Enhanced BASIC, extending CALL
CALL <address> calls a machine code routine at location address. While this in itself is useful it can be extended by adding parameters to the CALL and parsing them from within the routine.
This technique can also be used to pass extra parameters to the USR() function.
First you need to define the parameters for your CALL. This example is for an imaginary bitmapped graphic device.
|CALL PLOT,x,y||Set the pixel at x,y|
|x||x axis value, range 0 to 255|
|y||y axis value, range 0 to 64|
This will then be the form that the call will always take.
Now you need to write the code.
.include BASIC.DIS ; include the BASIC labels file. this allows you ; easy access to the internal routines you need ; to parse the command stream and access some of ; the internals of BASIC. It is usually output ; by the assembler as part of the listing or as a ; separate, optional, file. ; for now we'll put this in the spare RAM at $F400 *= $F400 PLOT JSR LAB_SCGB ; scan for "," and get byte STX PLOT_XBYT ; save plot x JSR LAB_SCGB ; scan for "," and get byte CPX #$40 ; compare with max+1 BCS PLOT_FCER ; if 64d or greater do function call error STX PLOT_YBYT ; save plot y ; now would be your code to perform the plot command ;. ;. ;. ;. ;. RTS ; return to BASIC ; does BASIC function call error PLOT_FCER JMP LAB_FCER ; do function call error, then warm start ; now we just need the variable storage PLOT_XBYT .byte $00 ; set default PLOT_YBYT .byte $00 ; set default END
Finally you need to set the value of PLOT in your BASIC program and use that to callit.
. 10 PLOT = $F400 . . 145 CALL PLOT,25,14 : REM set pixel .