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63 lines
2.7 KiB
Markdown
63 lines
2.7 KiB
Markdown
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[← Home](../../README.md) · [Hardware](../README.md) · [ECS](README.md)
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# Gary — A3000 System Controller
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## Overview
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**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:
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- **Bus controller**: Manages interaction between 68030/68882, chip bus, and Zorro III
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- **Auto-config controller**: Runs Zorro expansion enumeration at boot
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- **DMA arbitration**: Between 68030, custom chips, and Zorro III DMA masters
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- **SCSI interface glue**: Works with the A3000's built-in WD33C93 SCSI controller
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- **ROM decode**: Maps Kickstart ROM into the address space
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Gary is not directly programmable by user software; its configuration is set by hardware strapping and the ROM initialisation sequence.
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## Bus Arbitration
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Gary manages three bus masters:
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| Master | Priority | Description |
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|---|---|---|
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| Custom chips (DMA) | Highest | Agnus DMA for display, audio, disk — must never stall |
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| 68030 CPU | Normal | Program execution |
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| Zorro III cards | Lowest | Expansion bus-mastering DMA |
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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.
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## A3000 SCSI Integration
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The A3000 includes a built-in **WD33C93A** SCSI controller. Gary provides the glue logic between the SCSI chip and the system bus:
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| Feature | Details |
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|---|---|
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| SCSI chip | WD33C93A (SBIC) |
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| DMA | SDMAC — dedicated SCSI DMA controller (separate from the CDTV-style DMAC) |
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| Interface | A3000 uses a dedicated SDMAC chip, not the A2091-style DMAC |
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| AmigaOS driver | `scsi.device` in Kickstart ROM |
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> [!NOTE]
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> 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.
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## Machines Using Gary
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| Model | Gary variant | Notes |
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|---|---|---|
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| A3000 | Original Gary | 68030, Zorro III, WD33C93 SCSI |
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| A3000T | Gary (tower variant) | Same chip; tower form factor with more drive bays |
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The A4000 does **not** use Gary — it uses a different system controller chip called **Ramsey** along with **Budgie** and **Buster** for bus management.
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## References
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- Commodore A3000 Technical Reference Manual
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- ADCD 2.1 — Hardware Manual, A3000 chapter
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- NDK39: hardware headers (community-documented Gary behaviour)
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## See Also
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- [Gayle — IDE & PCMCIA](../common/gayle_ide_pcmcia.md) — A600/A1200 storage controller (different chip, different function)
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- [Zorro Bus](../common/zorro_bus.md) — Zorro II/III expansion managed by Gary
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- [ECS Chipset](chipset_ecs.md) — Super Agnus + ECS Denise (A3000)
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