Routines
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49490: Scale Scroll Speed
Used by the routine at UpdateStars.
Makes a scroll speed smaller for the stars of a layer further away, by shifting it right once for the second layer and twice for the third. For the third layer, on odd frames, a speed of 254 is first changed to zero and a speed of 2 is changed to 4.
Input
B Layer (one for the nearest)
H Scroll speed
Output
H Scaled scroll speed
ScaleScrollSpeed 49490 LD A,B A=B.
49491 CP 1 Return if A is 1.
49493 RET Z
49494 CP 2 Jump to ScaleScrollSpeed_Third if A is 2.
49496 JR Z,ScaleScrollSpeed_Third
49498 LD A,H A=H.
49499 CP 254 Jump to ScaleScrollSpeed_Third if A is not 254.
49501 JR NZ,ScaleScrollSpeed_Third
49503 LD A,(39524) The frame counter.
49506 AND %00000001 Keep the bits of A that are set in 1.
49508 JR Z,ScaleScrollSpeed_Third Jump to ScaleScrollSpeed_Third if the zero flag is set.
49510 LD H,0 Clear H.
49512 RET Return.
ScaleScrollSpeed_Third 49513 LD A,B A=B.
49514 CP 3 Jump to ScaleScrollSpeed_Shift if A is not 3.
49516 JR NZ,ScaleScrollSpeed_Shift
49518 LD A,H A=H.
49519 CP 2 Jump to ScaleScrollSpeed_Shift if A is not 2.
49521 JR NZ,ScaleScrollSpeed_Shift
49523 LD A,(39524) The frame counter.
49526 AND %00000001 Keep the bits of A that are set in 1.
49528 JR Z,ScaleScrollSpeed_Shift Jump to ScaleScrollSpeed_Shift if the zero flag is set.
49530 LD H,4 H=4.
ScaleScrollSpeed_Shift 49532 LD A,B A=B.
49533 DEC A Step A back.
ScaleScrollSpeed_Loop 49534 SRA H Shift H right keeping the sign.
49536 DEC A Step A back.
49537 JR NZ,ScaleScrollSpeed_Loop Jump to ScaleScrollSpeed_Loop if the zero flag is clear.
49539 RET Return.
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