Date   : Fri, 19 Nov 2010 23:19:05 +1100
From   : plexer@... (James McGill)
Subject: BBC file formats

The IMG I'm trying to disassemble is available here:
http://www.bbcmicrogames.com/software/acornsoft/monsters.zip

It's loaded into &1D00, so that JMP looks reasonable. What's
interesting is that there's clearly a small amount of tokenized BASIC
near the end of the file. It's possible that the machine code loads
and executes this - I'll keep looking into it.

If anyone's feeling generous I'd appreciate any other pointers. I'm
trying to ramp up on BBC development, but obviously struggling a bit
:)

Cheers,
James



On Fri, Nov 19, 2010@... PM, Michael Firth <mfirth@...> wrote:
> If you assume that the first two bytes (0D FF) are a BASIC end of
> program marker for some reason, the rest of what you've posted is
> plausable assembler:
>
> JMP &1DF5
> LDY #&05
> STY &22B1
> LDA #&81
>
> Without having the rest of the program, its hard to know if that is
> what it is or not.
>
> One other key thing for any assembler code is to know what the load
> and exec addresses are - do you have any way of finding out those?
>
> Alternatively, if you can upload the image file somewhere I'm sure
> that people will be able to decode it for you.
>
> Regards
>
> Michael
>
>
> Quoting James McGill <plexer@...>:
>
>> I strongly suspect that this is a tokenized Basic file (especially
>> given that it starts with &0D), but it isn't one that I'm able to
>> decode. I've tried a few different programs but they all die, and I
>> think I know why. The first few bytes of the program are (in hex):
>>
>> 0D FF 4C F5 ?1D A0 05 8C ?B1 22 A9 81
>>
>> Which as far as I know means: (newline) (end of program) (other stuff)
>>
>> If I ignore the end of program marker and try and decode the line
>> anyway I get:
>>
>> "Line number: 65356
>> "Length: 245"
>> "Data: ?EVALTHENPOS"LENDIVGET$RAD"EVALOSCLI REM????"? ?"? ?`?8? ?? L??? ? ?"
>>
>> I also tried disassembling this as if it were 6502 machine code and
>> the results appeared to be mostly garbage, but I haven't stared too
>> closely. Notably it involved chains of JMPS to out of bounds memory
>> locations, which suggests to me that this probably isn't machine code.
>>
>> Any thoughts? Is it possible that the program is somehow modified at
>> runtime so that it can be decoded correctly, or that the MONSTRS file
>> (used to CHAIN) loads only a subset of the original program somehow?
>>
>> Regards,
>> James
>>
>> On Fri, Nov 19, 2010@... AM, J.G.Harston <jgh@...> wrote:
>>> James McGill wrote:
>>>> The game I'm looking at is AcornSoft ?Monsters, and I have a copy on
>>>> my linux box in .IMG format (as well as a physical copy on cassette).
>>>> I used BBC Explorer ?to open the disk and extract two files - MONSTRS
>>>> and MONST-M, but now I'm not quite sure what I'm looking at. MONST-M
>>> ...
>>>> Is anyone aware of a way to disassemble or decode these files?
>>>
>>> If you can see them in BBC Explorer (or *INFO *.* when mounted by
>>> an emulator) it should show for load and execution addresses. BASIC
>>> will tend to be something like 001900/008023, FF1900/FF8023 or similar,
>>> or sometimes FFFBxx/xxxxxx if it's been RISC OS filestamped. Also BASIC
>>> files start with <cr> (&0D).
>>>
>>> The filenames suggest that MONSTRS is the file to CHAIN, and MONST-M
>>> is the machine code - suggested by the trailing "-M". If examining
>>> MONSTRS reveals a string that looks like a "*LOAD MONST-M" or
>>> "*RUN MONST-M" command that would confirm it.
>>>
>>> If you want to disassemble machine code, tools are available at
>>> http://mdfs.net/Software/Assembler/DisAssem
>>>
>>> --
>>> J.G.Harston - jgh@...
>>>
>>>
>>> _______________________________________________
>>> bbc-micro mailing list
>>> bbc-micro@...
>>> http://lists.cloud9.co.uk/mailman/listinfo/bbc-micro
>>>
>>
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