0 Switch transparency mode
1 Suspend/resume bitmap updates
2 Reveal/conceal
3 Enable/disable black foreground
This new subreason is part of Hi-Res teletext support, where the new typeface is designed on a 16×20 grid (previously RISC OS used 8×10), so the effective screen resolution becomes 640×500 pixels (rather than 320×250).
The larger Hi-Res font is not cached due to its increased size, and is instead generated on the fly, with a 25% speed hit - some of the new VDU sequences allow rendering to be temporarily disabled to avoid this impacting machine performance.
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