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Amiga Chip RAM
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".
== Direct memory access ==
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 CPU-only "fast" RAM (when present) or ROM.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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