I'm trying to mimic the behavior of a NOT gate using Jasmin. The behavior is as follows:
I've tried two different attempts at this but to no avail.
Attempt 1:
...(other code1)
ifeq 3 ; if the top of stack is 0, jump 3 lines down to "i_const1"
i_const0 ; top of stack was not 0, so we push 0
goto 2 ; jump 2 lines down to first line of (other code2)
i_const1
...(other code2)
Of course, the above example doesn't work because ifeq <offset> takes in a Label rather than a hard-coded integer as its offset. Is there a similar operation to ifeq that does accept integers as a parameter?
Attempt 2:
...
ifeq Zero ; top of stack is 0, so jump to Zero
i_const0 ; top of stack was 1 or greater, so we push 0
...
... (some code in between)
...
ifeq Zero ; top of stack is 0, so jump to Zero
i_const0 ; top of stack was 1 or greater, so we push 0
...
Zero:
i_const1 ; top of stack was 0, so push 1 to stack
goto <???> ; How do I know which "ifeq Zero" called this label?
The problem with this is that I've more than one place in my code making use of the NOT operation. I tried using ifeq with labels, but after I'm done how do I know which line to return to using goto? Is there a way to dynamically determine which "ifeq Zero" made the jump?
Any insight would be greatly appreciated.
Is there a similar operation to ifeq that does accept integers as a parameter?
Yes, you can specify relative offsets using $ sign.
BUT the relative offsets are counted in bytes, not in lines.
ifeq $+7 ; 0: jump +7 bytecodes forward from this instruction
iconst_0 ; +3
goto $+4 ; +4
iconst_1 ; +7
# ... ; +8
Is there a way to dynamically determine which "ifeq Zero" made the jump?
No. Use multiple different labels instead of single Zero.
Well, there is actually a pair of bytecodes (jsr/ret) that support dynamic return address. But these bytecodes are deprecated and are not supported in Java 6+ class files.