Routines
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B394: Calculate Screen Address
Used by the routines at CopyScoreToScreen, DrawChangedObjects and DisplayObjectsCarried.
Finds the address in the display file of the byte that holds a pixel. The pixel y position counts up from the bottom of the screen, so the first step is to invert it. A y position of C0 or more is above the top of the screen, and gives an address below 4000.
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 B394 LD A,C Divide C by eight.
B395 RRCA
B396 RRCA
B397 RRCA
B398 AND %00011111 Keep only the five bits of the column.
B39A LD E,A Set E to the column.
Find the line within the character square.
B39B LD A,B Invert the y co-ordinate, to count down from the top of the screen.
B39C CPL
B39D AND %00000111 Keep the three bits of the line within the character square.
B39F EX AF,AF' Copy it to A'.
Find the character row within the third of the screen, and merge it with the column to make the low byte of the address.
B3A0 LD A,B Fetch B and invert it.
B3A1 CPL
B3A2 RLCA Move the character row into bits 5 to 7.
B3A3 RLCA
B3A4 AND %11100000 Keep bits 5 to 7.
B3A6 OR E Combine them with the column in E.
B3A7 LD E,A Copy it to E.
Find the third of the screen and merge it with the line to make the high byte of the address.
B3A8 LD A,B Fetch B and invert it.
B3A9 CPL
B3AA RRCA Move the third into bits 3 and 4.
B3AB RRCA
B3AC RRCA
B3AD AND %00011000 Keep bits 3 and 4.
B3AF LD D,A Copy it to D.
B3B0 EX AF,AF' Fetch the line within the character square.
B3B1 OR D Merge D.
B3B2 ADD A,$38 Add 38, which corrects for the inverted y co-ordinate and adds the display file base of 40.
B3B4 LD D,A Copy it to D.
B3B5 RET Return.
View the equivalent code in;
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