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:
“Besides, our action on each other, good as well as evil, is so incidental and at random, that we can seldom hear the acknowledgments of any person who would thank us for a benefit, without some shame and humiliation. We can rarely strike a direct stroke, but must be content with an oblique one; we seldom have the satisfaction of yielding a direct benefit, which is directly received.”
—Ralph Waldo Emerson (1803–1882)
“She might have been old once and now, miraculously, young again—but with the memory of that other life intact. She seemed to know the world down there in the dark hall and beyond for what it was. Yet knowing, she still longed to leave this safe, sunlit place at the top of the house for the challenge there.”
—Paule Marshall (b. 1929)
“Power, in Case’s world, meant corporate power. The zaibatsus, the multinationals ..., had ... attained a kind of immortality. You couldn’t kill a zaibatsu by assassinating a dozen key executives; there were others waiting to step up the ladder; assume the vacated position, access the vast banks of corporate memory.”
—William Gibson (b. 1948)