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Old 20 June 2019, 00:06   #1
jotd
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is CIA port A bit 1 (filter) readable?

in some game I saw:


Code:
    TST.B    some_flag     
    BEQ.S    LAB_0598   
    BSET    #1,CIAA_PRA   
    BRA.S    LAB_0599   
LAB_0598:
    BCLR    #1,CIAA_PRA   
LAB_0599:

but is it possible to just toggle it ?:


Code:
  BCHG    #1,CIAA_PRA

I suppose that all (except 6 & 7 maybe) bits are readable as BSET/BCLR already read the whole byte to just change 1 bit. Is that bit readable too?
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Old 20 June 2019, 01:20   #2
Thomas Richter
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Quote:
Originally Posted by jotd View Post
Is that bit readable too?
Yes. CIA port bits are both read and write at the same address, unlike the custom chips where the direction is implicit in the address.
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Old 07 July 2019, 22:44   #3
Photon
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I have assumed no, since I've seen code that saves and restore hardware registers but not the state of this bit. Interesting.

Probably some structure in audio.device or elsewhere can be used to get the last state set by some library call, though?
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Old 08 July 2019, 08:05   #4
Thomas Richter
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Unfortunately, operating system support for this bit is zero. There is nothing, nowhere.
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