Routines
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B980: Find Horizontal Move
Used by the routines at CheckTerrainCollision, StartTerrainCheck and MoveObject.
Works out the horizontal position of an object after it has moved by its horizontal speed, which is wrapped around to the next map column, or the previous one, when it goes off the edge of the column. There are 60 columns.
Input
IX Address of the object
Output
H Map column
L Horizontal position
E Fraction of the horizontal position
FindHorizontalMove B980 LD H,(IX+$02) The horizontal position.
B983 LD L,(IX+$03) Read offset 03 of IX into L.
B986 LD D,(IX+$07) The high byte of the horizontal speed.
B989 LD E,(IX+$08) The low byte of the horizontal speed.
B98C ADD HL,DE HL gains DE.
B98D LD E,L E=L.
B98E LD L,H L=H.
B98F LD H,(IX+$01) The map column.
B992 LD C,(IX+$02) The horizontal position.
B995 LD A,L A=L.
B996 BIT 7,D Jump to FindHorizontalMove_Left if bit 7 of D is set.
B998 JR NZ,FindHorizontalMove_Left
FindHorizontalMove_Right B99A CP $E8 Jump to FindHorizontalMove_Next if A is E8 or more.
B99C JR NC,FindHorizontalMove_Next
B99E CP C Return if A is C or more.
B99F RET NC
FindHorizontalMove_Next B9A0 SUB $E0 Subtract E0 from A.
B9A2 LD L,A L=A.
B9A3 INC H Advance H.
B9A4 LD A,H A=H.
B9A5 SUB $60 Subtract 60 from A.
B9A7 RET NZ Return if the zero flag is clear.
B9A8 LD H,A H=A.
B9A9 RET Return.
FindHorizontalMove_Left B9AA CP $08 Jump to FindHorizontalMove_Previous if A is less than 08.
B9AC JR C,FindHorizontalMove_Previous
B9AE CP C Return if A is less than C.
B9AF RET C
B9B0 RET Z Return if the zero flag is set.
FindHorizontalMove_Previous B9B1 ADD A,$E0 Add E0 to A.
B9B3 LD L,A L=A.
B9B4 DEC H Step H back.
B9B5 LD A,H A=H.
B9B6 INC A Advance A.
B9B7 RET NZ Return if the zero flag is clear.
B9B8 LD H,$5F H=5F.
B9BA RET Return.
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