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Amiga Model-Specific ROM Differences

 

TOPIC

Q: Can ROMs be exchanged between different Amiga models?

 

DISCUSSION

A: To a limited degree, yes. Amiga system ROMs differ in size and in features.

The following list illustrates compatibility of different models with different ROM sizes:

  • Amiga 1000: 256 KB (copied from "Kickstart" disk)
  • Amiga 3000, 4000, 4000T: 256 KB, 512 KB (on two ROM chips)
  • Amiga 500, 500 Plus, 2000: 256 KB, 512 KB, 1 MB (single ROM chip)
  • Amiga 600: 256 KB, 512 KB, 1 MB, 2 MB (single ROM chip)
  • Amiga 1200: 256 KB, 512 KB, 1 MB, 2 MB (on two ROM chips)

Emulated systems may overcome some of the above limitations by supporting 1 MB and 2 MB ROMs on a wider range of emulated Amiga models.

Some hardware-specific features also differ between models. ROMs designed for AGA chipset Amiga systems (Amiga 1200, 4000) do not support certain ECS chipset features such as ECS Super Hires modes. Conversely, ROM versions prior to 3.0 do not support AGA chipset features. Amiga scsi.device functionality also differs between models: the Amiga 600 and 1200 share the same code, but the Amiga 3000, 4000 and 4000T each require a version tailored for the respective hardware. Amiga 3000 ROMs may further access hardware unique to the Amiga 3000, and will fail on other systems (emulated or "real").

The scsi.device found on Amiga 4000 (IDE) and 4000T (IDE and SCSI) ROMs checks for existing device names in Exec's DeviceList and sets itself to the first available free name, e.g. "scsi.device", "2nd.scsi.device", "3rd.scsi.device", etc. (up to "8th.scsi.device"). This may explain a change in device names when different ROMs are used.

The ROMs for the 68000-based systems (Amiga 1000, 500, 500 Plus, 2000, 600) are largely interchangeable. Amiga 600 ROMs additionally include scsi.device, card.resource and carddisk.device modules for built-in hard disk and PC Card support. These modules check for the presence of the Gayle custom chip (which in the Amiga 600 and 1200 replaced the Gary chip) and fail gracefully if it is not found. This is the case for example for the 3.1 and 3.X ROMs, where the 68000 build incorporates features for the Amiga 600, but is also designed to run on other models with the same CPU, like the Amiga 500 and Amiga 2000.

The ROMs for 68000-based systems (Amiga 1000, 500, 500 Plus, 2000, 600) can in general also be used on newer systems, except for their graphics.library not supporting AGA features and expansion.library not supporting 32-bit Fast RAM (unless such RAM came with its own ROM to add the RAM) and Zorro III cards. The Amiga 600 scsi.device should also work on the Amiga 1200 (but not on the Amiga 3000 or 4000).

Certain third-party peripherals like Phase 5 CPU cards limit support to 512 KB ROMs when using their "MapRom" option.

Other peripherals may also affect compatibility. For example, FFS partitions larger than 2 or 4 GB (depending on the controller and device) and hard disks larger than 4 or 8 GB are only reliably supported from scsi.device version 43 (3.X ROMs in Amiga Forever).

Amiga OS versions up to 1.3 used 256 KB ROMs. Starting from the "1.4" beta series (which was later officially released as 2.0), 512 KB ROMs were used. CDTV and CD³² systems use an additional expansion ROM (256 KB on the CDTV, 512 KB for the CD³²) to host CD access and other console-specific features.

In order to keep using 512 KB Amiga OS 3.1 ROMs on the Amiga 4000T ("Tower") model, which needed to fit additional code to support the dual IDE and SCSI interfaces in ROM, the workbench.library was moved from ROM to disk and a small "WBFind" module was added in its place, to load workbench.library from any disk. Amiga 4000T ROMs contain also work on the Amiga 4000, as long as there is a disk-based workbench.library.

Amiga Forever 512 KB "3.X" ROMs for the Amiga 3000, 4000 and 4000T employ the same technique used for the 4000T ROMs, freeing ROM space used by workbench.library to make room for other enhancements. Amiga Forever 1 MB "3.X" ROMs (for Amiga systems which support 1 MB ROMs) instead include both workbench.library and the other ROM enhancements.

Some ROMs included in early versions of Amiga Forever included "illegal opcodes" that were implemented in emulated systems since 1997, but which were not supported on the original hardware. ROMs included in newer versions of Amiga Forever do not use "illegal opcodes".

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Article Information
Article ID: 15-127
Platform: All
Products: Amiga Forever
Additional Keywords: Gary, Fat Gary
Last Update: 2014-09-27
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