Routines
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B234: Update Ship Speeds
Used by the routine at RunFrame.
Reads the controls, then changes the horizontal and vertical speeds of the ship by the amounts in the table at ShipAccelerationTable, depending on what is pressed and on the speed it already has, limits them, and works out the vertical scroll speed in VerticalSpeed with UpdateVerticalScrollSpeed.
Input
IX Address of the ship at ShipObject
UpdateShipSpeeds B234 CALL ReadControls Call ReadControls.
B237 XOR A Clear A.
B238 LD ($5C36),A Update the steering direction.
B23B LD DE,($B31A) Fetch the word at B31A into DE.
B23F LD A,($96AB) The crystal-delivery flag.
B242 AND A Jump to UpdateShipSpeeds_Drag if A is zero.
B243 JR Z,UpdateShipSpeeds_Drag
B245 LD A,$08 A=08.
B247 LD ($5C37),A Update the controls.
B24A LD A,$04 A=04.
B24C LD ($5C3F),A Update the vertical scroll limit.
B24F LD A,($CE58) The map row.
B252 AND A Jump to UpdateShipSpeeds_Sideways if A is not zero.
B253 JR NZ,UpdateShipSpeeds_Sideways
B255 LD ($CCBA),A Update the vertical scroll speed.
B258 RET Return.
UpdateShipSpeeds_Drag B259 LD A,(IX+$07) Read offset 07 of IX.
B25C NEG Negate A.
B25E AND A Jump to UpdateShipSpeeds_Sideways if A is zero.
B25F JR Z,UpdateShipSpeeds_Sideways
B261 LD DE,($B31E) Fetch the word at B31E into DE.
B265 BIT 7,A Jump to UpdateShipSpeeds_Sideways if bit 7 of A is set.
B267 JR NZ,UpdateShipSpeeds_Sideways
B269 LD DE,($B31C) Fetch the word at B31C into DE.
UpdateShipSpeeds_Sideways B26D LD A,($5C37) The controls.
B270 BIT 1,A Jump to UpdateShipSpeeds_Right if bit 1 of A is clear.
B272 JR Z,UpdateShipSpeeds_Right
B274 EX DE,HL Swap DE and HL.
B275 LD DE,($B318) Fetch the word at B318 into DE.
B279 LD A,$80 A=80.
B27B LD ($5C36),A Update the steering direction.
B27E LD A,H A=H.
B27F CP $FF Jump to UpdateShipSpeeds_Vertical if A is FF.
B281 JR Z,UpdateShipSpeeds_Vertical
B283 ADD HL,DE HL gains DE.
B284 EX DE,HL Swap DE and HL.
B285 JR UpdateShipSpeeds_Vertical Jump to UpdateShipSpeeds_Vertical.
UpdateShipSpeeds_Right B287 BIT 0,A Jump to UpdateShipSpeeds_Vertical if bit 0 of A is clear.
B289 JR Z,UpdateShipSpeeds_Vertical
B28B EX DE,HL Swap DE and HL.
B28C LD DE,($B316) Fetch the word at B316 into DE.
B290 LD A,$03 A=03.
B292 LD ($5C36),A Update the steering direction.
B295 LD A,H A=H.
B296 AND A Jump to UpdateShipSpeeds_Vertical if A is zero.
B297 JR Z,UpdateShipSpeeds_Vertical
B299 ADD HL,DE HL gains DE.
B29A EX DE,HL Swap DE and HL.
UpdateShipSpeeds_Vertical B29B LD A,($5C40) The ship was just hit.
B29E AND A Jump to UpdateShipSpeeds_Thrust if A is zero.
B29F JR Z,UpdateShipSpeeds_Thrust
B2A1 LD DE,$0000 DE=00.
UpdateShipSpeeds_Thrust B2A4 CALL UpdateHorizontalSpeed Call UpdateHorizontalSpeed.
B2A7 LD DE,($B312) Fetch the word at ShipAccelerationTable into DE.
B2AB LD A,($5C37) The controls.
B2AE AND %00001000 Keep the bits of A that are set in 08.
B2B0 JR Z,UpdateShipSpeeds_Gravity Jump to UpdateShipSpeeds_Gravity if the zero flag is set.
B2B2 LD DE,($B314) Fetch the word at B314 into DE.
UpdateShipSpeeds_Gravity B2B6 LD H,(IX+$1B) Read offset 1B of IX into H.
B2B9 LD L,(IX+$1C) Read offset 1C of IX into L.
B2BC LD A,($B832) The special-shot flag.
B2BF AND A Jump to UpdateShipSpeeds_Limit if A is zero.
B2C0 JR Z,UpdateShipSpeeds_Limit
B2C2 LD HL,$0000 HL=00.
B2C5 LD DE,$0000 DE=00.
B2C8 LD ($B320),HL Update the vertical scroll fraction.
UpdateShipSpeeds_Limit B2CB ADD HL,DE HL gains DE.
B2CC CALL LimitVerticalSpeed Call LimitVerticalSpeed.
B2CF LD (IX+$1B),H Write H to offset 1B of IX.
B2D2 LD (IX+$1C),L Write L to offset 1C of IX.
Continue on to UpdateVerticalScrollSpeed.
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