[stella] 6 Digit Routine Cont.

["Chris Larkin" <clarkin@xxxxxxxxxxxxx>]

The Thrust source contains a plethera of great 6 digit routines... However the concept of how these routines work still eludes me.  It's only a few lines of code and I just don't get it!  Ugh.

 

The 6502 is by no means my forte'... I still kind of just fumble my way through...  Can someone help spell this out for me?

 

To begin I have 6 seperate 8x8 graphics, is there a maximum height for these graphics? I would assume the entire graphic data has to fit on one page to keep cycle counts accurate?  I have questions in code.

 

*****************FROM THRUST SOURCE, ShowText SUB************************

ShowText SUBROUTINE
    ldy     MsgDelayTab-1,x
    beq     .skipWait
.loopWait:
    sta     WSYNC
    dey                         ; 2
    bne     .loopWait           ; 2³

 

^ Simple Enough

.skipWait:
    lda     Planet
    and     #~PLANETMASK
    bne     .NTSCMsgCol
    lda     MsgColTab_PAL,x     ; 4
    bne     .contMsgCol
    DEBUG_BRK
.NTSCMsgCol:
    lda     MsgColTab,x         ; 4
.contMsgCol:

 

^ How can this always produce the same amount of cycles?

^ Why arent all the instruction cycles counted in comments?

^ This code is obviously not needed for me, however I'm not sure how

^many other cycles exactly I have to replace it with.

    sta     COLUP0              ; 3
    sta     COLUP1              ; 3 = 10 <-- When I count, I get much more than 10?

    ldy     MsgTab,x            ; 4

^??????? Totally lost here |Load height of graphic?

 

    lda     LetterPtrTab,y      ; 4
    sta     digitPtr            ; 3
^LetterPtrTab,Y = Address of first 8x8 Graphic?

 

    lda     LetterPtrTab+1,y    ; 4

^Here is where I get lost? I don't understand how this gets compiled
^If LetterPtrTab = $1200 and Y is $05, then this returns $1200 + 1 + 5? ($1206)???

^Shouldn't this get the last byte of the second graphic? I'm confused how it does this?

 

    sta     digitPtr+2          ; 3

^digitPtr is in Zero Page... So the graphic data is loaded into RAM...

    lda     LetterPtrTab+2,y    ; 4
    sta     digitPtr+4          ; 3
    lda     LetterPtrTab+3,y    ; 4
    sta     digitPtr+6          ; 3
    lda     LetterPtrTab+4,y    ; 4
    sta     digitPtr+8          ; 3
    lda     LetterPtrTab+5,y    ; 4
    sta     digitPtr+10         ; 3 = 46

 

 

 

    ldy     #5                  ; 2
;    ldy     MsgHeightTab-1,x
    txa                         ; 2         don't change C!
    eor     #MSG_C_2000         ; 2
    bne     .msgLoop            ; 2³
    ldy     #7                  ; 2 = 7/8
.msgLoop:

 

^ All graphics are 5 height, except Copyright which is 7...

^ These are all wasted cycles for me... in order for this to work for me however,

^ I have to get this exact cycle count?

    dey                         ; 2     61
    sty     tmpVar              ; 3     64
    lda     (digitPtr+$8),y     ; 5     69
    tax                         ; 2     71
    sta     WSYNC               ; 3 --- 76 ---
    lda     (digitPtr),y        ; 5      5
    sta     GRP0                ; 3      8
    lda     (digitPtr+$2),y     ; 5     13
    sta     GRP1                ; 3     16
    lda     (digitPtr+$4),y     ; 5     21
    sta     GRP0                ; 3     24
    lda     (digitPtr+$6),y     ; 5     29
    sta     tmpVar2             ; 3     32
    lda     (digitPtr+$a),y     ; 5     37
    ldy.w   tmpVar2             ; 4     41
    sty     GRP1                ; 3     44
    stx     GRP0                ; 3     47
    sta     GRP1                ; 3     50
    sta     GRP0                ; 3     53
    ldy     tmpVar              ; 3     56
    bne     .msgLoop            ; 2³    59

 

^This all makes sense

 

    sty     GRP0
    sty     GRP1
    sty     GRP0

^Why is GRP0 cleared twice?
    rts 

******************************************************************************

The $10,000 question...

 

So assuming my graphics data is as follows, How do I get this subroutine to draw it correctly?

   

    ORG $1200

Graphic0    .byte $00,$00,$00,$00,$00,$00,$00,$00

Graphic1    .byte $00,$00,$00,$00,$00,$00,$00,$00

Graphic2    .byte $00,$00,$00,$00,$00,$00,$00,$00

Graphic3    .byte $00,$00,$00,$00,$00,$00,$00,$00

Graphic4    .byte $00,$00,$00,$00,$00,$00,$00,$00

Graphic5    .byte $00,$00,$00,$00,$00,$00,$00,$00