Date   : Fri, 05 Jan 1996 11:06:06 GMT
From   : David Alan Gilbert <gilbertd@...>
Subject: Re: Beeb emulator snapshot format

> David Alan Gilbert wrote:
> 
> Very true.  Are you suggesting a completely ASCII format for the file,
> or binary data separated by ASCII keywords ?

In some ways it would be nice; but for things like memory it would be silly.
I'm thinking of something similar to the way some image file formats work.
Besides the header you could have a number of blocks of data of the form:
  4 character ID (e.g. MEMO, PROC, UVIA, SVIA, - a special one to mean
end of file etc.)
  length of this block (counted from the start of the ID field - i.e.
offset to next ID)
  data

For anything with multibyte values the endianness should be well defined
(I suggest
little endian for sake of simplicity compared to beeb - and because we
all know
that it is the CORRECT endianness :-)

> Out of interest, why do you keep track of who has requested an interrupt ?

What I have is a word for IRQ's and a word for NMI's; if the appropriate
word is
none zero then its time to cause an interrupt.  Each interrupting device has
a bit allocated to it in the word.If it wants to cause an interrupt it
just OR's
its bit in; if it no longer wants to cause it it just AND's it out.
The actual execution is not changed by which device is interrupting (other
than by
the OS figuring out by looking at device registers).
This way a device can assert the interrupt line without worrying if it
was already doing
so. 

Cheers,

Dave