Routines
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B8AD: Calculate Screen Address
Used by the routines at HallOfFame, MenuWriteText, PrintStringColour, PrepDraw23, PrepDraw2 and DrawRoom.
Converts pixel coordinates into the address of the byte in the display file that holds the pixel.
Input
H Y coordinate
L X coordinate
Output
HL Screen address
The low byte is made up of the column (bits 0-4) and the character row within the third of the screen (bits 5-7).
ScreenAddress B8AD LD A,L A=L.
B8AE RRCA Divide the X coordinate by 08 to get the column.
B8AF RRCA
B8B0 RRCA
B8B1 AND %00011111 Keep only bits 0-4.
B8B3 LD L,A L=A.
B8B4 LD A,H A=H.
B8B5 RLCA Multiply the Y coordinate by 04 which moves bits 3-5 into bits 5-7.
B8B6 RLCA
B8B7 AND %11100000 Keep only bits 5-7.
B8B9 OR L Merge in the column.
B8BA LD L,A L=A.
The high byte is made up of the third of the screen (bits 3-4) and the pixel line within the character (bits 0-2).
B8BB LD A,H A=H.
B8BC AND %00000111 Keep only bits 0-2 (the pixel line).
B8BE EX AF,AF' Store the pixel line in AF'.
B8BF LD A,H A=H.
B8C0 RRCA Divide the Y coordinate by 08.
B8C1 RRCA
B8C2 RRCA
B8C3 AND %00011000 Keep only bits 3-4 (the third of the screen).
B8C5 OR %01000000 Set bit 6 to point to the display file.
B8C7 LD H,A H=A.
B8C8 EX AF,AF' Fetch the pixel line from AF'.
B8C9 OR H Merge in the third of the screen.
B8CA LD H,A H=A.
B8CB RET Return.
View the equivalent code in;
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