SCRN function for Hires screen from BASIC



you call this from basic. it returns the state of a pixel on the hires screen (1 for on, 0 for off) BRUN it from BASIC (this sets up the ampersand command) and then 6040 X = 100:Y = 100 6050 & A = X,Y 6060 PRINT A call it from BASIC. it uses the ampersand command & for interface to BASIC. the above code will set BASIC variable A to the state of the pixel at X,Y 

   ;HIRES SCRN FUNCTIN
   ;FROM ROBOTIC ARM PROJECTS
   ;BOOK.
   ;SEPTEMBER 9 2003 IN SATAN HELLE
          ORG $30A
FRMEVL    EQU $DD7B
FRMNUM    EQU $E752
LOW       EQU $50
HIGH      EQU $51
CHRGOT    EQU $B1
POS       EQU $F411
INDEX     EQU $E5
MASK      EQU $30
SETFAC    EQU $EAF9
NEWSTT    EQU $D7D2
AMPER     EQU $3F6
PTRGET    EQU $DFE3
FORPNT    EQU $85
SYNCHR    EQU $DEC0
BASE      EQU $26
FLTACC    EQU $EB93
SETFOR    EQU $EB27
HIRCOR    EQU $F6B9
COLD      LDA #START
          STA AMPER+1
          RTS
START     JSR PTRGET     ; GET VAR
          STA FORPNT
          STY FORPNT+1
          LDA #$D0       ;=
          JSR SYNCHR     ;CHECK FOR =
          JSR HIRCOR     ;READ COORDINATES
          JSR POS        ;CONVERT TO ADDRESS
          LDY INDEX
          LDA (BASE),Y   ;GET HIRES BYTE
          AND MASK       ;CHECK PROPER BYTE
          AND #$7F       ;CLEAR COLOR BIT
          BEQ PTONE
          LDA #1         ;SHOW SET
PTONE     JSR FLTACC     ;FLOAT ACC
          JSR SETFOR     ;MOVE ACC TO VARIABLE
          RTS            ;BACK TO BASIC
                                     

to hplot a pixel from assembly:  https://rich12345.tripod.com/prog/hplotassy.html                                                                                                                                                                                                                                                    
		  Problem with & under prodos
DSK image of Dos 3.3 Merlin with source code
there is the disk.
dos3.3 DSK image
load the dsk image in an emulator, BOOT
Type L to load
Type HIRESCRN 
Type E
Look at the source code
Type CONTROL-Q
type ASM
type Q
type O
Type HIRESCRN 
exit merlin
]BRUN HIRESCRN
]call -151
*0800 L
 
*3f4L
3f4: 4c 0b 08   JMP  $080b
the ampersand vector is correctly initialized
in DOS3.3
I copied this object code into PRODOS disk
using copy II+
The HIRESCRN gets BRUN'ed.  It correctly
sets up the ampersand vector at 3f4-3f6
When calling the code from basic:
&a=x,y
Prodos Basic System must be getting in
the way, as BASIC variable A always returns
with the value it started at, and does
not get modified by the code.