Re: FPGA (was: 6809 / 6702 puzzle)

From: Michał Pleban <>
Date: Tue, 24 Apr 2012 17:43:38 +0200
Message-ID: <>
Hello! wrote:

> My suggestion, therefore, is to print out all three schematics and to
> cross-check them with each other and find out which two agree with each
> other! I also came across an issue associated with data bus termination
> - or lack of - stating that the addition of 10 kOhm pull-up and
> pull-down resistors on the data bus cured some unreliability issues. Any
> redesign should incorporate any known bugfixes.

I did not look at Ruud's schematics yet, but this definitely will need 
to be done :-)

> However, I also believe that other software is downloaded from the host
> processor to the 8088 which will probably do 'bit twiddling' on the
> hardware to implement functionality that is not coded in the EPROM - so
> analysing the EPROM code for the functionality of the peripheral chips
> may not identify what is actually required. For example, the TOD
> functionality of the 6526 is clearly used as someone has gone to the
> trouble of wiring up the TOD input - this feature is not activated from
> within the EPROM. The 6526 also appears to generate interrupts as the
> IRQ pin is wired. So the question is do we go for a minimal emulation of
> the 6525/6526 and expand it as we find functionality that is used that
> we have not implemented or do we go for a full-blown emulation and find
> that there is unused functionality - or do we just implement what
> appears easy and leave the rest for later?

I have the disassembly of MS-DOS bootloader laying somewhere, so I will 
check whether it contains any code to interface directly with I/O chips. 
My understanding was that it does not, leaving everything to the IPC 
routines in EPROM.

> The EPROM code appears to also do 'strange' things. For instance, the
> Stack Pointer (SP) is initialised when reset - but not the Stack Segment
> (SS) as I would have expected. The default is for SS to be initialised
> to 0 on a reset. Interrupts are clearly used by the EPROM code - but I
> have not yet found anywhere where the 8088 interrupt vectors are
> actually initialised (unless this is initially done by the host
> processor). I have found some code in the EPROM which does play with the
> interrupt vectors - but this only seems to be executed later on once an
> interrupt has already occured. I will print out the source code listing
> from tomorrow and have a read - it may shed some
> more light on this issue.

Interrupt pointers and other stuff, are set up by the host, and only 
then an interrupt is sent by it to the 8088.


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Received on 2012-04-24 16:00:33

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