$Id: Help,v 1.4 2002/01/14 23:54:48 james Exp $

Beeb-related questions I'd like answered
========================================

There are lots of things you need to know to write an emulator that
aren't sufficient well explained in the User Guides and manuals.
If you know the answer to any of these questions, please e-mail me
<james@cloud9.co.uk>.


o What are the initial states of the 6850 ACIA registers ?

  That is, when the machine is powered up, before the CPU starts
  executing the OS.  Presumably they have a fixed power-up state ?


o When the 6850 ACIA does a master reset, are the parity error and
  framing error flags cleared ?


o What values are returned when the 6845 CRTC registers are read ?

  The Advanced User Guide says that the register 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 ?


o What's the initial configuration of the Analogue-to-Digital converter
  on power-up reset ?

  That is, what channel does it start doing a conversion on, is it in
  ten- or eight-bit conversion mode, is the ADC busy and is a conversion
  marked as complete ?


o What are the initial values of all the CRTC registers ?

  Before the OS sets them all up correctly so that it's actually possible
  to see the display.


o What's the power-up state of the 8271 FDC ?


o What's the power-up state of the Econet controller ?


o What's the power-up state of the serial ULA ?


o In the 6522 VIA, the manuals say that the T2 interrupt can only be
  re-enabled (after it has been triggered) by writing to T2CH or reading
  from T2CL.  Is that because these actions also reset the interrupt flag
  or does this happen completely independently ?


o And another...  Read/Write of the ORA clears the interrupt flag
  in the IFR.  Does Read/Write of the non-handshaking copy of ORA do the
  same thing ?


o What is the power-up state of the Tube ULA registers ?


o What is the power-up state of the Video ULA registers ?


o What values are returned when the Video ULA registers are read ?

  The Advanced User Guide says that the registers are write-only.


Revision History
================

$Log: Help,v $
Revision 1.4  2002/01/14 23:54:48  james
Updates for v0.4

Revision 1.3  2000/09/08 00:05:58  james
Change from old hermione.demon.co.uk e-mail address

Revision 1.2  1996/09/24 22:47:20  james
Some clarification of questions.

Revision 1.1  1996/09/24 22:43:21  james
Moved HELP to doc/Help.

