Programming overview of version 0.10
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Need storage locations for 4 AD port values (8-bits each),
Fire bits can be read on the spot a la digital


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SWI
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On SWI entry, determine i/f type (D or A). If D, do existing
code & exit.

If A, is it the first call to this SWI? If so, set the 
ticky routine going.

Read the values from the magic memory & exit.

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Ticky routine
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On entry, it checks all the AD port 'stop' bits.
For each port with the stop bit set, it updates their
'magic' location, & resets it to do the next conversion.
End

NB: To 'reset' the conversion, write a 0 to the cntcl bit and
a 1 to the dchg bit. After a delay (unknown as yet), write a
0 to the dchg bit and a 1 to the cntcl bit.