[Coco] Bit 5 of DSKREG

[Robert Gault]

tim lindner wrote:
> Brad Grier <bradgrier at cox.net> wrote:
> 
> 
>>I downloaded the MESS source (I should have done this earlier). There's
>>a comment in cococart.c that bit 5 of the control register at $ff40 not
>>only controls floppy density but is an "NMI enable flag".
> 
> 
> I wrote that.
> 
> I discovered it from my reading of the various schematics for various
> floppy disk controllers.
> 
> I could be wrong, I am not an expert reading schematics.
> 
> Maybe Mark Marlette could chime in to settle this question.
> 
> Since this bit is doeumented to enable/disable the single density mode,
> I thought I might ask a question regarding that:
> 
> Does anyone have a single density I/O kernal? I am in the process of
> adding single density support to the CoCo driver in MESS and would like
> some _real_world_ code to test.
> 
> I have been thinking of tracking down a copy of Flex.
> 

Tim,

There is most definitely a problem with making bit 5 both a density flag 
and an NMI flag. This is after all a single bit so it can only have two 
states, on or off. Making this both a density and NMI flag implies that 
the bit can have 4 states which is just not possible.

There is no indication of this bit being an NMI flag/switch in the 
"Unravelled" series, "The Facts for the TRS-80 Color Computer" (an 
earlier unraveled version), or the Disk Basic owners manual.