Routines
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E556: Calculate Screen Address
Used by the routines at PickUpItem and PrintString.
Works out the address on the screen of a position given in pixels, measured from the bottom left of the screen.
Input
H Y position (from the bottom of the screen)
L X position
Output
HL Screen address
CalculateScreenAddress E556 PUSH DE Keep DE, and put the position in it.
E557 EX DE,HL
E558 LD A,E Work out the column from the X position.
E559 RRCA
E55A RRCA
E55B RRCA
E55C AND $1F
E55E LD L,A
E55F LD A,D Work out the character row within a third of the screen, from the Y position measured from the top.
E560 CPL
E561 RLCA
E562 RLCA
E563 AND $E0
E565 OR L
E566 LD L,A
E567 LD A,D Work out the third of the screen.
E568 CPL
E569 RRCA
E56A RRCA
E56B RRCA
E56C AND $18
E56E LD H,A
E56F LD A,D Work out the pixel row within the character, and make the high byte of the address.
E570 CPL
E571 AND $07
E573 OR H
E574 ADD A,$38
E576 LD H,A
E577 POP DE Get DE back and return.
E578 RET
View the equivalent code in;
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