Amiga Chip RAM

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Chip RAM is the name given to RAM in the Amiga computer that could be accessed by the Amiga Original chipset as well as the CPU. The custom chipset was able to perform DMA transfers to and from this RAM, and would even lock out the CPU while doing so.

By default, most Amiga computers only came with chip RAM, but could be expanded with RAM (called Fast RAM) that only the CPU could access, even when the custom chipset was simultaneously accessing chip RAM.

The amount of chip RAM varied between different Amiga models, although generally newer models had more chip RAM than older ones. The original Amiga 1000 was the only model to only have 256 kilobytes of chip RAM on the motherboard; it was expanded to 512 KB by the addition of a daughterboard under a cover in the centre front of the machine. Its more popular successors, the Amiga 500 and Amiga 2000, had 512 kilobytes. The Amiga 500 Plus, Amiga 600, CDTV, later versions of the A2000, and the Amiga 3000 had a full megabyte of chip RAM, and the last models, the Amiga 1200, Amiga CD32, and Amiga 4000, had two megabytes. All models, except the A1000, were upgradable to 2 MB of Chip RAM. The Amiga 4000 motherboard includes a jumper that claims to accommodate 8 megabytes of chip RAM, but this jumper is unimplemented - regardless of its position, the computer will only recognize two megabytes due to the limitations of the Alice chip.[1]

The ability to handle various amounts of Chip RAM was dependent on the version of the Agnus chip fitted to the Amiga. The original AGNUS chip was a 48-pin DIP package able to address 512 KB Chip RAM. This was fitted to the A1000 and first A2000. Subsequent versions of the AGNUS were in an 84-pin PLCC package and this effectively cut earlier owners off from the later upgrades. This led to the Phoenix and Rejuvenator projects to enable A1000 owners to access greater amounts of Chip RAM. Sales of Phoenix Boards were over 600 units, in spite of delays, so there was obviously some interest on the part of A1000 owners.

The original Amiga 500 had a trapdoor on the underside of the machine that took an upgrade to 1 megabyte of RAM. The extra 512 kilobytes was classified as Fast RAM, but was sometimes referred to as Slow RAM since due to the design of the trap door slot bus this RAM is actually on the graphics bus. The trap door expansion usually included a clock that was battery-backed up. All versions of the A500 could have the trap door RAM configured as Chip RAM by a simple hardware modification, which mainly involved fitting of a later model of AGNUS chip.

All versions of the A500, and all A500 based versions of the A2000, could be upgraded to 2 MB Chip RAM by fitting the 8372B Agnus chip and adding extra memory. The A1000 Phoenix replacement motherboard featured 2 MB of Chip RAM.

The UAE (Unix Amiga Emulator) emulates up to 8 MB of Chip RAM.

References

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