Stella Debugger Trapping a Bit

[+Karl G]

I was wondering if anyone had any clever ways to trap writes only to a specific bit of a variable?

 

[+stephena]

IIRC, this functionality is already built in.  It basically makes use of the '&' operator to look at specific bits.  There are built-in functions that use it, so you could write your own expression to do it too.  This is from the 'help' command in the debugger:

Quote

Built-in functions:
_joy0left   {!(*SWCHA & $40)}   Left joystick moved left
_joy0right  {!(*SWCHA & $80)}   Left joystick moved right
_joy0up     {!(*SWCHA & $10)}   Left joystick moved up
_joy0down   {!(*SWCHA & $20)}   Left joystick moved down
_joy0button {!(*INPT4 & $80)}   Left joystick button pressed
_joy1left   {!(*SWCHA & $04)}   Right joystick moved left
_joy1right  {!(*SWCHA & $08)}   Right joystick moved right
_joy1up     {!(*SWCHA & $01)}   Right joystick moved up
_joy1down   {!(*SWCHA & $02)}   Right joystick moved down
_joy1button {!(*INPT5 & $80)}   Right joystick button pressed
_select     {!(*SWCHB & $02)}   Game Select pressed
_reset      {!(*SWCHB & $01)}   Game Reset pressed
_color      {*SWCHB & $08   }   Color/BW set to Color
_bw         {!(*SWCHB & $08)}   Color/BW set to BW
_diff0b     {!(*SWCHB & $40)}   Left diff. set to B (easy)
_diff0a     {*SWCHB & $40   }   Left diff. set to A (hard)
_diff1b     {!(*SWCHB & $80)}   Right diff. set to B (easy)
_diff1a     {*SWCHB & $80   }   Right diff. set to A (hard)

Note the "& $xx" parts above.  This is only looking at those specific bits.  Since this is using a conditional expression, you need to use the 'IF' versions of trap: trapif, trapreadif, trapwriteif.  In your case, you would use the very last one.  As always, the manual is your friend ?