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Programming / Why would you do this? (SCD 68k)
« on: March 05, 2012, 11:38:21 am »
So I'm trying to figure out the graphics compression for a Sega CD game. It seems like pretty basic stuff, only a handful of lines, but I'm kind of stumped as to a certain bit in there. It seems to consist of a lot of strings of values, preceded by a four-byte control code.
Is this right, that D7 is being shifted zero bits to the left? What could be the possible benefit of that? And then immediately overwriting it with a new byte? Is "#0" just some kind of weird secret debugger code for "eight"?
Also is there some kind of ridiculously verbose mode for Gens Tracer? I'm quite the beginner when it comes to assembly, and it would be nice to see this code being processed over and over again to see what it does with different values. The only trace that Gens Tracer shows for me is for the initial control code of 00000000, which isn't helpful.
Code: [Select]
FF:55FC 1E 18 MOVE.b (A0)+,D7
FF:55FE E1 8F LSL.L #0,D7
FF:5600 1E 18 MOVE.b (A0)+,D7
FF:5602 E1 8F LSL.L #0,D7
FF:5604 1E 18 MOVE.b (A0)+,D7
FF:5606 E1 8F LSL.L #0,D7
FF:5608 1E 18 MOVE.b (A0)+,D7
Is this right, that D7 is being shifted zero bits to the left? What could be the possible benefit of that? And then immediately overwriting it with a new byte? Is "#0" just some kind of weird secret debugger code for "eight"?

Also is there some kind of ridiculously verbose mode for Gens Tracer? I'm quite the beginner when it comes to assembly, and it would be nice to see this code being processed over and over again to see what it does with different values. The only trace that Gens Tracer shows for me is for the initial control code of 00000000, which isn't helpful.
