Disassembling

Basic and Machine Language

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ldxstx
Vic 20 Dabbler
Posts: 71
Joined: Tue Nov 27, 2012 4:09 pm
Website: http://www.nonsolo8bit.it
Location: VICenza (Italy)
Occupation: NitroWheels

Disassembling

Postby ldxstx » Fri Apr 07, 2017 7:09 am

Hi, I am studying the disassembly of some games (i.e. Amok, Astro Panic, etc) to see instruction after instruction how they specifically
implemented the scrolling at single pixel level, on the unexpanded Vic (at least for the moment I like to focus on this configuration).
I am searching for buddies to proceed in this adventure joining our forces. Btw I've already completed the analysis of other portions of them.
As a following step I would like to develop a new simple ML game for the unexpanded machine.

I like to be here writing again after such a long time (Jeff I've sent you a PM :) ) Long live to this wonderful forum.

Thanks
-Enrico

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Kakemoms
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Joined: Sun Feb 15, 2015 8:45 am

Re: Disassembling

Postby Kakemoms » Fri Apr 07, 2017 4:49 pm

Well, looking at Astro-Panic I would say that the x-displacements are probably pre-calculated and then the gfx is applied onto the screen bitmap using a mask (AND) and ORA to put in the graphics.

You could of course use ROL or ROR to bit-shift the different "sprites" to their x-position in real time, but from my experience with soft sprites on the vic, it takes a lot of cycles to do that. You can look at the source of my mouse pointer for the Vic-20 here to see that (set Vice to NEOS mouse emulation). If links are broken, send me a note and I will try to update them (again!).

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ldxstx
Vic 20 Dabbler
Posts: 71
Joined: Tue Nov 27, 2012 4:09 pm
Website: http://www.nonsolo8bit.it
Location: VICenza (Italy)
Occupation: NitroWheels

Re: Disassembling

Postby ldxstx » Sat Apr 08, 2017 3:20 am

Hi Kakemoms, thank you for your reply and pointers :)

I've been talking about the scrolling matter - and got useful tips&hints as always - with Mike and Nippur72; what I'm searching for at the moment, possibly, is some "crazy" buddy as me :P to specifically look into some asm portions of these games, to see their specific implementation, this as a reversing exercise too. Maybe, if nobody can join or if this seems like a total waste of time && (||) energy, as an alternative I will paste some snippets instead, for example regarding Amok.

Have a nice day, cheers
Enrico


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