Routines
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C152: 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 FE is first changed to zero and a speed of 02 is changed to 04.
Input
B Layer (one for the nearest)
H Scroll speed
Output
H Scaled scroll speed
ScaleScrollSpeed C152 LD A,B A=B.
C153 CP $01 Return if A is 01.
C155 RET Z
C156 CP $02 Jump to ScaleScrollSpeed_Third if A is 02.
C158 JR Z,ScaleScrollSpeed_Third
C15A LD A,H A=H.
C15B CP $FE Jump to ScaleScrollSpeed_Third if A is not FE.
C15D JR NZ,ScaleScrollSpeed_Third
C15F LD A,($9A64) The frame counter.
C162 AND %00000001 Keep the bits of A that are set in 01.
C164 JR Z,ScaleScrollSpeed_Third Jump to ScaleScrollSpeed_Third if the zero flag is set.
C166 LD H,$00 Clear H.
C168 RET Return.
ScaleScrollSpeed_Third C169 LD A,B A=B.
C16A CP $03 Jump to ScaleScrollSpeed_Shift if A is not 03.
C16C JR NZ,ScaleScrollSpeed_Shift
C16E LD A,H A=H.
C16F CP $02 Jump to ScaleScrollSpeed_Shift if A is not 02.
C171 JR NZ,ScaleScrollSpeed_Shift
C173 LD A,($9A64) The frame counter.
C176 AND %00000001 Keep the bits of A that are set in 01.
C178 JR Z,ScaleScrollSpeed_Shift Jump to ScaleScrollSpeed_Shift if the zero flag is set.
C17A LD H,$04 H=04.
ScaleScrollSpeed_Shift C17C LD A,B A=B.
C17D DEC A Step A back.
ScaleScrollSpeed_Loop C17E SRA H Shift H right keeping the sign.
C180 DEC A Step A back.
C181 JR NZ,ScaleScrollSpeed_Loop Jump to ScaleScrollSpeed_Loop if the zero flag is clear.
C183 RET Return.
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