disassembly of 6803 code

[Ludis Langens]

Arnaud Westenberg wrote:
> 
> I downloaded the data sheet for a MC6803E processor. I assume there will
> be some differences between the processors but also that they use the
> same instruction set?

6803E?  My data books don't show such an animal.  There is a 6809E, but
that is a totally different animal with a mostly different instruction set.

If you can't find data on a 6803, look for 6801 stuff.  The '01 and '03
are the same, except that the '01 contains a small amount of internal ROM.

> Where/how do I start to disassemble the code? Would a disassembler be of
> any good?

A disassembler would be very useful.  If you find a 68HC11 disassembler,
you can probably use it.  The 6801/03 are a subset of the 'HC11.  One
thing to watch out for is that the 'HC11 has more interrupt vectors than
the older chips.

> Here are the specs:
> CPU, MC6803U4CPR running at 4MHz in mode 2

Are you sure it is at 4MHz?  That's awful fast for a 6803.  More likely
you have a 4MHz crystal and the CPU runs at 1 MHz.  The divide by 4 is a
normal feature of the 6803.

> but
> how to figure out that the timer is at $2000-$2007, for example?

What do you mean?  Are you asking how the hardware set those addresses? 
If so, there will be several small logic chips that decode these
addresses.  Most likely one or more of them will be a 74LS138 or 74LS139.

-- 
Ludis Langens                               ludis (at) cruzers (dot) com
Mac, Fiero, & engine controller goodies:  http://www.cruzers.com/~ludis/


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