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:
“Authority and power are two different things: power is the force by means of which you can oblige others to obey you. Authority is the right to direct and command, to be listened to or obeyed by others. Authority requests power. Power without authority is tyranny.”
—Jacques Maritain (1882–1973)
“For my name and memory I leave it to men’s charitable speeches, and to foreign nations, and the next ages.”
—Francis Bacon (1561–1626)
“Make thick my blood,
Stop up th’ access and passage to remorse,
That no compunctious visitings of nature
Shake my fell purpose.”
—William Shakespeare (1564–1616)