Routines
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D990: Adjust Edge Address
Used by the routine at DrawVerticalEdge.
Moves the screen address in HL one cell to the left when the horizontal speed is negative. The last four instructions, which would move it back and count a step less, cannot be reached because the speed is known to be negative.
Input
C Number of steps left
HL Screen address
Output
C Number of steps left
HL Screen address
AdjustEdgeAddress D990 LD A,($CCBB) The horizontal scroll speed.
D993 BIT 7,A Return if bit 7 of A is clear.
D995 RET Z
D996 DEC L Step L back.
D997 LD A,($CE44) The segment row.
D99A CP $5F Return if A is 5F or more.
D99C RET NC
D99D LD A,($CCBB) The horizontal scroll speed.
D9A0 NEG Negate A.
D9A2 BIT 7,A Return if bit 7 of A is clear.
D9A4 RET Z
D9A5 INC L Advance L.
D9A6 DEC C Step C back.
D9A7 RET Return.
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