Routines
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47488: 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 96 columns.
Input
IX Address of the object
Output
H Map column
L Horizontal position
E Fraction of the horizontal position
FindHorizontalMove 47488 LD H,(IX+2) The horizontal position.
47491 LD L,(IX+3) Read offset 3 of IX into L.
47494 LD D,(IX+7) The high byte of the horizontal speed.
47497 LD E,(IX+8) The low byte of the horizontal speed.
47500 ADD HL,DE HL gains DE.
47501 LD E,L E=L.
47502 LD L,H L=H.
47503 LD H,(IX+1) The map column.
47506 LD C,(IX+2) The horizontal position.
47509 LD A,L A=L.
47510 BIT 7,D Jump to FindHorizontalMove_Left if bit 7 of D is set.
47512 JR NZ,FindHorizontalMove_Left
FindHorizontalMove_Right 47514 CP 232 Jump to FindHorizontalMove_Next if A is 232 or more.
47516 JR NC,FindHorizontalMove_Next
47518 CP C Return if A is C or more.
47519 RET NC
FindHorizontalMove_Next 47520 SUB 224 Subtract 224 from A.
47522 LD L,A L=A.
47523 INC H Advance H.
47524 LD A,H A=H.
47525 SUB 96 Subtract 96 from A.
47527 RET NZ Return if the zero flag is clear.
47528 LD H,A H=A.
47529 RET Return.
FindHorizontalMove_Left 47530 CP 8 Jump to FindHorizontalMove_Previous if A is less than 8.
47532 JR C,FindHorizontalMove_Previous
47534 CP C Return if A is less than C.
47535 RET C
47536 RET Z Return if the zero flag is set.
FindHorizontalMove_Previous 47537 ADD A,224 Add 224 to A.
47539 LD L,A L=A.
47540 DEC H Step H back.
47541 LD A,H A=H.
47542 INC A Advance A.
47543 RET NZ Return if the zero flag is clear.
47544 LD H,95 H=95.
47546 RET Return.
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