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Main loop: Main flight loop (Part 4 of 16)

[NES version, Bank 0]

Name: Main flight loop (Part 4 of 16) [Show more] Type: Subroutine Category: Main loop Summary: For each nearby ship: Copy the ship's data block from K% to the zero-page workspace at INWK Deep dive: Program flow of the main game loop Ship data blocks Splitting the main loop in the NES version
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * FlightLoop4To16 calls via MAL1

The main flight loop covers most of the flight-specific aspects of Elite. This section covers the following: * Start looping through all the ships in the local bubble, and for each one: * Copy the ship's data block from K% to INWK * Set XX0 to point to the ship's blueprint (if this is a ship)
Other entry points: MAL1 Marks the beginning of the ship analysis loop, so we can jump back here from part 12 of the main flight loop to work our way through each ship in the local bubble. We also jump back here when a ship is removed from the bubble, so we can continue processing from the next ship
.MAL1 STX XSAV ; Store the current slot number in XSAV STA TYPE ; Store the ship type in TYPE SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure ; the PPU to use nametable 0 and pattern table 0 JSR GINF ; Call GINF to fetch the address of the ship data block ; for the ship in slot X and store it in INF. The data ; block is in the K% workspace, which is where all the ; ship data blocks are stored ; Next we want to copy the ship data block from INF to ; the zero-page workspace at INWK, so we can process it ; more efficiently LDY #NI%-1 ; There are NI% bytes in each ship data block (and in ; the INWK workspace, so we set a counter in Y so we can ; loop through them .MAL2 LDA (INF),Y ; Load the Y-th byte of INF and store it in the Y-th STA INWK,Y ; byte of INWK DEY ; Decrement the loop counter BPL MAL2 ; Loop back for the next byte until we have copied the ; last byte from INF to INWK SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure ; the PPU to use nametable 0 and pattern table 0 LDA TYPE ; If the ship type is negative then this indicates a BMI MA21 ; planet or sun, so jump down to MA21, as the next bit ; sets up a pointer to the ship blueprint, and then ; checks for energy bomb damage, and neither of these ; apply to planets and suns CMP #SST ; If this is not the space station, jump to main32 BNE main32 LDA spasto ; Copy the address of the space station's ship blueprint STA XX0 ; from spasto(1 0) to XX0(1 0), which we set up in NWSPS LDA spasto+1 ; when calculating the correct station type (Coriolis or STA XX0+1 ; Dodo) LDY #SST * 2 ; Set Y = ship type * 2 BNE main33 ; Jump to main33 (this BNE is effectively a JMP as Y is ; never zero) .main32 ASL A ; Set Y = ship type * 2 TAY LDA XX21-2,Y ; The ship blueprints at XX21 start with a lookup STA XX0 ; table that points to the individual ship blueprints, ; so this fetches the low byte of this particular ship ; type's blueprint and stores it in XX0 LDA XX21-1,Y ; Fetch the high byte of this particular ship type's STA XX0+1 ; blueprint and store it in XX0+1 .main33 ; We now check whether this ship prevents us from ; performing an in-system jump CPY #ESC * 2 ; If this is an escape pod, jump to main36 to skip the BEQ main36 ; following, as it doesn't prevent jumping CPY #TGL * 2 ; If this is a Thargon, jump to main36 to skip the BEQ main36 ; following, as it doesn't prevent jumping CPY #SST * 2 ; If this is the space station, jump to main35 to check BEQ main35 ; whether it is hostile LDA INWK+32 ; If bit 7 of the ship's byte #32 is clear, then AI is BPL main36 ; not enabled, so jump to main36 to skip the following, ; as it doesn't prevent jumping CPY #MSL * 2 ; If this is a missile, jump to main34 to skip the BEQ main34 ; aggression level check (as this doesn't apply to ; missiles) AND #%00111110 ; If bits 1-5 of the ship's byte #32 are clear, then the BEQ main36 ; ship's aggression level is zero, so jump to main36 to ; skip the following, as it doesn't prevent jumping .main34 LDA INWK+31 ; If either bit 5 or 7 of the ship's byte #31 are set, AND #%10100000 ; then the ship is exploding or has been killed, so jump BNE main36 ; to main36 to skip the following, as it doesn't prevent ; jumping .main35 LDA NEWB ; If bit 2 of the ship's NEWB flag is clear then the AND #%00000100 ; ship is not hostile, so jump to main36 to skip the BEQ main36 ; following, as it doesn't prevent jumping ; If we get here then this is one of the following: ; ; * A missile ; ; * A hostile space station ; ; * An aggressive ship with AI enabled ; ; and it isn't an escape pod or Thargon, and it hasn't ; been killed and isn't exploding ; ; So this ship prevents us from performing an in-system ; jump, so we need to set bit 7 of allowInSystemJump ; to do just that ASL allowInSystemJump ; Set bit 7 of allowInSystemJump to prevent us from SEC ; being able to perform an in-system jump ROR allowInSystemJump .main36