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:
“England and France, Spain and Portugal, Gold Coast and Slave Coast, all front on this private sea; but no bark from them has ventured out of sight of land, though it is without doubt the direct way to India.”
—Henry David Thoreau (1817–1862)
“It must be a peace without victory.... Victory would mean peace forced upon the losers, a victor’s terms imposed upon the vanquished. It would be accepted in humiliation, under duress, at an intolerable sacrifice, and would leave a sting, a resentment, a bitter memory upon which the terms of peace would rest, not permanently, but only as upon quicksand.”
—Woodrow Wilson (1856–1924)
“Knowledge in the form of an informational commodity indispensable to productive power is already, and will continue to be, a major—perhaps the major—stake in the worldwide competition for power. It is conceivable that the nation-states will one day fight for control of information, just as they battled in the past for control over territory, and afterwards for control over access to and exploitation of raw materials and cheap labor.”
—Jean François Lyotard (b. 1924)