Routines
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47277: 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 47277 LD A,L A=L.
47278 RRCA Divide the X coordinate by 8 to get the column.
47279 RRCA
47280 RRCA
47281 AND %00011111 Keep only bits 0-4.
47283 LD L,A L=A.
47284 LD A,H A=H.
47285 RLCA Multiply the Y coordinate by 4 which moves bits 3-5 into bits 5-7.
47286 RLCA
47287 AND %11100000 Keep only bits 5-7.
47289 OR L Merge in the column.
47290 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).
47291 LD A,H A=H.
47292 AND %00000111 Keep only bits 0-2 (the pixel line).
47294 EX AF,AF' Store the pixel line in AF'.
47295 LD A,H A=H.
47296 RRCA Divide the Y coordinate by 8.
47297 RRCA
47298 RRCA
47299 AND %00011000 Keep only bits 3-4 (the third of the screen).
47301 OR %01000000 Set bit 6 to point to the display file.
47303 LD H,A H=A.
47304 EX AF,AF' Fetch the pixel line from AF'.
47305 OR H Merge in the third of the screen.
47306 LD H,A H=A.
47307 RET Return.
View the equivalent code in;
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