6502 Software Application and Usage Notes

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  2013-11-17 Started this page to show how to efficiently use the new features.

• Accessing local, static and global variables

The idea of the B register, together with the stack-/PC- and B-relative addressing modes comes from this use case: how do I access local, static and global variables? (static in the Java sense - local to a class, i.e. all instances at the same time)

The idea behind is that:

• The stack contains local variables, i.e. variables available to this thread only. The code would look about this:

  	.(		; start of subroutine
  	SBS #20		; make place for local variables
  	STA (S,0)	; use values on stack with stack-relative addressing
  	...
  	ADS #20		; clean up stack
  	RTS
  	.)
  

• The B-register would point to a parameter block. This would be the parameters for the method. Variables there would be accessed by

  	...
  	LDA (B,12)	; load parameter
  	...
  

• The code itself is compiled to a specific address, and "static" variables belonging to this code are compiled into a fixed offset (or one that is adjusted ad load time). So variables accessible to all threads running this code could use:

  	...
  	LDA (P,10)	; load shared variable
  	...
  

• Finally, global variables can be accessed via non-offset addresses as ususal:

  	...
  	LDA $1234
  	...