Debugging

[britlion]

Is a pain.

Right now I've got a program that:

Code:

FOR n=40 to 50                                                                    
FastPlot (n,50)
next n

draws pixels in all the wrong places So my fastplot is wrong?

Code:

FOR n=40 to 50                                                                    
FastPlot (n,50)
pause 0
next n

Ah! That fixed it. Now it plots all the pixels, with pauses, in all the right places.

*headscratch*

Presumably fastplot is wrong. I think. But...huh? How does that make it work better?!?

Code:

SUB FastPlot (x as uByte, y as uByte)

ASM
ld HL,FastPlotYValue+1
LD E,(IX+7) ; y
LD (HL),E

call FastPlotASMStart
END ASM
return

ASM
FastPlotASMStart:
    ld      hl,BLtabpow2
    ld      (FastPlotSaveX+1),A
    and     7       ; x mod 8
    ;ld      b,0
    ;ld      c,a
    ;add     hl,bc
    add a,l
    ld l,a
    jr nc, FastPlotSkip1
       inc h
    FastPlotSkip1:
    
    ;ld      a,(hl)    ; optimize - load e with (hl) directly, not (hl) -> a -> e.
    ld      e,(hl)     ; e contains one bit set

    ld      hl,BLTabLinIdx
END ASM
FastPlotYValue:
ASM
FastPlotYValue:
    ld      a,0 ; Self modding code that puts y value here
    ;ld      b,0
    ;ld      c,a
    ;add     hl,bc
    add a,l
    ld l,a
    jr nc, FastPlotSkip2
       inc h
    FastPlotSkip2:
    ld      a,(hl)      ; table lookup

    ;ld      h,0
    ;ld      l,a
    ;add     hl,hl
    ;add     hl,hl
    ;add     hl,hl
    ;add     hl,hl
    ;add     hl,hl       ; x32 (16 bits) (this multiply routine is 66 t states)

    rrca
    rrca
    rrca
    ld h,a
    and %11100000
    xor h
    or %01000000
    ld h,a   ;- this is faster at 35 T states to do hl=a*32

    ;set     6,h         ; adds the screen start address (16384) (moved up to previous and %010 instruction)

FastPlotSaveX:
    ld      a, 255 ; X Self Modifying - this is written into to save X earlier.
    ;srl     a
    ;srl     a
    ;srl     a           ; x/8 24 T states
    rrca
    rrca
    rrca
    and %00011111        ;x / 8 19 T states
    
    or      l
    ld      l,a         ; + x/8.

    ld      a,(hl)
    or      e           ; or = superposition mode.
    ld      (hl),a      ; set the pixel.
    ret

END ASM


#ifndef BLTabLineIdxPresent
#define BLTabLineIdxPresent
ASM
    ;; small screen lines lookup table

BLTabLinIdx:
    defb    0,8,16,24,32,40,48,56,1,9,17,25,33,41,49,57
    defb    2,10,18,26,34,42,50,58,3,11,19,27,35,43,51,59
    defb    4,12,20,28,36,44,52,60,5,13,21,29,37,45,53,61
    defb    6,14,22,30,38,46,54,62,7,15,23,31,39,47,55,63

    defb    64,72,80,88,96,104,112,120,65,73,81,89,97,105,113,121
    defb    66,74,82,90,98,106,114,122,67,75,83,91,99,107,115,123
    defb    68,76,84,92,100,108,116,124,69,77,85,93,101,109,117,125
    defb    70,78,86,94,102,110,118,126,71,79,87,95,103,111,119,127

    defb    128,136,144,152,160,168,176,184,129,137,145,153,161,169,177,185
    defb    130,138,146,154,162,170,178,186,131,139,147,155,163,171,179,187
    defb    132,140,148,156,164,172,180,188,133,141,149,157,165,173,181,189
    defb    134,142,150,158,166,174,182,190,135,143,151,159,167,175,183,191

BLtabpow2:
    ;; lookup table with powers of 2
    defb    128,64,32,16,8,4,2,1    

END ASM
#endif    
        
END SUB


FOR n=40 to 50                                                                    
FastPlot (n,60)
next n

FOR n=40 to 50                                                                    
FastPlot (n,50)
pause 0
next n

Ideas accepted

(Not sure any are in the right places, but the non pause version had them non-contiguous, which was very weird).

[boriel]

Hi, Britlion

Sorry for the delay! (as always)
Are you using this FastPlot implementation?
https://zxbasic.readthedocs.io/en/docs/l.../fastplot/

Ok. You're using the 2nd implementation (the Wiki one does not include the lookup table?)

Update: In the original code, even with PAUSE 0, the code plotted in the wrong region of the screen for me...
Update: I've fastcalled your routine. Now it does not hang, but still the plots are a bit odd

Code:

SUB Fastcall FastPlot (x as uByte, y as uByte)

    ASM
    pop hl ; hl = RETURN ADDRESS
    ex (sp), hl ; Callee H = y
    ex de, hl   ; d = y

    ld HL,FastPlotYValue+1
    ld (hl), d

    FastPlotASMStart:
        ld      hl,BLtabpow2
        ld      (FastPlotSaveX+1),A
        and     7       ; x mod 8
        add a,l
        ld l,a
        jr nc, FastPlotSkip1
        inc h

    FastPlotSkip1:
        ld      e,(hl)     ; e contains one bit set
        ld      hl,BLTabLinIdx

    FastPlotYValue:
        ld      a,0 ; Self modding code that puts y value here
        add a,l
        ld l,a
        jr nc, FastPlotSkip2
        inc h

    FastPlotSkip2:
        ld      a,(hl)      ; table lookup
        ; OR a      <= CLEAR CARRY FLAG missing??

        rrca
        rrca
        rrca
        ld h,a
        and %11100000
        xor h
        or %01000000
        ld h,a   ;- this is faster at 35 T states to do hl=a*32

    FastPlotSaveX:
        ld      a, 255 ; X Self Modifying - this is written into to save X earlier.
        rrca
        rrca
        rrca
        and %00011111        ;x / 8 19 T states
      
        or      l
        ld      l,a         ; + x/8.

        ld      a,(hl)
        or      e           ; or = superposition mode.
        ld      (hl),a      ; set the pixel.

    END ASM
    return

    #ifndef BLTabLineIdxPresent
    #define BLTabLineIdxPresent
    ASM
        ;; small screen lines lookup table

    BLTabLinIdx:
        defb    0,8,16,24,32,40,48,56,1,9,17,25,33,41,49,57
        defb    2,10,18,26,34,42,50,58,3,11,19,27,35,43,51,59
        defb    4,12,20,28,36,44,52,60,5,13,21,29,37,45,53,61
        defb    6,14,22,30,38,46,54,62,7,15,23,31,39,47,55,63

        defb    64,72,80,88,96,104,112,120,65,73,81,89,97,105,113,121
        defb    66,74,82,90,98,106,114,122,67,75,83,91,99,107,115,123
        defb    68,76,84,92,100,108,116,124,69,77,85,93,101,109,117,125
        defb    70,78,86,94,102,110,118,126,71,79,87,95,103,111,119,127

        defb    128,136,144,152,160,168,176,184,129,137,145,153,161,169,177,185
        defb    130,138,146,154,162,170,178,186,131,139,147,155,163,171,179,187
        defb    132,140,148,156,164,172,180,188,133,141,149,157,165,173,181,189
        defb    134,142,150,158,166,174,182,190,135,143,151,159,167,175,183,191

    BLtabpow2:
        ;; lookup table with powers of 2
        defb    128,64,32,16,8,4,2,1  

    END ASM
    #endif  
          
END SUB

FOR n=40 to 50                                                                  
FastPlot (n,60)
next n

FOR n=40 to 50                                                                  
FastPlot (n,50)
pause 0
next n

[boriel]

More updates: I've located a possible bug (signaled in the code above):

• Did you forgot to clear the Carry Flag (with an OR a) before doing RRCA (see the mark above, after the label FastPlotSkip2.
  
• I think you also forgot to subtract the y value from 191 as in the original plot routines, so the Y position is reversed or eve n out of the screen.
  

Can you check that? 

[britlion]

I can.

I rarely can get here, since the site refuses to allow me to access it with my VPN running, which is a pain.

Thanks for looking - I'll go dig on that.

[britlion]

Ok. You're using the 2nd implementation (the Wiki one does not include the lookup table?)

The wiki should probably link. It does say "HRPrintFast also uses the same table - it's important to only include it once" - however, the screen tables are enormous in full - so even HRPrintFast links to the saved file in this forum... - I couldn't see a way to attach a file to the wiki.