Screwball (Blue Ribbon)

[Wrathchild]

I came across an email I'd sent to AtariMania back in 2007 regarding this title as at that time I'd made a CAS dump, verified that both sides produced the same results, but the image crashes under emulation. Here's the email:

 

Saw this on Ebay and that it wasn't in the collection

 

The image however crashes under emulation? This maybe, I suspect, something to do with the ROM/BASIC images I'm using and so a different combination may work?

 

The problem seems to be that this title uses RUN "C:" in order to load the game and so achieves this by using a little routine it loads at $600. This looks like:

 

0600 LDX #$FD

0602 TXS

0603 LDA #$B7

0605 PHA

0606 LDA #$54

0608 PHA

0609 LDA #$04

060B JSR $BBB6

060E LDA #$FF

0610 JMP $BB04

 

So assumes entry points into the Basic ROM but seems to work for Rev.C

 

So what gets loaded looks like a mixture of basic and machine code. It looks like, after an initialisation routine is called (located at $9708 which could be from a USR call in basic) the stack pops leading to jumping to rubbish code and so the system crashes.

 

My suspicion is that, if it is a USR call, then the number of args is on the stack and this is not popped by the machine code routine and so one of the RTS instructions using this as part of the return address. However, testing this after a "break 9708" with a "c 971a ea 68" (NOP/PLA to replace a LDA #$00) the crash disappears but still the game doesn't start.

 

Probably best to post this up on AtariAge and see if someone can unravel why (Also I haven't had the chance to try this direct on a real Atari yet, maybe later this week).

 

So here is the image if anyone fancies a go at finding out what is wrong we can provide the missing image from AtariMania.

 

Regards,

Mark

SCREWBAL.zip

[Kr0tki]

The image doesn't load under emulation, and probably won't load on a real Atari - an inter-record gap between records 105 and 106 is too short. Here is the fixed image. It works with and without SIO patch (at least under Atari800 v2.1.0).

Screwball (1985)(Blue Ribbon Software).cas.zip