Routines
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D826: Calculate Screen Address
Used by the routines at DisplayObjectsCarried, CopyAreaToScreen and DrawChangedObjects.
Finds the address on the screen of the byte that holds a pixel.
Input
B Pixel Y coordinate (from the bottom of the screen)
C Pixel X coordinate
Output
DE Address on the screen
Build the display-file address from the pixel X and Y.
CalculateScreenAddress D826 LD A,C Fetch the pixel X coordinate and move it right by three bits.
D827 RRCA
D828 RRCA
D829 RRCA
D82A AND %00011111 Keep the byte of the line.
D82C LD E,A Stash it in E.
D82D LD A,B Fetch the pixel Y coordinate, inverted to get the line from the top.
D82E CPL
D82F AND %00000111 Keep the line within the character row.
D831 EX AF,AF' Stash it in the alternate A.
D832 LD A,B Invert the pixel Y coordinate again and move it left by two bits.
D833 CPL
D834 RLCA
D835 RLCA
D836 AND %11100000 Keep the character row within the third, in the high bits.
D838 OR E Add the byte of the line.
D839 LD E,A Store the low byte of the address in E.
D83A LD A,B Invert the pixel Y coordinate again.
D83B CPL
D83C RRCA Move the third of the screen into bits 3 and 4.
D83D RRCA
D83E RRCA
D83F AND %00011000 Keep it.
D841 LD D,A Stash it in D.
D842 EX AF,AF' Fetch the line within the character row.
D843 OR D Add the third of the screen.
D844 ADD A,$38 Add 38, to make the high byte of the address.
D846 LD D,A Store the high byte of the address in D.
D847 RET Return.
View the equivalent code in;
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