Editing Amstrad PCW (section)

=== 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.