Date   : Fri, 01 Apr 2016 22:45:14 +0100 (WET-DST)
From   : bbcmicro@... (Peter Coghlan)
Subject: Emulation

>
> My mistake, I mean to say I'm setting bit 0 of $FE4D but bit 0 on &FE4E is
> zero...
>

Bit 0 of the IER should be set to 1 by the time anything is to be done with the
keyboard.  This will have been done by something like LDA #&81: STA &FE4E

Note that 6522 registers (and most other I/O device registers) at a particular
address may behave differently depending on whether they are being read from
or written to.  Some devices have two completely different registers at the same
addresses, depending on whether they are being read or written,  status and
command registers for example.

LDA &FE4E should return the current contents of the IER with bit 7 always being
returned as zero.

STA &FE4E will set or clear zero or more bits in the IER depending on whether
bit 7 of the value stored is set or clear.  The next read of &FE4E might well
return a different value to the value that was just written.

All this is explained in the Advanced User Guide.

>
> I'll read & digest JGH's reply....
>

(I sent a reply saying more or less what JGH said but I sent it to you directly
instead of to the list by mistake.  I didn't bother to forward my reply on to
the list as JGH's reply had then appeared on the list.)

Regards,
Peter Coghlan.