Routines
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D135: Calculate Screen Address
Used by the routines at UpdateLightYears, DisplayObjectsCarried, CopyAreaToScreen and DrawChangedObjects.
Finds the address on the screen of the byte that holds a pixel. The pixel y position is from the bottom of the screen.
Input
B The pixel y position
C The pixel x position
Output
DE The address on the screen
Work out the byte from the position.
CalculateScreenAddress D135 LD A,C Divide C by eight.
D136 RRCA
D137 RRCA
D138 RRCA
D139 AND %00011111 Keep bits 0 to 4.
D13B LD E,A Copy it to E.
D13C LD A,B Fetch B and invert it.
D13D CPL
D13E AND %00000111 Keep bits 0 to 2.
D140 EX AF,AF' Copy it to A'.
D141 LD A,B Fetch B and invert it.
D142 CPL
D143 RLCA Move the character row into bits 5 to 7.
D144 RLCA
D145 AND %11100000 Keep bits 5 to 7.
D147 OR E Merge E.
D148 LD E,A Copy it to E.
D149 LD A,B Fetch B and invert it.
D14A CPL
D14B RRCA Move the third into bits 3 and 4.
D14C RRCA
D14D RRCA
D14E AND %00011000 Keep bits 3 and 4.
D150 LD D,A Copy it to D.
D151 EX AF,AF' Fetch A'.
D152 OR D Merge D.
D153 ADD A,$38 Add 38.
D155 LD D,A Copy it to D.
D156 RET Return.
View the equivalent code in;
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