ZXpand Amusing Incompatibility

Discussions about Sinclair ZX80 and ZX81 Hardware
XorA
Posts: 98
Joined: Thu May 10, 2012 9:14 am
Location: Glasgow, Scotland, UK
Contact:

ZXpand Amusing Incompatibility

Post by XorA »

I thought I was going insane as zxpand didn't quite work right on my first zx81 so I always assumed it was me being stupid.

Turns out it doesn't quite work right on Issue 1 boards with the two chip ROM mod on CPU (no idea what this mod is called).
RWAP
Posts: 1348
Joined: Thu May 08, 2008 8:42 am
Location: Stoke-on-Trent, UK
Contact:

Re: ZXpand Amusing Incompatibility

Post by RWAP »

Can you post a picture? And what happens when you try to use the ZXpand?

Presumably the mod was to correct an issue with some Z80 chips
XorA
Posts: 98
Joined: Thu May 10, 2012 9:14 am
Location: Glasgow, Scotland, UK
Contact:

Re: ZXpand Amusing Incompatibility

Post by XorA »

Forum already has picutures and description of the mod.

viewtopic.php?t=95

You can load/save/delete/cat as normal, but even the simplest program like

10 PRINT "GRAEME"
20 GOTO 10

Will print the first GRAEME then error with a wierd code. I can get the exact codes at some later time!

As soon as you do CONFIG "X" to turn off zxpand programs work as expected!
sirmorris
Posts: 2811
Joined: Thu May 08, 2008 5:45 pm

Re: ZXpand Amusing Incompatibility

Post by sirmorris »

ZXpand uses an overlay which is a modification of the existing ROM. The addresses corrected by the fix aren't changed in the overlay however; so I don't know what kind of effect is in evidence here.

Was the very first ROM radically different to the later release? I thought it was just a few bytes - however if there were functions which overlap the changed areas in the overlay then this would cause trouble...

If the ROMs are essentially the same then the top-of-my-head offer would be that the additional circuitry is loading the bus somehow... Sorry I can't be more illuminating :(

C
sirmorris
Posts: 2811
Joined: Thu May 08, 2008 5:45 pm

Re: ZXpand Amusing Incompatibility

Post by sirmorris »

I've been off and found copies of the original and improved ROM online. They appear to be radically different. This means that the hardware fix is removing opcodes that it shouldn't be... I wonder if I could hack around this. What addresses are affected - is it definitely $1733-$1735? I may be able to jump around them..

C
XorA
Posts: 98
Joined: Thu May 10, 2012 9:14 am
Location: Glasgow, Scotland, UK
Contact:

Re: ZXpand Amusing Incompatibility

Post by XorA »

Its not a major issue for me as my other 5 zeddys work fine.

If you wanted to be completist then I think your in UK so I could always lend you the machine for a couple of weeks!

G
sirmorris
Posts: 2811
Joined: Thu May 08, 2008 5:45 pm

Re: ZXpand Amusing Incompatibility

Post by sirmorris »

I think it may be worth noting the fact that it doesn't work with the rare 1st issue machines in the documentation or sales blurb. I'm sure it can be fixed in the ROM but I don't think it's worth putting the fix into the mainline ROM as it will slow down some math operations. And we all know it's speed we need, not slows :)

If there is a call for a compatible ROM I'll be happy to look into it. For the moment I don't think I'll need a machine (thanks for the offer XorA) but I may call on you for testing..!

C
RWAP
Posts: 1348
Joined: Thu May 08, 2008 8:42 am
Location: Stoke-on-Trent, UK
Contact:

Re: ZXpand Amusing Incompatibility

Post by RWAP »

I should imagine that there is a very small percentage of ZX81s which have this fix!
gozzo
Posts: 452
Joined: Fri Jul 08, 2011 8:52 pm

Re: ZXpand Amusing Incompatibility

Post by gozzo »

According to information in 'understanding your zx81 rom' by Dr Ian Logan the differences between the early rom with the maths bug and pause crash, and the later version is ..

between 0000 and 0EE9 - unchanged, between 0F20 and 1022 - moved up 3 bytes, between 1046 and 1716 - moved up 4 bytes, between 1737 and 1DE1 - moved up 1 byte. (no mention of any other changes)

so maybe this could be a cause of the problem...???? as for the 'hardware fix' , Dr Logan 'thinks' (he said that sinclair had not confirmed this) that it corrects the maths bug by patching out an instruction - changing "LD H,A" to "DAA" at 1735..... obviously this would not move the routines as shown above..
gozzo
Posts: 452
Joined: Fri Jul 08, 2011 8:52 pm

Re: ZXpand Amusing Incompatibility

Post by gozzo »

none of my issue ones (4 or 5, I think !)has a hardware fix, so maybe this was only done with a very few before the new rom came out ..)
Post Reply