Gary — A3000 System Controller

Overview

Gary is the custom system controller chip in the Amiga 3000. It consolidates functions that are discrete ICs on the A2000 into a single gate array:

Bus controller: Manages interaction between 68030/68882, chip bus, and Zorro III
Auto-config controller: Runs Zorro expansion enumeration at boot
DMA arbitration: Between 68030, custom chips, and Zorro III DMA masters
SCSI interface glue: Works with the A3000's built-in WD33C93 SCSI controller
ROM decode: Maps Kickstart ROM into the address space

Gary is not directly programmable by user software; its configuration is set by hardware strapping and the ROM initialization sequence.

Bus Arbitration

Gary manages three bus masters:

Master	Priority	Description
Custom chips (DMA)	Highest	Agnus DMA for display, audio, disk — must never stall
68030 CPU	Normal	Program execution
Zorro III cards	Lowest	Expansion bus-mastering DMA

When a custom chip DMA cycle occurs, Gary holds the 68030 off the bus until the cycle completes. This is the fundamental source of "DMA contention" slowdown on all Amiga models.

A3000 SCSI Integration

The A3000 includes a built-in WD33C93A SCSI controller. Gary provides the glue logic between the SCSI chip and the system bus:

Feature	Details
SCSI chip	WD33C93A (SBIC)
DMA	SDMAC — dedicated SCSI DMA controller (separate from the CDTV-style DMAC)
Interface	A3000 uses a dedicated SDMAC chip, not the A2091-style DMAC
AmigaOS driver	`scsi.device` in Kickstart ROM

Note

The A3000's SDMAC is a different chip from the A2091/CDTV DMAC, despite both interfacing with WD33C93 SCSI controllers. The register layouts are incompatible.

Machines Using Gary

Model	Gary variant	Notes
A3000	Original Gary	68030, Zorro III, WD33C93 SCSI
A3000T	Gary (tower variant)	Same chip; tower form factor with more drive bays

The A4000 does not use Gary — it uses a different system controller chip called Ramsey along with Budgie and Buster for bus management.

References

Commodore A3000 Technical Reference Manual
ADCD 2.1 — Hardware Manual, A3000 chapter
NDK39: hardware headers (community-documented Gary behavior)

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