As I described in a previous entry, the "floating bus" is an effect that occurs when an unattached port is read during display update, returning not the normal &FF, but instead the pixel- or attribute data which is currently being delivered to the CRT by the ULA. This effect was used in some games for the earlier Spectrum models to synchronize screen updates with the CRT beam position. When the Amstrad +2A/+3 models appeared on the scene it was understood that they did not have this effect, and programmers used other techniques to avoid flickering graphics. However, thanks to Cesar Hernandez, Ast A. Moore, Mark Woodmass and others it is now not only known that there actually is a floating bus effect on the +2A/+3 machines; the effect has also been described in detail. Suggested reading:
Emulation The floating bus effect on the +2A/+3 models works according to the following principle:
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