Routines
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8A4E: Calculate screen address from pixel coordinates.
Converts X,Y pixel coordinates to a ZX Spectrum screen memory address.
Input
B Vertical coordinate of the object in pixels.
C Horizontal coordinate of the object in pixels.
Output
B Horizontal coordinate of the object in pixels relative to its tile.
HL Screen address corresponding to the coordinates.
calculate_pixel_address 8A4E LD DE,$0800
8A51 LD HL,$3800
8A54 LD A,B Load the number of the third of the screen corresponding to the vertical coordinate of the object into A.
8A55 RLCA
8A56 RLCA
8A57 AND $03
8A59 INC A
calc_pixel_screen_third_loop 8A5A ADD HL,DE Load the starting address of the third of the screen into HL.
8A5B DEC A
8A5C JP NZ,calc_pixel_screen_third_loop
8A5F LD A,B Leave only the 6 lowest bits in B which define the coordinate of the object relative to its third of the screen.
8A60 AND $3F
8A62 LD B,A
8A63 AND $38 Unset the 3 lowest bits, so now A contains the coordinate of starting tile relative to its third of the screen.
8A65 PUSH HL
8A66 LD H,$00 Multiply the value of A by 4 and put into DE which makes the offset of the starting tile address from its third of the screen.
8A68 LD L,A
8A69 ADD HL,HL
8A6A ADD HL,HL
8A6B EX DE,HL
8A6C POP HL Now HL contains the screen address of the tile.
8A6D ADD HL,DE
8A6E LD A,B Leave only the 3 lowest bits in B which define the coordinate of the object relative to it tile.
8A6F AND $07
8A71 LD B,A
8A72 INC B Prepare for row loop.
8A73 DEC H
calc_pixel_row_loop 8A74 INC H Increment H (row within tile) B times.
8A75 DJNZ calc_pixel_row_loop
8A77 LD A,C
8A78 SRL A Add column offset and extract bit position.
8A7A SRL A
8A7C SRL A
8A7E LD D,C
8A7F LD C,A
8A80 ADD HL,BC
8A81 LD A,D
8A82 AND $07
8A84 LD B,A
8A85 RET
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