Using DF_CC with ASM

[swensont]

I want to be able to see what character is at X,Y location on the screen. The system variable DF_CC contains the address of the next location to print. If I do a PRINT AT, but don't print, then the value at the location pointed by DF_CC should be at X,Y. For some reason I can't get it to work in ASM, but I can get it to work in BASIC.

Here is the BASIC program:

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10 print at 10,10;"*"
15 print at 10,10;
20 let x = peek 16398 + 256*peek 16399
30 let y = peek(x)
40 print at 0,0;x
50 print at 1,0;y

The result prints "23" at the end, which is the code for asterisk.

Here is the ASM:

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     LD     BC,$0A0A    ; set print position
     CALL   PRINTAT
     LD     A,$17       ; print asterisk
     CALL   PRINT

     LD     BC,$0A0A
     CALL   PRINTAT     ; set new PRINTAT location
     LD     A,(16398)   ; Get value at new position
     PUSH   AF
     LD	BC,$0000    ; set print to 0,0
     CALL	PRINTAT
     POP    AF
     CALL   PRINT       ; print at 0,0 what is in DF_CC
			
     RET                ; Return to BASIC

This does not print an asterisk, but instead I get a "\". I'm not sure what is going wrong.

Thanks,

Tim

[dr beep]

You miss 1 step

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     LD     BC,$0A0A
     CALL   PRINTAT     ; set new PRINTAT location
     LD     HL,(16398)   ; Get new position
     LD	A,(HL)	; get value

[swensont]

The doctor has spoken. I guess I don't need a second opinion.

Tim

[XavSnap]

Hi Tim,

Yes, your code is wrong:

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     LD     BC,$0A0A
     CALL   PRINTAT     ; set new PRINTAT location
     LD     A,(16398)   ; Get value at new position

The value at 16398 is "20", from the DF_CC value... ($42"20" ? or something else.)
LD A,(16398) ; Get value at new position
Don't get the indirect value at the DF_CC target address, but at 16398 !

Dr.Beep use the indirect HL value :

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     LD     HL,(16398)   ; Get new position
     LD	A,(HL)	; get value

HL=Target and A, the value in this target.

LD A,(16398)= the byte in 16398 in the memory !
You had to retrive the offset in 16398, and extract the value in HL.
LD A,(HL) ; only HL,BC,DE or IX/IY/SP register are suitable to get an indirect value.

LD A,(16398 = the value at the 16398 memory offset.
LD A,(HL)= the value at the HL=16398 memory offset, the displayed character in the screen memory location from DF_CC.

[dr beep]

You miss 1 step

Code: Select all

     LD     BC,$0A0A
     CALL   PRINTAT     ; set new PRINTAT location
     LD     HL,(16398)   ; Get new position
     LD	A,(HL)	; get value

After the CALL to PRINTAT the HL-register is already pointing to the screenposition.
so the 3rd line is not needed at all.