Date   : Mon, 04 Dec 1995 21:51:16 +0000 (GMT)
From   : James Fidell <james@...>
Subject: Reading the CRTC registers

One of my list of "Questions needing answers" (it's a long list -- see
http://www.OiT.co.uk/~james/BBCMicro/Xbeeb/Questions.html) was :

    What values are returned when the CRTC registers are read ?

To which I'd added the following commentary :

    The Advanced User Guide says that the registers R0-R13 are
    write-only. There's some software (Repton, Pharoah's Curse, Penguin)
    that does try to read these registers however. I've been told that
    R12/R13 can be used to read whatever value was written into them. Is
    this true for all the registers ? If not, what values are returned
    when reading the write-only registers ?

I've done some investigation into this and I know believe the case to be
as follows :

    R00 to R11    Write only.  Reading returns zero.
    R12 to R13    Read/Write.  Not like the AUG says, write-only.
    R14 to R15    Read/Write as per the AUG
    R16 to R17    Read only as per the AUG.

I can't guarantee that these are right, but having run a few tests on
my Beeb, I'm prepared to use them in my emulator.

James.

-- 
 "Yield to temptation --             | Work: james@...      
  it may not pass your way again"    | Play: james@...                 
                                     | http://www.OiT.co.uk/~james/
        - Lazarus Long               |              James Fidell