '''BootX''' – Loads Linux kernels from within Mac OS.
** '''Quik / iQuik''' – Forth-based bootloaders that run under Open Firmware, bypassing Toolbox.
** Special floppy images – Used to deceive Toolbox into launching non-Mac OS kernels (common in Linux PPC distributions).
* 68k-based Macs and NuBus Power Macs require Mac OS to be present to boot any secondary operating system, including Apple’s own [[A/UX]].

== Transition and Identification ==
* Old World ROM machines can be identified by the absence of built-in USB ports. Only New World models introduced USB as factory standard.
* '''Mac OS X Support:''' Officially supported only on the Beige Power Mac G3 and PowerBook G3 Series (Wallstreet). Later systems required New World ROM.
* Transition to New World ROM marked a fundamental architectural shift: Toolbox code was loaded from disk by Open Firmware rather than residing in physical ROM.

== See also ==
* [[Macintosh Toolbox]]

== External links ==
*{{webarchive |url=https://web.archive.org/web/20020621112135/http://docs.info.apple.com/article.html?artnum=11609 |date=June 21, 2002 |title=Macintosh: ROM Size for Various Models}}

[[Category:Vintage Apple]]