Routines
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30106: Calculate Screen Address
Used by the routines at MenuWriteText, UpdateFlower, DisplayPlayerLives, PrintStringColour and SetUpPreviousSprite.
Converts a pixel position into an address in the screen buffer.
Input
H Y position (pixel row)
L X position (pixel column)
Output
HL Screen buffer address
ScreenAddress 30106 LD A,L A=the X position.
30107 RRCA Divide by 8 (to give the character column).
30108 RRCA
30109 RRCA
30110 AND %00011111 Keep only bits 0-4.
30112 LD L,A Store the column in L.
30113 LD A,H A=the Y position.
30114 RLCA Multiply by 4 so that bits 3-5 of the Y position are in bits 5-7.
30115 RLCA
30116 AND %11100000 Keep only bits 5-7 (the character row within the third).
30118 OR L Combine with the column.
30119 LD L,A Store the result in L.
30120 LD A,H A=the Y position.
30121 AND %00000111 Keep only bits 0-2 (the pixel row within the character cell).
30123 EX AF,AF' Switch to the shadow AF register to keep the result.
30124 LD A,H A=the Y position.
30125 RRCA Divide by 8.
30126 RRCA
30127 RRCA
30128 AND %00011000 Keep only bits 3-4 (the third of the screen).
30130 OR %01000000 Set bit 6 to point at the screen buffer.
30132 LD H,A Store the result in H.
30133 EX AF,AF' Switch back to the saved pixel row.
30134 OR H Combine with the third of the screen.
30135 LD H,A Store the result in H.
30136 RET Return.
View the equivalent code in;
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