Editing Amstrad PCW (section)

=== Memory ===
The Z80 could only access 64&nbsp;[[kibibyte|KB]] of [[RAM]] at a time. Software could work around this by [[bank switching]], accessing different banks of memory at different times but this made programming more complex and slowed the system down. The PCW divided the Z80 memory map into four 16&nbsp;KB banks. In [[CP/M]], the memory used for the display was switched out while programs were running, giving more than 60&nbsp;KB of usable RAM. While the Joyce architecture was designed with configurations of 128&nbsp;KB and 256&nbsp;KB of RAM in mind, no PCW was ever sold with 128&nbsp;KB of RAM.

Each PCW's [[CP/M]] application could not use more than 64&nbsp;KB so the system used the rest of the RAM for a [[RAM drive]].<ref name="Osborne2007RememberingPCW" /> On the other hand, the [[standalone program|standalone]] [[Locoscript]] word processor program was reported as using up to 154&nbsp;KB as normal memory and the rest as a RAM disk.<ref name="OldComputersPCW8xxx" />

Unusually, the Z80 CPU in the PCW 8256, 8512, 9512, 9256 and 9512+ had no directly connected [[read-only memory|ROM]], which most computers used to start the [[Booting|boot process]]. Instead, at startup, the [[ASIC]] (customised circuit) at the heart of the PCW provided access to part of the 1k ROM  within the [[Intel MCS-48|Intel 8041 microcontroller]] used to drive the printer. The Z80 would copy 256 bytes via the ASIC into RAM, providing sufficient instructions to load the first [[Disk sector|sector]] from a floppy.<ref>{{cite web|url=http://www.chiark.greenend.org.uk/~jacobn/cpm/pcwboot.html|title=PCW boot sequence|access-date=2011-02-28}}</ref> The ROM-based code cannot display text, being too small to support character generation; instead, it displays a bright screen which is progressively filled by black stripes as the code is loaded from the floppy.