Editing Amstrad PCW (section)

==Technical design==
The PCW 8256, 8512, 9512, 9256 and 9512+ share a common architecture. Other than the Z80 CPU, the PcW16 does not share any hardware with the original PCW series and should be considered to be a completely different machine.

=== CPU ===
All PCW models, including the PcW16, used the [[Zilog Z80]] range of [[CPU]]s. A 4&nbsp;MHz Z80A was used in the 8256, 8512, 9512, 9256, 9512+ and PCW10;<ref name="OldComputersPCW9xxxx" /><ref name="LuxsoftFAQ" /> and a 16&nbsp;MHz Z80 in the PCW16.<ref name="OldcomputersPWC16" />

=== 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.

=== Printer ===
To make the [[Computer printer|printer]] cheap enough to be included with every PCW, Amstrad placed the majority of its drive electronics inside the PCW cabinet.  The printer case contained only  electromechanical components and high-current driver electronics; its power was supplied via a [[coaxial power connector]] socket on the monitor casing, and rather than using a traditional [[IEEE 1284|parallel interface]], pin and motor signals were connected directly by a 34-wire [[ribbon cable]] to an 8041 [[microcontroller]] on the PCW's [[mainboard]].<ref name="UserGuideSettingUp">{{cite book|last=Gilmour|first=J.|title=PCW8256/8512: User Guide - CP/M Logo & Word Processor Manual|publisher=Amstrad Consumer Electronics plc|year=1985|pages=1–10|chapter=Setting Up|url=http://amstrad.cpc.free.fr/amstrad/manuals/pcw8256/pcw8256.pdf|access-date=2009-05-07|archive-url=https://web.archive.org/web/20110720221017/http://amstrad.cpc.free.fr/amstrad/manuals/pcw8256/pcw8256.pdf|archive-date=2011-07-20|url-status=dead}}</ref>

Most models of PCW were bundled with a 9-pin [[dot matrix printer]] mechanism, with the later 9512 and 9512+ models using a [[daisywheel]] (with a different cable; the printers were not interchangeable with the dot matrix models).  These PCW printers could not be used on other computers, and the original PCW lacked a then-standard [[Centronics]] printer port.  Instead, the [[Z80]] bus and video signals were brought to an [[edge connector]] socket at the back of the cabinet.  Many accessories including [[parallel port|parallel]] and [[serial port|serial]] ports were produced for this interface. Some of the later models included a built-in [[parallel port]]; these could be bundled with either the dedicated Amstrad printer or a [[Canon Inc.|Canon]] [[Bubblejet]] model.

=== Video system ===
The PCWs were not designed to play [[video game]]s, although some software authors considered this a minor detail, releasing games like ''[[Batman (1986 video game)|Batman]]'', ''[[Head Over Heels (game)|Head Over Heels]]'', and ''[[Bounder (video game)|Bounder]]''.  The PCW video system was not at all suited to games.  In order that it be able to display a full 80-column page plus margins, the display's addressable area was 90 columns and the display had 32 lines.  The display was [[monochrome]] and [[bitmapped]] with a resolution of 720 by 256 pixels.  At 1 bit per pixel, this occupied 23&nbsp;KB of RAM which was far too large for the Z80 CPU to [[Scrolling|scroll]] in software without ripple and tearing of the display.  Instead, the PCW implemented a [[Display list|Roller RAM]] consisting of a 512-byte area of RAM that held the address of each line of display data. The screen could now be scrolled either by changing the Roller RAM contents or by writing to an I/O port that set the starting point in Roller RAM for the screen data. This allowed for very rapid scrolling. The video system also fetched data in a special order designed so that plotting a character eight scan lines high would touch eight [[wikt:contiguity|contiguous]] addresses.  This meant that the [[Z80]]'s concise block copy instructions, such as [[Zilog_Z80#Instruction_set_and_encoding|LDIR]], could be used.  Unfortunately, it also meant that drawing lines and other shapes could be very complicated.