Date   : Sun, 30 Oct 2005 16:58:00 +0000
From   : Tom Seddon <tom@...>
Subject: Re: BBC using 3.5 high density format

Tom Seddon wrote:

> The NMI routine would go a bit like this:
> 
>     \ +7 (7) -- NMI overhead
> &D00    LDA FDC_DATA \ +6 (13)
>     STA (&C0),Y \ +6 (19)
>     INY \ +2 (21)
>     BNE NO_BUMP:INC &C1 \ +7 (worst case) 28 <-- point A
>     .NO_BUMP TXS \ +2 (30)
>     NOP \ +2 (32)
>     NOP:NOP:NOP \ +6 (cater for best-case timings)
>     NOP \ the lucky NOP
>     JMP READ_END
> 

My calculations are incorrect, I think. The STA could potentially take 
10 cycles writing to the I/O area, because you have 1 read from the 
target address (whilst the MSB of the EA is fixed up) and 1 write. 
Assuming the high byte doesn't need fixing, that's 2 x 1MHz accesses.

(This may be 9, actually, because if one read takes an extra 2 then the 
next one takes an extra 1... or something like that! It's been a while 
since I looked at this stuff :)

--Tom