Память Chip RAM в компьютерах Amiga: Архитектура и особенности доступа.

Chip RAM is a commonly used term for the integrated RAM used in Commodore's line of Amiga computers. Chip RAM is shared between the central processing unit (CPU) and the Amiga's dedicated chipset (hence the name). It was also, rather misleadingly, known as "graphics RAM".

Under the Amiga architecture, the direct memory access (DMA) controller is integrated into the Agnus (Alice on AGA models) chip. Both the CPU and other members of the chipset have to arbitrate for access to shared RAM via Agnus. This allows the custom chips to perform video, audio, or other DMA operations independently of the CPU. As the 68000 processor used in early Amiga systems usually only accesses memory on every second memory cycle, Agnus operates a system where the "odd" clock cycle is allocated to time critical custom chip access and the "even" cycle is allocated to the CPU: thus, for average DMA demand, the CPU is not typically blocked from memory access and may run without interruption. However, certain chipset DMA, such as high resolution graphics with a larger color palette, Copper, or blitter operations, can use any spare cycles, effectively blocking cycles from the CPU. In such situations CPU cycles are only blocked while accessing shared RAM, but never when accessing Fast (CPU only) RAM (when present) or ROM. However, the software emulator UAE can emulate an Amiga system with the design limit of up to 8 MiB of Chip RAM.

Amiga systems can also be expanded with Fast RAM, which is only accessible to the CPU. This improves execution speed, as CPU cycles are never blocked even when the custom chipset is simultaneously accessing Chip RAM. Adding Fast RAM to systems with 32 bit CPUs roughly doubles the instruction speed, as the more advanced 68020, '030, and '040 CPUs can utilize more memory cycles than the earlier 68000. Confusingly, a system may have several different kinds and speeds of Fast RAM. For example, an Amiga 3000 may contain 16 bit Zorro II expansion RAM, 32 bit Zorro III expansion RAM, 32 bit motherboard RAM, and 32 bit CPU card RAM simultaneously (in increasing speed order). Automatically configured RAM is prioritized by the system, so the fastest memory is used first. Early versions of the Amiga 2000B, and the most common "trapdoor memory expanded" configuration of the Amiga 500, contain 512 KiB pseudo fast RAM ("slow RAM") controlled by Agnus with the same limitations as Chip RAM, yet unusable as such due to register limitations. Numerous budget trapdoor expansions for the 500 extended this "controllerless" concept to up to 1.8 MB slow RAM (requiring a Gary adapter for addressing).