CompuServe Messages

Ok, here's the deal… The FAT Agnus (8370) contains no RAM, but addresses a 512K section of RAM located on the motherboard. With most A500's this was the ONLY RAM the A500 shipped with. This RAM is called CHIP RAM because the Agnus CHIP can access this RAM directly, and does not need to asks the CPU to feed it the info from the RAM. The Agnus & CPU take turns accessing CHIP RAM, and share it. The RAM plugged into the side of A500 (thru the 86-pin Expansion slot), is connected only to the CPU, and only used by the CPU. The Agnus doesn't know of this RAM, can't access this RAM, and doesn't care about went it is being accessed. This RAM is called FAST RAM. As is, the A501-Slot of the A500 allows upto 512K of FAST RAM to be added. Normally, only the CPU uses this RAM, but there is an option were it can be re-configured as CHIP RAM so that the Agnus can access & use it. However, once re-configured, the A501 expander becomes a requirement, and the Amiga can't boot without it. The AmigaOS allows upto 2-megs (2048K) of CHIP RAM, but the A500 only has the necessary hardware for 1-meg (1024K) of CHIP RAM — 512K on the motherboard plus another 512K in the A501-slot. The FAT AGNUS (8370) could access only 512K of RAM, so only the 512K of motherboard RAM was needed. The ECS Agnus (8372A) could be toggled to access 1024K or 512K of RAM. This allows the ECS Agnus to be dropped into an unexpanded A500. If the ECS's 1024K option is used, the A501 must be re-configured into being CHIP RAM (by cutting a solder jumper connected to the GARY). And the ECS Agnus must also be re-configured by toggling a solder jumper near it from 512K mode to 1024K mode. Ron.