Routines
Prev: $3047 Up: Map Next: $3113
$3072: Shift the play area window left
Used by the routine at $35EF.
$3072 SEI Disable interrupts.
$3073 LDA $58 Pick up the x-coordinate of the leftmost column of the skool on screen from $58.
$3075 CMP #$08 Is it 8 or more?
$3077 BCS $307C Branch if so.
$3079 JMP $3111 Otherwise return now.
$307C LDA #$08 Set the destination colour information index at $55 to 8.
$307E STA $55
$3080 LDA #$00 Set the source colour information index at $54 to 0.
$3082 STA $54
$3084 LDA #$F8 Store $E0F8 at $5A. This screen bitmap address corresponds to the 32nd tile from the left in the top row of the screen.
$3086 STA $5A
$3088 LDA #$E0
$308A STA $5B
$308C LDA #$1F Store $CC1F at $5C. This colour information address corresponds to the 32nd tile from the left in the top row of the screen.
$308E STA $5C
$3090 LDA #$CC
$3092 STA $5D
$3094 LDX #$20 Initialise the column counter in X.
$3096 DEX Decrement the column counter in X.
$3097 TXA Save the column counter on the stack temporarily.
$3098 PHA
$3099 LDA $5A Copy the screen bitmap address from $5A to $4E. This will be the source address.
$309B STA $4E
$309D LDA $5B
$309F STA $4F
$30A1 LDA $5C Copy the colour information address from $5C to $50. This will be the source address.
$30A3 STA $50
$30A5 LDA $5D
$30A7 STA $51
$30A9 LDA $4E Copy the screen bitmap address from $4E to $52.
$30AB STA $52
$30AD LDA $4F
$30AF STA $53
$30B1 LDA $52 Add 64 to the screen bitmap address at $52. This corresponds to the tile that is 8 cells to the right in the same row, and will be the destination address.
$30B3 CLC
$30B4 ADC #$40
$30B6 BCC $30BA
$30B8 INC $53
$30BA STA $52
$30BC JSR $2FB6 Move a column of screen tiles right.
$30BF LDA $5A Subtract 8 from the screen bitmap address at $5A. This corresponds to the next tile to the left in the top row of the screen.
$30C1 CLC
$30C2 ADC #$F8
$30C4 CMP #$F8
$30C6 BNE $30CA
$30C8 DEC $5B
$30CA STA $5A
$30CC DEC $5C Subtract 1 from the colour information address at $5C. This corresponds to the next tile to the left in the top row of the screen.
$30CE PLA Restore the column counter to X.
$30CF TAX
$30D0 BNE $3096 Branch back to deal with the remaining columns.
The 32 leftmost columns of the skool on screen have been shifted right. Time to draw the leftmost 8 columns of the skool.
$30D2 LDA #$38 Store $E038 at $5A. This screen bitmap address corresponds to the 8th tile from the left in the top row of the screen.
$30D4 STA $5A
$30D6 LDA #$E0
$30D8 STA $5B
$30DA LDA #$07 Update the colour information address at $5C to $CC07. This corresponds to the 8th tile from the left in the top row of the screen.
$30DC STA $5C
$30DE JSR $9868 Move sprites #1-#6 right by 64 pixels.
$30E1 NOP
$30E2 LDX #$08 There are 8 new play area columns to draw in the gap left by the shifting window.
$30E4 TXA Save this column counter temporarily.
$30E5 PHA
$30E6 DEC $58 Decrement the x-coordinates of the leftmost column of the skool on screen and off.
$30E8 DEC $59
$30EA LDY $58 Pick up the x-coordinate of the leftmost column of the skool on screen.
$30EC LDA $5A Save the screen bitmap address on the stack.
$30EE PHA
$30EF LDA $5B
$30F1 PHA
$30F2 LDA $5C Save the colour information address on the stack.
$30F4 PHA
$30F5 LDA $5D
$30F7 PHA
$30F8 JSR $2F8C Print the play area tiles in a single column of the screen.
$30FB PLA Restore the saved colour information address to $5C.
$30FC STA $5D
$30FE PLA
$30FF STA $5C
$3101 PLA Restore the saved screen bitmap address to $5A and subtract 8 from it. This points at the next tile to the left in the top row of the screen.
$3102 STA $5B
$3104 PLA
$3105 SEC
$3106 SBC #$08
$3108 STA $5A
$310A DEC $5C Subtract 1 from the colour information address at $5C. This points at the next tile to the left in the top row of the screen.
$310C PLA Restore the column counter to X.
$310D TAX
$310E DEX Decrement the column counter.
$310F BNE $30E4 Branch back to deal with the remaining columns.
$3111 CLI Re-enable interrupts.
$3112 RTS
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