The ZX Spectrum Reverse Engineering and Clone Desgin Blog


A site dedicated to the reverse engineering of the ZX Spectrum and related projects.

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The Spectrum FLASH Mode

Sep 4, 2007

The ZX Spectrum produces the FLASH attribute mode by toggling the INK and PAPER bits once every 32 display frames. This is an extremely simple mode to implement, as the following equation shows:

DisplayPixelOut = DisplayPixel XOR ( AttributeFlash • Frame0-31 )

i.e. If the current frame number is between 0 and 31 and the attribute has FLASH mode set, then invert the display pixel.

A single counter, clocked by VCrst, is used to count the frames. Whenever its Q4 is high and the FLASH attribute bit is set, the current output pixel is inverted.

The sound interface can been viewed in the video output schematic version 1.16.