Posted By
IstvanV
on 2011-07-04
08:44:47
|  Re: FLD and $FFFF
The $FFFF value is read and displayed as bitmap data during FLD because the TED does not enable its address bus output for bitmap reading outside the "active" screen area. If I recall correctly, this is enabled with the first "second" DMA (i.e. reading of character codes, or color bytes in bitmap modes), and disabled at line 204. There is also a horizontal condition for enabling the address output that is active for 40 characters.
The enable/blank screen bit ($FF06 & $10) has an effect only at the beginning of the screen (lines 0, and 4 or 8), you cannot continuously toggle the enabled/blank state mid-screen with it.
Setting either ECM+bitmap, or ECM+MCM mode will result in a black screen. The black color is constant, it does not seem to depend on any other data that is read from memory. It is a sort of "hardware bug" in the TED. I think the VIC-II has this same effect, too.
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