Direct Memory Access
Under the Amiga architecture, the Agnus (Alice on AGA models) coprocessor is the direct memory access (DMA) controller. 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 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 the CPU is not typically blocked from memory access and may run without interruption. However, certain chipset DMA, such as 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 external RAM or ROM.
Read more about this topic: Amiga Chip RAM
Famous quotes containing the words direct, memory and/or access:
“Long as I have lived, and many blasphemers as I have heard and seen, I have never yet heard or witnessed any direct and conscious blasphemy or irreverence; but of indirect and habitual, enough. Where is the man who is guilty of direct and personal insolence to Him that made him?”
—Henry David Thoreau (1817–1862)
“Language was not powerful enough to describe the infant phenomenon. “I’ll tell you what, sir,” he said; “the talent of this child is not to be imagined. She must be seen, sir—seen—to be ever so faintly appreciated.”... The infant phenomenon, though of short stature, had a comparatively aged countenance, and had moreover been precisely the same age—not perhaps to the full extent of the memory of the oldest inhabitant, but certainly for five good years.”
—Charles Dickens (1812–1870)
“Oh, the holiness of always being the injured party. The historically oppressed can find not only sanctity but safety in the state of victimization. When access to a better life has been denied often enough, and successfully enough, one can use the rejection as an excuse to cease all efforts. After all, one reckons, “they” don’t want me, “they” accept their own mediocrity and refuse my best, “they” don’t deserve me.”
—Maya Angelou (b. 1928)