Routines
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30435: Calculate Screen Address
Used by the routines at PrintTileBlock, MenuWriteText, DisplayPlayerLives, PrintStringColour, PrepareOldSprite and PrepareActorSprite.
Converts the pixel co-ordinates to the address in the screen buffer of the byte which contains that pixel.
Input
HL Pixel co-ordinates (horizontal in L, vertical in H)
Output
HL Screen address
ScreenAddress 30435 LD A,L Fetch the horizontal co-ordinate.
30436 RRCA Divide by 8 (to get the column).
30437 RRCA
30438 RRCA
30439 AND %00011111 Keep only bits 0-4.
30441 LD L,A Store the column in L.
30442 LD A,H Fetch the vertical co-ordinate.
30443 RLCA A=A * 4.
30444 RLCA
30445 AND %11100000 Keep only bits 5-7.
30447 OR L Combine with the column in L.
30448 LD L,A Store the result in L.
30449 LD A,H Fetch the vertical co-ordinate.
30450 AND %00000111 Keep only bits 0-2 (the pixel row within the character cell).
30452 EX AF,AF' Switch to the shadow AF register to hold the pixel row.
30453 LD A,H Fetch the vertical co-ordinate.
30454 RRCA Divide by 8 (to get the third of the screen).
30455 RRCA
30456 RRCA
30457 AND %00011000 Keep only bits 3-4.
30459 OR %01000000 Set bit 6.
30461 LD H,A Store the result in H.
30462 EX AF,AF' Switch to the shadow AF register to fetch the pixel row.
30463 OR H Combine with H.
30464 LD H,A Store the result in H.
30465 RET Return.
View the equivalent code in;
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