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⌂ ∞ ▽ Elite on the 6502

Main loop: FlightLoop4To16

[NES version, Bank 0]

       Name: FlightLoop4To16                                         [Show more]
       Type: Subroutine
   Category: Main loop
    Summary: Display in-flight messages, call parts 4 to 12 of the main flight
             loop for each slot, and fall through into parts 13 to 16
  Deep dive: Splitting the main loop in the NES version
             The NES combat demo

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * DEATH calls FlightLoop4To16
             * Main flight loop (Part 3 of 16) calls FlightLoop4To16
             * RunDemoFlightLoop calls FlightLoop4To16




.main24

 DEC DLY                ; Decrement the delay counter in DLY, which is used to
                        ; control how long flight messages remain on-screen

 BMI main27             ; If DLY is now negative, jump to main27 to set DLY to
                        ; zero and skip the following, as there is no flight
                        ; message to display

 BEQ main25             ; DLY is now zero so it must have been non-zero before
                        ; we decremented it, so jump to main25 to remove the
                        ; flight message from the screen, as its timer has run
                        ; down

                        ; DLY is non-zero, so we need to redraw any flight
                        ; messages we may have, so they remain on-screen while
                        ; DLY is still ticking down

 JSR PrintFlightMessage ; Print the current in-flight message, if there is one

 JMP main26             ; Jump to main26 to display the message we just printed
                        ; and continue with the rest of the main loop

.main25

 JSR CLYNS              ; Clear the bottom two text rows of the upper screen,
                        ; and move the text cursor to the first cleared row

.main26

 JSR DrawMessageInNMI   ; Configure the NMI to update the in-flight message part
                        ; of the screen (which is either the current in-flight
                        ; message, or the part of the screen that the call to
                        ; CLYNS just cleared)

 JMP MA16               ; Jump to MA16 to skip the following and continue with
                        ; the rest of the main loop

.FlightLoop4To16

 LDA QQ11               ; If this is not the space view (i.e. QQ11 is non-zero),
 BNE main24             ; jump to main24 to print the flight message for
                        ; non-space views, rejoining the main subroutine at MA16
                        ; below

 DEC DLY                ; Decrement the delay counter in DLY, which is used to
                        ; control how long flight messages remain on-screen

 BMI main27             ; If DLY is now 0 or negative, jump to main27 to set DLY
 BEQ main27             ; to zero and skip the following, as there is no flight
                        ; message to display

                        ; DLY is non-zero, so we need to redraw any flight
                        ; messages we may have, so they remain on-screen while
                        ; DLY is still ticking down

 JSR PrintFlightMessage ; Print the current flight message, if there is one

 JMP MA16               ; Jump to MA16 to skip the following and continue with
                        ; the rest of the main loop

.main27

 LDA #0                 ; Set DLY to 0 so that it doesn't decrement below zero
 STA DLY

.MA16

 LDA ECMP               ; If our E.C.M is not on, skip to MA69, otherwise keep
 BEQ MA69               ; going to drain some energy

 JSR DENGY              ; Call DENGY to deplete our energy banks by 1

 BEQ MA70               ; If we have no energy left, jump to MA70 to turn our
                        ; E.C.M. off

.MA69

 LDA ECMA               ; If an E.C.M is going off (ours or an opponent's) then
 BEQ MA66               ; keep going, otherwise skip to MA66

 LDA #128               ; Set K+2 = 128 to send to the DrawLightning routine as
 STA K+2                ; the x-coordinate of the centre of the lightning, so
                        ; it is centred on-screen

 LDA #127               ; Set K = 127 to send to the DrawLightning routine as
 STA K                  ; half the height of the lightning, so it fills the
                        ; whole screen width

 LDA halfScreenHeight   ; Set K+3 to the y-coordinate of the centre of the
 STA K+3                ; screen in halfScreenHeight, to send to the
                        ; DrawLightning routine as the y-coordinate of the
                        ; centre of the lightning, so it is centred on-screen

 STA K+1                ; Set K+1 to the y-coordinate of the centre of the
                        ; screen in halfScreenHeight, to send to the
                        ; DrawLightning routine as half the height of the
                        ; lightning, so it fills the whole screen height

 JSR DrawLightning_b6   ; Draw the lightning effect of the E.C.M. going off

 DEC ECMA               ; Decrement the E.C.M. countdown timer, and if it has
 BNE MA66               ; reached zero, keep going, otherwise skip to MA66

.MA70

 JSR ECMOF              ; If we get here then either we have either run out of
                        ; energy, or the E.C.M. timer has run down, so switch
                        ; off the E.C.M.

.MA66

 LDX #0                 ; We are about to work our way through all the ships in
                        ; the bubble, calling MAL1 (parts 4 to 12 of the main
                        ; flight loop) for each of them, so set X as a ship slot
                        ; counter

 LDA FRIN               ; If slot 0 is empty, jump to main28 to move on to the
 BEQ main28             ; next slot

 JSR MAL1               ; Call parts 4 to 12 of the main flight loop to update
                        ; the ship in slot 0

.main28

 LDX #2                 ; We deal with the sun/space station in slot 1 below, so
                        ; we now skip to slot 2 by setting X accordingly

.main29

 LDA FRIN,X             ; If slot X is empty then we have reached the last slot,
 BEQ main30             ; so jump to main30 to stop updating the slots

 JSR MAL1               ; Call parts 4 to 12 of the main flight loop to update
                        ; the ship in slot X

 JMP main29             ; Loop back until we have updated all the ship slots

.main30

 LDX #1                 ; We now process the sun/space station in slot 1, so we
                        ; set X as the slot number

 LDA FRIN+1             ; If slot 1 is empty then there is no sun or space
 BEQ MA18               ; station (which can happen in witchspace), so jump to
                        ; part 13 of the main loop as we are done updating the
                        ; ship slots

 BPL main31             ; If bit 7 of the ship type is clear, then this is the
                        ; space station rather than the sun, so jump to main31
                        ; to skip the following

 LDY #0                 ; Set the "space station present" flag to 0, as we are
 STY SSPR               ; no longer in the space station's safe zone

.main31

 JSR MAL1               ; Call parts 4 to 12 of the main flight loop to update
                        ; the sun or space station in slot 2

                        ; Fall through into part 13 to continue working through
                        ; the main flight loop