Editing Amiga Chip RAM (section)

== Fast RAM ==
Amiga systems can also be expanded with Fast RAM, which is only accessible to the CPU.  This improves execution speed, as CPU cycles are never blocked even when the custom chipset is simultaneously accessing Chip RAM.  Adding Fast RAM to systems with [[32-bit]] CPUs roughly doubles the [[instructions per second|instruction speed]], as the more advanced [[68020]], [[68030|'030]], and [[68040|'040]] CPUs can utilize more memory cycles than the earlier 68000.

Confusingly, a system may have several different kinds and speeds of Fast RAM.  For example, an Amiga 3000 may contain 16-bit [[Zorro II]] expansion RAM, 32-bit [[Zorro III]] expansion RAM, 32-bit motherboard RAM, and 32-bit CPU card RAM simultaneously (in increasing speed order).  Automatically configured RAM is prioritized by the system, so the fastest memory is used first.

Early versions of the Amiga 2000B, and the most common "trapdoor memory expanded" configuration of the Amiga 500, contain 512&nbsp;KiB pseudo-fast RAM ("slow RAM") controlled by Agnus with the same limitations as Chip RAM, yet unusable as such due to [[hardware register|register]] limitations.  Numerous budget trapdoor expansions<ref>{{cite web | url=http://amiga.resource.cx/search.pl?product=&company=&amiga=500&intf=trap&mid=&base=dec&pid= | title=List of A500 Trapdoor Expansions | accessdate=3 November 2018}}</ref> for the 500 extended this "controllerless" concept to up to 1.8&nbsp;MB<!-- what's the exact amount? --> slow RAM (requiring a [[Amiga custom chips#Gary|Gary]] adapter for addressing).