Routines
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BFE3: Update Stars
Used by the routine at ScrollWorld.
Moves and draws the stars in the sky, in three layers of six stars. Only does anything when the ship is in one of the four zones at the top of the planet. The stars in the nearer layers move faster than the ones in the further layers when the screen scrolls, and are bigger. This is a best reading of the code.
UpdateStars BFE3 LD A,($CE58) The map row.
BFE6 CP $04 Return if A is 04 or more.
BFE8 RET NC
BFE9 LD BC,$0006 BC=06.
BFEC LD HL,$C184 The star offsets.
BFEF LD DE,$C18A The second star offset.
BFF2 LDIR Copy BC bytes from HL to DE.
BFF4 LD B,$03 B=03.
BFF6 LD IX,$C184 The star offsets.
UpdateStars_Loop BFFA LD HL,($CCBA) Fetch the word at VerticalSpeed into HL.
BFFD CALL ScaleScrollSpeed Call ScaleScrollSpeed.
C000 LD A,H A=H.
C001 ADD A,(IX+$00) Add the byte at offset 00 of IX to A.
C004 LD (IX+$00),A Write A to offset 00 of IX.
C007 LD H,L H=L.
C008 INC IX Advance IX.
C00A CALL ScaleScrollSpeed Call ScaleScrollSpeed.
C00D LD A,H A=H.
C00E ADD A,(IX+$00) Add the byte at offset 00 of IX to A.
C011 LD (IX+$00),A Write A to offset 00 of IX.
C014 INC IX Advance IX.
C016 DJNZ UpdateStars_Loop Decrease B and jump to UpdateStars_Loop until none remain.
UpdateStars_Draw C018 LD HL,$C0EA The star positions.
C01B LD ($C0E8),HL The address of the star's position.
C01E LD IX,$C1A3 The star layer types.
C022 LD HL,$C184 The star offsets.
C025 LD ($C0E6),HL The address of the star's offset.
C028 LD B,$03 B=03.
UpdateStars_Layer C02A PUSH BC Stash BC on the stack.
C02B CALL UpdateStarLayer Call UpdateStarLayer.
C02E POP BC Bring BC back off the stack.
C02F DJNZ UpdateStars_Layer Decrease B and jump to UpdateStars_Layer until none remain.
C031 RET Return.
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