Wednesday, January 16, 2013

Reverse engendering of 680x0 assembly code.

Trying to make sense of it all, IRA the disassembler has been really help full to gives some clues; by providing EQU constants for hardware registers, and some other stuff I don't know what is.


It is worth to point out that any access to hardware registers are illegal under AmigaOS4.1, unless your running on old hardware, so we need to replace it whit system friendly code, so we need to look for any references to this constants, that I have marked, and replace the code.


Next thing that is a issue is to understand what the code does, there are some clues, see the lines that starts whit JSR, JSR is short for Jump To Sub Routine. In front of A6 (address register 6), you have a negative value, this is a offset value we call LVO, A6 is loaded whit library base address, we just need to find what library and compare the LVO values to that library offsets, LVO number are not unique to one library they can be the same for number of libraries, so we can't just auto replace the values whit constants.

Another thing that will help making it possible to understand some thing is to look at bottom of the code, this where you find data, like strings.


DC and DS is defines data space, DC is for values you enter, while DS just reserves chunk of space, it is the DC that we are most interested, in front of DC there is label it represent a reference to the data, the label has been generated by IRA disassembler and not human understandable, we need to replace the label name whit some thing we understand, so we replace LAB_0F71 whit LAB_STERO_LEVEL, and LAB_0F6B whit LAB_VOLUME_BOOST, we do that for all readable strings, we most be careful to replace every reference of LAB_0F6B and LAB_0F71.

2 comments:

  1. Interesting read!

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  2. Well hopefully by writing some word here, I might inspire some one. Nobody need a classic Amiga to write code in 680x0 Assembly language, and it is possible to write system fremdly code in 680x0 Assembly language, so there is no excuse not doing so,

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