Xe03 Spectrum

The Spectrum is an interesting beast because it basically has no hardware and you need to do everything in software. When rendering the best way to do things is to render behind the raster that is sending everything to the TV screen.

The only synchronisation that is available is with the vertical sync of the raster, but this leaves a lot of time (in terms of machine cycles) before the start of the screen, where we want to start rendering to allow us to draw just behind the raster, if we render before hand then we will see flicker, just behind and the display will be rock solid (which is what we are aiming for).

Fortunately on the Spectrum there is a little known way that this can be done, once again a technique which was used on Cobra (a technical masterpiece), it uses the floating bus on the Spectrum (see http://www.ramsoft.bbk.org/floatingbus.html for an explanation). The code is relatively simple

SyncToScreen: ld bc,#3f28
STS_lp:       ld a,c 
              in a,(#ff) 
              cp b 
              jr nc,STS_lp 
              ret

This routine basically waits until an attribute is being output from the ULA of the Spectrum, which if it is started during the border region of the display, then it will wait for the first line of the screen... exactly what we need!

So now our armoury of techniques is filling up I really need to get on and write some real code...