Chip RAM
From Free net encyclopedia
Chip RAM is the name given to RAM in the Amiga computer that could be accessed by the custom 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. Its more popular successors, the Amiga 500 and Amiga 2000, had 512 kilobytes. The Amiga 500 Plus, Amiga 600, later versions of the A2000, and the Amiga 3000 had a full megabyte of chip RAM, and the last models, the Amiga 1200 and Amiga 4000, had two megabytes. The A500+, A600 and A3000 were upgradable to 2Mb of Chip RAM. The Amiga 4000 motherboard includes a jumper that claims to accommodate eight megabytes of chip RAM, but this jumper is broken - regardless of its position, the computer will only recognise two megabytes.
The amount of addressable Chip RAM is dependent on the Agnus chip revision.
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 Fast RAM, but was sometimes referred to as Slow RAM since due to the design of the Trap door slot this RAM was slower than true Fast RAM. The trap door expansion usually included a clock that was battery-backed up. Later revisions of the A500 could have the trap door RAM configured as Chip RAM by a simple hardware modification.
Most revisions of the A2000 and A500 could be upgraded to 2Mb Chip RAM using a third party upgrade. The A1000 replacement motherboard "Phoenix" featured 2Mb of Chip RAM.
The UAE Amiga emulator allows the emulation of up to 8Mb of Chip RAM.