Note: This question is not likely NES specific, but I have not yet seen this happen elsewhere

So I have been looking in FCEUX Debugger (NES) just for fun and came across this stange thing inside, so far, 3 different games.
This example is not from any of the games, but just an illustration of the exact same procedure.



What purpose could this jump possibly have? If this is residual code from development, why not 3 NOP's instead?
Any reason to do this sort of thing

JMP absolute is the easiest way to burn three bytes and three cycles with the same instruction. Maybe that's what they were trying to do?

Mike

The last line of a macro expansion? Ie., the source code doesn't show it looking like that, but that's what the object code looks like.

Could F4F2 be the end of an If type statement with several Elses?

Another possibility is that it's a "generic" code sequence used in the common part of a bankswitch ROM, and the subsequent part is different in every other code bank. It could be an artifact of whatever development toolset they are using linking multiple files together. As Mike suggested, it could be to burn a specific number of cycles, as certain effects on the NES require fairly precise timing with respect to the raster position. A final possibility is incompetance (never discount this possibility, there is some spectacularly bad code in some of the NES games out there).

It could be an output from a compiler for an IF-THEN-ELSE statement, after the THEN section jumping over a non-existant ELSE section. Some compilers are quite good at this sort of unnecessary code.

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Shift to the left,
Shift to the right,
Mask in, Mask Out,
BYTE! BYTE! BYTE!

I too belive this was made by a compiler or by mistake in the source. Like a macro or a IF-THEN.. loop. Also, all of your suggestions could be more or less true as this routine is present in 3 differently specified ROM's. Anyways, off for some practise assembler coding. Thanks alot!

Another possibility is that the code was written using a relocatable assember and the JMP transfers execution between two routines written in different code modules but which have been linked into consecutive memory locations.

For other Nintendo platforms the Intelligent Systems development suites were the officially recommended toolset (see http://www.intsys.co.jp/english/tools/index.html). These look like relocatable assemblers and C compilers to me.

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Andrew Jacobs
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