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⌂ ∞ ▽ Elite on the BBC Micro and NES

Save and load: DrawSmallLogo

[NES version, Bank 4]

       Name: DrawSmallLogo                                           [Show more]
       Type: Subroutine
   Category: Save and load
    Summary: Set the sprite buffer entries for the small Elite logo on the Save
             and Load screen

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * DrawSmallLogo_b4 calls DrawSmallLogo




 Arguments:

   CNT                  Offset of the first free sprite block within the sprite
                        buffer, which we can use for drawing the logo



.DrawSmallLogo

 LDA #1                 ; Set XC = 1 so we draw the top-left corner of the logo
 STA XC                 ; at text column 1 (plus the pixel value in X that we
                        ; set below)

 ASL A                  ; Set YC = 1 so we draw the top-left corner of the logo
 STA YC                 ; on text row 2 (plus the pixel value in Y that we
                        ; set below)

 LDX #8                 ; Set K = 8 so we draw 8 tiles in each row
 STX K

 STX K+1                ; Set K+1 = 8 so we draw 8 rows in total

 LDX #6                 ; Set X = 6 so we draw the logo at a point 6 pixels
                        ; into text column 1 (i.e. on the sixth pixel along the
                        ; x-axis in the character block in column 1)

 LDY #6                 ; Set Y = 6 so we draw the logo at a point 6 pixels
                        ; into text row 2 (i.e. on the sixth pixel down the
                        ; y-axis in the character block in row 2)

 LDA #67                ; Set K+2 = 67 to use as the pattern number of the first
 STA K+2                ; pattern for the small logo

 LDA CNT                ; Set K+3 = CNT / 4, which we use below when rounding
 LSR A                  ; down the sprite buffer offset to a multiple of four
 LSR A
 STA K+3

 LDA #HI(smallLogoTile) ; Set V(1 0) = smallLogoTile so we draw the small
 STA V+1                ; Elite logo in the following
 LDA #LO(smallLogoTile)
 STA V

 LDA #%00000001         ; Set S to use as the attribute for each of the sprites
 STA S                  ; in the logo, so each sprite is set as follows:
                        ;
                        ;   * Bits 0-1    = sprite palette 1
                        ;   * Bit 5 clear = show in front of background
                        ;   * Bit 6 clear = do not flip horizontally
                        ;   * Bit 7 clear = do not flip vertically

 LDA XC                 ; Set SC = XC * 8 + X
 ASL A                  ;        = XC * 8 + 6
 ASL A                  ;
 ASL A                  ; So SC is the pixel x-coordinate of the top-left corner
 ADC #0                 ; of the logo we want to draw, as each text character in
 STA SC                 ; XC is 8 pixels wide and X contains the x-coordinate
 TXA                    ; within the character block
 ADC SC
 STA SC

 LDA YC                 ; Set SC+1 = YC * 8 + 6 + Y
 ASL A                  ;          = YC * 8 + 6 + 6
 ASL A                  ;
 ASL A                  ; So SC+1 is the pixel y-coordinate of the top-left
 ADC #6+YPAL            ; corner of the logo we want to draw, as each text row
 STA SC+1               ; in YC is 8 pixels high and Y contains the y-coordinate
 TYA                    ; within the character block
 ADC SC+1
 STA SC+1

 LDA K+3                ; Set X = K+3 * 4
 ASL A                  ;       = CNT / 4 * 4
 ASL A                  ;
 TAX                    ; So X contains the offset of the sprite's four-byte
                        ; block in the sprite buffer, as each sprite consists
                        ; of four bytes, so this is now the offset within the
                        ; sprite buffer of the first sprite we can use

 LDA K+1                ; Set T = K+1 to use as a counter for each row in the
 STA T                  ; logo

 LDY #0                 ; Set a tile counter in Y to increment as we draw each
                        ; tile, starting with Y = 0 for the first tile at the
                        ; start of the first row, and counting across and down

.drsm1

 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure
                        ; the PPU to use nametable 0 and pattern table 0

 LDA SC                 ; Set SC2 to the pixel x-coordinate for the start of
 STA SC2                ; each row, so we can use it to move along the row as we
                        ; draw the logo

 LDA K                  ; Set ZZ to the number of tiles in each row of the logo
 STA ZZ                 ; (in K), so we can use it as a counter as we move along
                        ; the row

.drsm2

 LDA (V),Y              ; Fetch the pattern number for the Y-th tile in the logo
                        ; from the smallLogoTile table at V(1 0), which gives us
                        ; the pattern number for this tile from the patterns
                        ; in the smallLogoImage table, which is loaded at the
                        ; pattern number in K+2

 INY                    ; Increment the tile counter to point to the next tile

 BNE drsm3              ; If we just incremented Y past a page boundary and back
 INC V+1                ; to zero, increment the high byte of V(1 0) to point to
                        ; the next page

.drsm3

 CMP #0                 ; If the pattern number is zero, then this is the
 BEQ drsm4              ; background, so jump to drsm4 to move on to the next
                        ; tile in the logo

 ADC K+2                ; Set the pattern for sprite X to A + K+2, which is the
 STA pattSprite0,X      ; pattern number in the PPU's pattern table to use for
                        ; this part of the logo

 LDA S                  ; Set the attributes for sprite X to S, which we set
 STA attrSprite0,X      ; above as follows:
                        ;
                        ;   * Bits 0-1    = sprite palette 1
                        ;   * Bit 5 clear = show in front of background
                        ;   * Bit 6 clear = do not flip horizontally
                        ;   * Bit 7 clear = do not flip vertically

 LDA SC2                ; Set the x-coordinate for sprite X to SC2
 STA xSprite0,X

 LDA SC+1               ; Set the y-coordinate for sprite X to SC+1
 STA ySprite0,X

 TXA                    ; Add 4 to the sprite number in X, to move on to the
 CLC                    ; next sprite in the sprite buffer (as each sprite
 ADC #4                 ; consists of four bytes of data)

 BCS drsm5              ; If the addition overflowed, then we have reached the
                        ; end of the sprite buffer, so jump to drsm5 to return
                        ; from the subroutine, as we have run out of sprites

 TAX                    ; Otherwise set X to the offset of the next sprite in
                        ; the sprite buffer

.drsm4

 LDA SC2                ; Set SC2 = SC2 + 8
 CLC                    ;
 ADC #8                 ; So SC2 contains the x-coordinate of the next tile
 STA SC2                ; along the row

 DEC ZZ                 ; Decrement the tile counter in ZZ as we have just drawn
                        ; a tile

 BNE drsm2              ; If ZZ is non-zero then we still have more tiles to
                        ; draw on the current row, so jump back to drsm2 to draw
                        ; the next one

 LDA SC+1               ; Otherwise we have reached the end of this row, so add
 ADC #8                 ; 8 to SC+1 to move the y-coordinate down to the next
 STA SC+1               ; tile row (as each tile row is 8 pixels high)

 DEC T                  ; Decrement the number of rows in T as we just finished
                        ; drawing a row

 BNE drsm1              ; Loop back to drsm1 until we have drawn all the rows in
                        ; the image

.drsm5

 RTS                    ; Return from the subroutine