Sinclair ZX80 / ZX81 / Z88 Forums

Hi all,

ZXMMC includes an overlay ROM. I have placed my bootstrap code in the bytes normally occupied by the character generator. As the internal ROM is accessed during display generation this isn't an issue. When a program accesses the character map data however, it will address the overlay and hence see the program code as garbage.

With this in mind I am building in the option to be able to switch out the overlay after loading so that programs which use the internal character table will work.

Because the overlay ROM is only patched in a couple of bytes then the switch should happen transparently. If, however, the internal ROM is itself hacked or is the 'old' ROM then there's going to be trouble as I can't know ahead of time what the program counter is going to be when the banks switch.

How widespread is the 'old' ROM? Does anyone have one? Or even information about it?? Google has failed me.

Charlie

Hi Charlie

I have two old rom machines (one with the hardware mod) I also have Ian Logans book Understanding Your Zx81 rom, which explains the differences between the two roms, the two relevent pages are here.

Moggy

You are a *STAR* Thanks

Curiouser:

There are 3 ROMs in the MESS set.

the original, the improved, and a third. The third only has 7 bytes changed, a simple re-ordering of instructions:

V2 'PAUSE' code: V3 'PAUSE' code: Any ideas about this, anyone? Was this ROM quietly slipped into production when the bug fix was discovered to be itself bugged??

Or is it a user hack???

Thanks

C

Hi,
In all documentations , the 'old' Rom is the Zx80 Rom.

Or is it a user hack???

A user hack !
Zx81a.Rom and Zx81b.Rom Roms are users hack.
Zx81.Rom is the original Zx81 rom... Or the original TK85's Rom !

>>viewtopic.php?f=6&t=190

XavSnap wrote:Hi,
In all documentations , the 'old' Rom is the Zx80 Rom.

Or is it a user hack???

A user hack !
Zx81a.Rom and Zx81b.Rom Roms are users hack.
Zx81.Rom is the original Zx81 rom... Or the original TK85's Rom !

>>viewtopic.php?f=6&t=190

In Ian Logans books on the subject He refers to "old" ZX81 roms and "new" ZX81 roms Meaning 8K not the 4k zx80 roms and I think Sir Morris is talking about real hardware roms for his new project rather than emulator hacks , I will check my roms for any difference in the pause command I have three types , the old 8k with sqr bug the second with bug and factory hardware mod and the "new" 8k.

Hi Charlie
Did a quick decimal listing and there is a difference in the two roms, bit pushed for time at the moment so will post when I can . In the meantime you might find this useful (if You haven't already got it)

Regards Moggy

Hi all,

just to clarify -

The MESS emulator includes:

zx81.rom - with srq bug and other problems - the 'old' rom.
zx81a.rom - with bugs fixed - the 'new' rom.
zx81b.rom - as above with the code transposition detailed above - 'new rom II'.

MESS doesn't include user hacks, generally. Otherwise we'd be seeing the 'shoulders of giants' ROM amongst others...

I'll single-step a & b next week and see what the effect is. I suspect the 'new-new rom' was quietly slipped out, after discovering the original fix also had its problems, to prevent any further damage to the reputation of Sinclair..

C

Was there a difference between the zx81 and the ts1000 roms? I Think there should have been at least one for the number of cycles per second (50 vs 60) and the 2k versus the 1k installed (as I type I think of more). Was there any bug fixes with these changes?. I recall Wilf documented a number of changes between the ts1000 and the ts1500 but don't recall anyone looking for differences between the zx81 and the ts1000.

Bill H

Um, wind forward to 2021 and read this topic

Mark