I realized JSR binary form is this format:
0100 | 1110 | 10 Mode Mode | Mode Registr Register Regiser|
Mode are bits for the Mode Register are bits for the register
I realized that there are only so few combinations. What I realized was:
0100 1110 (0x4E) is always the highest byte.
The Next bytes is next... bit 7 and 6 are 10. bit 5 and 4 are only a few combinations for JSR. This makes this bytes upper nyble have only a few combinations:
1001 (0x9)
1010 (0xA)
1011 (0xB)
So, for the instruction to be some form of JSR: ( Relating to said instructions binary form )
1. The first byte is 0x4E
Condition is b@(pc) == 4E
2. The next bytes upper two bits ( bits 7 and 6 ) are 0010 or 0x2
So to get these upper two bits
First I isolate the upper nyble of this second byte like so
b@(pc + 1) >> 4
To extend on this, I want to isolate just the upper two bits of this so I do
(b@(pc + 1) >> 4) >> 2
So I used the expressions that MESS allows to set a watchpoint for the entier ROM (say 0FFFFF) , to break only when these two conditions are met like so:
wpset 0,0fffff, rw,b@(pc) == 4E && ((b@(pc+1))>> 4 >>2) == 2
This could easily be applied to other instructions as well
.What do ya'll think how it could be applied. Is there easier ways to search for a certain code in assembly ( say you are doing a dissasembly ) like a main game loop and its array where you jsr Arrray(pc,d0) in sonic the hedgehog mainloops?
You can also set it to print program counter address every time the instruction is JSR to a text file.