Can't hack Alien Returns

[Pac Munchkin]

I tried hacking Alien Returns but even changing it slightly makes the screen go all haywire.

 

I was going to attempt to change Alien Returns into a Turtles like game, there would be differences of course because in Alien Returns there is an order the parts of the ship that must be grabed.

 

Col'n seems easier to hack but it seems off, like the bottom of the screen is below the viewing area.

 

Maybe this just can't be done?

 

Wade

[Nukey Shay]

I tried hacking Alien Returns but even changing it slightly makes the screen go all haywire.

 

I was going to attempt to change Alien Returns into a Turtles like game, there would be differences of course because in Alien Returns there is an order the parts of the ship that must be grabed.

 

Col'n seems easier to hack but it seems off, like the bottom of the screen is below the viewing area.

 

Maybe this just can't be done?

 

Wade

 

Anything can be done

Chances are, the areas you were changing were being shared by something else...or the bitpattern you used caused an unconditional branch to fail. Regarding the latter...consider the following:

 

Just suppose GFX for a given sprite is always expected to be a non-zero value for every scanline used. A program might use a BNE instruction (branch if not zero) after loading the bitmap line...in which case, the branch would behave similar to a JMP instruction - it's always taken (and saves a byte, because BNE uses 1 less than JMP). If you tried to make a sprite smaller in such a program by filling parts of the bitmap with zeros, that branch would no longer work...and the program would spill over into the next following instructions or data (major disaster if you happen to be in the display kernal - trying to display much more than a 2600 game is supposed to).

 

The way that these problems are identified/corrected is through disassembly.