Prevent ROM paging if interrupt bits are set. #15
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According to recent investigations, the ULA will only change the selected ROM if the four upper bits of &FE05, used for changing interrupt-related state, are clear. This allows software to conveniently clear interrupts without inadvertently changing the selected ROM or needing to engage with the ROM selection logic. See: https://stardot.org.uk/forums/viewtopic.php?t=27791
It appears that to select ROM 0..7, ROM 12..15 must first be selected, this enabling the full range of ROMs to be selected.
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According to recent investigations, the ULA will only change the selected ROM if the four upper bits of &FE05, used for changing interrupt-related state, are clear. This allows software to conveniently clear interrupts without inadvertently changing the selected ROM or needing to engage with the ROM selection logic.
See: https://stardot.org.uk/forums/viewtopic.php?t=27791