Combat demo: PlayDemo

       Name: PlayDemo                                                [Show more]
       Type: Subroutine
   Category: Combat demo
    Summary: Play the combat demo
  Deep dive: The NES combat demo
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * PlayDemo_b0 calls PlayDemo


.PlayDemo

 JSR RES2               ; Reset a number of flight variables and workspaces

 JSR ResetCommander_b6  ; Reset the current commander to the default "JAMESON"
                        ; commander

 LDA #0                 ; Set the fuel level to zero so we can't hyperspace out
 STA QQ14               ; of the demo

 STA CASH               ; Zero the two lowest bytes of the cash reserves so no
 STA CASH+1             ; missions are triggered

 LDA #$FF               ; Give our ship an E.C.M.
 STA ECM

 LDA #1                 ; Give our ship an energy unit
 STA ENGY

 LDA #POW+128           ; Give our ship a beam laser
 STA LASER

 LDA #$FF               ; Set demoInProgress = $FF to indicate that we are
 STA demoInProgress     ; playing the demo

 JSR SOLAR              ; Set up data blocks and slots for the planet and
                        ; sun

 LDA #0                 ; Set our ship's speed to zero
 STA DELTA

 STA ALPHA              ; Set ALPHA and ALP1 to 0, so our roll angle is 0
 STA ALP1

 STA QQ12               ; Set QQ12 = 0 to indicate that we are not docked

 STA VIEW               ; Set the space view to the front view

 JSR TT66               ; Clear the screen and set the view type in QQ11 to $00
                        ; (Space view with no fonts loaded)

 LSR demoInProgress     ; Clear bit 7 of demoInProgress

 JSR CopyNameBuffer0To1 ; Copy the contents of nametable buffer 0 to nametable
                        ; buffer

 JSR SetupFullViewInNMI ; Configure the PPU to send tiles for the full screen
                        ; during VBlank

 JSR SetupSpaceView     ; Set up the NMI variables for the space view

 JSR FixRandomNumbers   ; Fix the random number seeds to a known value so the
                        ; random numbers generated are always the same when we
                        ; run the demo

 JSR SetupDemoShip      ; Set up the ship workspace for a new ship that's to our
                        ; upper left and in front of us, pointing into the
                        ; screen

 LDA #6                 ; Set the ship's pitch counter to 6 to make it pitch
 STA INWK+30            ; slightly in a positive direction (pitch down), so it
                        ; starts diving gently towards the middle of the screen

 LDA #24                ; Set the ship's roll counter to 24 to make it roll in
 STA INWK+29            ; a positive direction (clockwise), for just under a
                        ; quarter roll (24 * 1/16 radians = 1.5 radians = 86
                        ; degrees)

 LDA #18                ; Call NWSHP with A = 18 to add a new Mamba ship to our
 JSR NWSHP              ; local bubble of universe

 LDA #10                ; Run ten iterations of the main flight loop so the
 JSR RunDemoFlightLoop  ; Mamba flies into the screen for a while

 LDA #$92               ; Set the ship's pitch counter to -18 to make it pitch
 STA K%+2*NIK%+30       ; slightly in a negative direction (pull up), so it
                        ; starts flying gently towards the top-left of the
                        ; screen
                        ;
                        ; The ship will have been spawned in ship slot 2, so
                        ; this directly updates byte #30 in the ship's data
                        ; block in K%, where each data block is NIK% bytes long

 LDA #1                 ; Set the ship's acceleration to 1 to make it accelerate
 STA K%+2*NIK%+28       ; away from us
                        ;
                        ; The ship will have been spawned in ship slot 2, so
                        ; this directly updates byte #28 in the ship's data
                        ; block in K%, where each data block is NIK% bytes long

 JSR SetupDemoShip      ; Set up the ship workspace for a new ship that's to our
                        ; upper left and in front of us, pointing into the
                        ; screen

 LDA #6                 ; Set the ship's pitch counter to 6 to make it pitch
 STA INWK+30            ; slightly in a positive direction (pitch down), so it
                        ; starts diving gently towards the middle of the screen

 ASL INWK+2             ; Flip the sign of x_sign so the ship is now to our
                        ; upper right and in front of us

 LDA #$C0               ; Set the ship's roll counter to -64 to make it roll in
 STA INWK+29            ; a negative direction (anti-clockwise), for two-thirds
                        ; of a roll (64 * 1/16 radians = 4.0 radians = 229
                        ; degrees)

 LDA #KRA               ; Call NWSHP to add a new Krait ship to our local bubble
 JSR NWSHP              ; of universe

 LDA #6                 ; Run six iterations of the main flight loop so the
 JSR RunDemoFlightLoop  ; Krait flies into the screen for a while and the Mamba
                        ; starts to pull away from the middle of the screen

 JSR SetupDemoShip      ; Set up the ship workspace for a new ship that's to our
                        ; upper left and in front of us, pointing into the
                        ; screen

 LDA #6                 ; Set the ship's pitch counter to 6 to make it pitch
 STA INWK+30            ; slightly in a positive direction (pitch down), so it
                        ; starts diving gently towards the middle of the screen

 ASL INWK+2             ; Flip the sign of x_sign so the ship is now to our
                        ; upper right and in front of us

 LDA #0                 ; Set x_lo = 0 so the ship is directly above us
 STA INWK

 LDA #70                ; Set z_lo = 70 so the ship starts out a little further
 STA INWK+6             ; in front than the others

 LDA #SH3               ; Call NWSHP to add a new Sidewinder ship to our local
 JSR NWSHP              ; bubble of universe

 LDA #5                 ; Run five iterations of the main flight loop
 JSR RunDemoFlightLoop  ; Sidewinder flies into the screen for a while

 LDA #$C0               ; Set the ship's pitch counter to -64 to make it pitch
 STA K%+4*NIK%+30       ; strongly in a negative direction (pull up), so it
                        ; starts flying towards the top-middle of the screen
                        ;
                        ; The ship will have been spawned in ship slot 4, so
                        ; this directly updates byte #30 in the ship's data
                        ; block in K%, where each data block is NIK% bytes long

 LDA #11                ; Run 11 iterations of the main flight loop so all three
 JSR RunDemoFlightLoop  ; ships pull away from the centre of the screen

 LDA #50                ; Set the NMI timer so it starts counting down from 50,
 STA nmiTimer           ; so the (nmiTimerHi nmiTimerLo) will tick up to one
                        ; second after 50 VBlanks (which is one second on PAL
                        ; systems or 0.83 seconds on NTSC)

 LDA #0                 ; Set the NMI timer in (nmiTimerHi nmiTimerLo) to zero
 STA nmiTimerLo         ; so we can use it to count how long the combat demo
 STA nmiTimerHi         ; runs for (i.e. how long it takes for us to eliminate
                        ; all three ships)

 JSR SIGHT_b3           ; Draw the laser crosshairs

 LSR allowInSystemJump  ; Clear bit 7 of allowInSystemJump to allow in-system
                        ; jumps, so the call to UpdateIconBar displays the
                        ; fast-forward icon (though choosing this in the demo
                        ; doesn't do an in-system jump, but skips the rest of
                        ; the demo instead)

 JSR UpdateIconBar_b3   ; Update the icon bar to show the correct buttons for
                        ; the weapons we have given our ship (i.e. missiles and
                        ; E.C.M.)

 LDA tuneSpeedCopy      ; Set tuneSpeed = tuneSpeedCopy, to ensure that the
 STA tuneSpeed          ; music speed is set correctly for the current tune (so
                        ; this resets any speed-ups that have been applied,
                        ; such as in the combat demo)

 LDA #16                ; Set our ship's speed to 16, so we start the demo by
 STA DELTA              ; flying forwards

 JMP MLOOP              ; Jump to MLOOP to run the main game loop for the
                        ; duration of the combat demo
                        ;
                        ; Part 15 of the main flight loop contains a check to
                        ; see whether the demo is enabled (which it is) and if
                        ; so, whether we have destroyed all three ships, in
                        ; which case it jumps to ShowScrollText with A = 1 to
                        ; show the scroll text with the results of combat
                        ; practice