[cctalk] Re: For Fred, especially: "Everything I know about floppy disks"

On Sun, Sep 10, 2023 at 05:44:59PM +0100, Liam Proven via cctalk wrote: [...] One unusual and interesting thing about the Amstrad PCW is how it uses a display list system, unlike basically any other home computer of the era apart from the Atari 8-bits and the Amiga. This lets it have a nice high-resolution bitmap display and do *partial* scrolling quickly without having to get the CPU to copy the whole screen around. This is obviously useful for a word processor which mostly just inserts and delete lines and makes the thing so much smoother to use. Various of its contemporaries such as the Beeb, CPC or PC video cards used or were inspired by the MC6845 CRTC that can only scroll the entire screen. This is fine for a dumb terminal experience such as the MS-DOS shell, but has to fall back to CPU rendering for partial scrolls. You can see the difference on a Beeb or emulator. Go into MODE 1 and type in any old junk such as "*HELP" to print a wodge of text. Towards the bottom, do "COLOUR 129" to set the text background to red (not strictly necessary, but it makes the changed area bigger and more obvious) and do that "*HELP" or whatever to make it scroll. Watch how the screen scrolls quickly with no tearing. That's using 6845 hardware scrolling. Now repeat this but do VDU 28,1,30,38,1 to set up a text viewport which is one character in from each edge and do the same. Look how it scrolls slowly and tears. That's software rendering making a pig's ear of it because there's just not enough memory bandwidth to copy a screen's worth of memory within a 20ms frame. This hardware scrolling support is more useful to the PCW than the CPU. If it had a 68000 in it, but no display list, it'd be much slower at editing text. Just look at wordprocessors on the original 128k Mac.