Routines
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6ECB: Screen Address One Pixel Above
Used by the routines at HandlerTerrainDots, DrawPlayer, MaskSprite and DrawTwoBytes.
Input
HL Current position
Output
HL Address for new position
Calculates the new address for writing a sprite pixel, in an upward direction, taking into consideration the screen memory layout.
ScreenPos1PixelAbove 6ECB DEC H Decrement H to move up one pixel on screen.
6ECC LD A,H Store the inverted result in A.
6ECD CPL
6ECE AND %00000111 Keep only bits 0-2.
6ED0 RET NZ If a character line has not been crossed then return.
6ED1 LD A,L Else subtract 20 from L.
6ED2 SUB $20
6ED4 LD L,A
6ED5 CPL Invert the bits in A.
6ED6 AND %11100000 Keep only bits 5-7.
6ED8 RET Z If a new section of the screen has not been crossed then return.
6ED9 LD A,H Else add 08 to H and return.
6EDA ADD A,$08
6EDC LD H,A
6EDD RET
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