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Is it possible to hack Amiga Alice chip and use more than 2MB CHIP RAM? It's a long shot, but I assume if it was possible someone would have done it already?

The maximum amount of Chip RAM in any model is 2 MiB. The Amiga 4000 motherboard includes a non-functional jumper that anticipated later chips and is labeled for 8 MiB of Chip RAM—regardless of its position, the system only recognizes 2 MiB due to the limitations of the Alice chip.

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    The pinout at megaburken.net/~patrik/pinout_temp/Alice.gif shows only 19 address pins, so I doubt it. 19 pins multiplied by a four-byte data bus does superficially give 2mb total but I'd like to figure out whether that's a proper interpretation before posting something that claims to be an answer. The 68020 onwards appear to give a byte address plus a size rather than a 68000-esque word address plus byte select lines so I'm not completely sold on that arithmetic.
    – Tommy
    May 20, 2021 at 14:54

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TL;DR The Amiga AGA Alice chip can only support 2MiB of CHIP RAM because it has 20 address lines, and thus allows 2^20 (1M) by 16-bit memory access.

The first Agnus chip found in the A1000 only supported 512KiB of CHIP RAM. But this was soon replaced by "Fat Agnus" in the A2000 that supported 512K x 16 (or 1MiB) of CHIP RAM using 19 address lines.

The ECS Agnus, introduced with the A3000, extended Agnus with an A20 line. So, now you could have 1M x 16 (or 2MiB) of CHIP RAM. This was not fundamentally changed with the AGA Alice chip, which still has A1-A20 and a 16-bit data bus.

Note: AGA has some fancy memory access abilities to use Fast Page Mode and 32-bit bitplane data access. But the Copper and Blitter remain 16-bit only, with no possibility to exceed the 2MiB limitation.

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  • To reconcile this with the comment I posted above, and respective to the pinout posted, I hadn't spotted A20 in the top right!
    – Tommy
    May 21, 2021 at 16:34
  • On emulated Amigas 4MB & 8MB Chip RAM is possible; big-box emulated Amiga (A4000T with 68040 CPU, Kickstart 3.1, Workbench 3.1, BetterWB pack by lilliput used, mentioned because it adds several patches which may have provided this ability also) on FS-UAE shows 4MB or 8MB graphics mem available. How is this possible?
    – Krackout
    May 30, 2021 at 9:05
  • @Krackout OP's question was about the Alice chip. In a software emulator, hardware limitations like number of lines for address bus are easily ignored. So, I'd conclude that the Amiga OS didn't prevent >2MiB of CHIP RAM; only its hardware.
    – Brian H
    May 30, 2021 at 15:12
  • Vampire Standalone gives a hardware 12 meg of chip
    – Zenzizenzizenzic
    Aug 20, 2021 at 12:52

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