Hi Mychess,
great job - a lot of work for only 3 bits!

Question 1: we have many H8 engines in CB-Emu - don't any other of these engines (apart from Centurion/Cougar) have this H8 bug?

Question 2: there's a difference in the Fun level bugfix between the 2 versions you've posted in your first thread and today:
66d0 : 7F 52 72 60 41 62 7F 52 72 70 5A 00 50 62 (from 31.03.2025)
66d0 : 7f 52 72 60 41 50 7f 52 72 70 5a 00 50 62 (from today)
So which one is correct?

Regards,
Franz

__________________
Meine Webseite: https://fhub.jimdofree.com/

Hi Fhub,


Question 1 : The list of computers with H8 bug come from Spacious-Mind : https://hiarcs.net/forums/viewtopic.php?f=1&t=6021,
so no doubt !

Question 2 : Both are correct, this byte is a dummy byte for checksum control when you power on the computer.
In the new version, it is modified to take account of the patch for H8 bug.

Code:

patch for H8 bug   :  3288 : 6b01 fe60 1c19
patch for Fun bug  :  505e : 5a00 66d0                                    
                      66d0 : 7f 52 72 60 41 50 7f 52 72 70 5a 00 50 62

Cordialy,

mychess

Hi Mychess,
ok, but there's still one thing that I don't understand with your Fun-level bugfix:
66d0 : 7f 52 72 60 41 50 7f 52 72 70 5a 00 50 62
I've disassambled the code at this address with the debugger and got:
Since I don't understand H8 assembly code I've searched the Internet for the H8 instruction set and found, this 'bf' (at 66D4) means 'branch if false' - but I don't know 'if WHAT is false' (I guess it's usually any condition/compare before this command)!?

Now I wonder why did you put this 'checksum correction' byte(s) 41 50 WITHIN your bugfix code - couldn't it happen that this 'WHAT' condition is indeed false when this code is executed, and in this case the program would jump/branch to an address (6726) with no valid code?
So why didn't you just put these correction bytes (41 50) at the end of your bugfix code, which would definitely never be executed?

Regards,
Franz

__________________
Meine Webseite: https://fhub.jimdofree.com/

Hi Fhub,


You can put the good byte for a good checksum at the end of the patch.
However, a curious programmer (Fh..) could be astonished by a byte never adressed by the patch.

So I prefer to use a dummy short instruction (yes, with 2 clocks CPU lost) with the good byte as immediate value.
The BF is a "branch never", as BT is a branch always, without testing any code condition.
So 41 50 is a 2 clocks cpu lost instruction who only make PC = PC+2, and 50 is the byte for a good checksum.

You can make a best performance patch with :

66D0: 7F52 7260 bclr #6, @h'ff52
66D4: 7F52 7270 bclr #7, @h'ff52
66D8: 5A00 5062 jmp h'5062
66DC: 91 ; (41+50)

Cordialy,

Mychess

Yes, I'm a VERY curious programmer!
Ok, now I understand that this jump is never done.
But what is an instruction BF good for, when it actually never does anything?

The 2 clocks CPU lost is of course no problem, because I think this bugfix code is only executed when the user changes the level (to or away from a Fun level).

__________________
Meine Webseite: https://fhub.jimdofree.com/

And from your "66DC: 91 ; (41+50)" I guess the program uses only a 1-byte checksum, correct?
Then why not just change the checksum byte instead of adding such a dummy byte?

__________________
Meine Webseite: https://fhub.jimdofree.com/

Yes, only a one byte checksum.
However, the checksum is not stored, it should be 00, so I add a byte to obtain 00.

Another solution would be to delete the checksum call from the rom, but I'm follower of "Entia non sunt multiplicanda praeter necessitatem", or principle of parsimony.


Cordialy,

Mychess

Well, a 1-byte checksum is a quite simple method - how does the program react when it detects a wrong checksum?
I remember such a situation for any other ROM that I've patched, and it actually returned an error message (I've just skipped this error routine).

__________________
Meine Webseite: https://fhub.jimdofree.com/

A continuous error message (E: ron) with beep.
However, "enter" bypass the message and the computer is ready.

Here my little list concerning the H8-Bug

H8-Bug
Centurion
Cosmos
Cougar
Mephisto Chess Challenger (Admiral)
Mephisto Explorer Pro
Mephisto Expert Travel Chess
Mystery

No H8-Bug
Gk 2100
Saitek Präsident/President
Saitek Travel Champion 2100
Saitek Brute Force Modul
MM VI
RadioShack Champion 2250XL
Mephisto Schachakademie

Mephisto Talking Chess Academy ?

I still have a question mark over Mephisto Talking Chess Academy by Saitek.
Talking Chess Academy is actually very close related to Mephisto Schachakademie, so there shouldn't be a H8-Bug.

On the other hand I once read somewhere, that the Talking Chess Academy from Saitek supposedly does have the H8-Bug.
Does anyone know exactly what is right here ?

Greetings
Hans-Jürgen