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Elite on the BBC Micro

Demo: DEMON [6502SP version]

Name: DEMON [Show more] Type: Subroutine Category: Demo Summary: Show the demo Deep dive: The 6502 Second Processor demo mode Secrets of the Executive version
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * BR1 (Part 1 of 2) calls DEMON * TITLE calls DEMON * TT170 calls DEMON

See the deep dive on "The 6502 Second Processor demo mode" for details of how the demo is implemented.
.DEMON LDA #1 \ Clear the top part of the screen, draw a white border, JSR TT66 \ and set the current view type in QQ11 to 1 JSR RESET \ Call RESET to initialise most of the game variables LDA #0 \ Set ALPHA and ALP1 to 0, so our roll angle (i.e. that STA ALPHA \ of the camera) is 0 STA ALP1 STA DELTA \ Set DELTA to 0, so our current speed (i.e. that of the \ camera) is 0 STA scacol+CYL \ Set the scanner colour for the Cobra Mk III to colour \ 0 (black), so it doesn't appear on the scanner during \ the demo JSR DOVDU19 \ Send a #SETVDU19 0 command to the I/O processor to \ switch to the mode 1 palette for the space view, \ which is yellow (colour 1), red (colour 2) and cyan \ (colour 3) JSR nWq \ Call nWq to create a random cloud of stardust LDX #LO(acorn) \ Set (Y X) to the address of acorn, which contains the LDY #HI(acorn) \ text: "ACORNSOFT PRESENTS" (or, in the Executive \ version: "PIZZASOFT PRESENTS") JSR SLIDE \ Call SLIDE to display the Star Wars scroll text JSR ZINF2 \ Call ZINF2 to reset INWK and the orientation vectors, \ with nosev pointing into the screen. We are about to \ add the Elite logo to the universe, and the logo's \ nosev points out of the top of the logo, so this will \ spawn the logo with it tilted back so it appears \ on-edge, with the bottom of the logo pointing towards \ the viewer LDA #%10000000 \ Set y_sign to be negative STA INWK+5 LDA #100 \ Set y_lo = 100 STA INWK+3 LDA #LGO \ Set the ship type to the Elite logo STA TYPE JSR NWSHP \ Add a new Elite logo to the local bubble (in this \ case, the demo screen), pointing INF to the new ship's \ data block in K% LDA #150 \ Set a loop counter in MCNT to 150 for the grand STA MCNT \ entrance of the logo, which takes it from z = 0 (right \ up against the camera) to z = 450 (a fair distance in \ front of us), moving the logo a distance of 3 away \ from us with each iteration .FLYL1 LDA INWK+6 \ Set (z_hi z_lo) = (z_hi z_lo) + 3 CLC \ ADC #3 \ starting with the low bytes STA INWK+6 LDA INWK+7 \ And then adding the high bytes ADC #0 STA INWK+7 JSR LL9 \ Call LL9 to draw the logo on-screen DEC MCNT \ Decrement the main loop counter BNE FLYL1 \ Loop back to FLYL1 until we have done 150 iterations, \ after which (z_hi z_lo) = 150 * 3 = 450 .FLYL2 LDA INWK+6 \ Set (z_hi z_lo) = (z_hi z_lo) + 2 CLC \ ADC #2 \ starting with the low bytes STA INWK+6 LDA INWK+7 \ And then adding the high bytes ADC #0 STA INWK+7 LDA #%10000000 \ Call TWIST2 with a negative A to pitch the logo by a JSR TWIST2 \ small angle in a negative direction \DEC INWK+3 \ This instruction is commented out in the original \ source, but it would decrement y_lo, moving the logo \ down the screen JSR LL9 \ Call LL9 to draw the logo on-screen LDA INWK+14 \ Loop back to FLYL2 to keep pitching, until nosev_z_hi BPL FLYL2 \ is negative (i.e. the logo has pitched forward through \ 90 degrees, as nosev starts out by pointing into the \ screen towards the camera, in a positive direction, \ and turns negative when it's reached the vertical). \ In other words, we loop until the logo has tilted \ towards the camera and is fully vertical in front of \ the viewer JSR STORE \ Call STORE to copy the ship data block at INWK back to \ the K% workspace at INF, so the logo becomes the first \ ship in the K% block JSR ZINF2 \ Call ZINF2 to reset INWK and the orientation vectors, \ with nosev pointing into the screen LDA #108 \ Set y_lo = 108, so the Cobra we are about to spawn STA INWK+3 \ appears at the top of the screen LDA #40 \ Set z_lo = -40, so it appears a small distance behind STA INWK+6 \ the camera (the negative part is achieved by setting LDA #%10000000 \ z_sign in INWK+8 to be negative, so the ship is behind STA INWK+8 \ the camera) LDA #CYL \ Set the ship type to a Cobra Mk III STA TYPE JSR NWSHP \ Add a new Cobra Mk III to the local bubble (in this \ case, the demo screen), pointing INF to the new ship's \ data block in K% LDA #1 \ Set the Cobra's speed to 1 STA INWK+27 STA HIMCNT \ Set HIMCNT = 1 to act as an outer counter for the \ following loop LDA #90 \ Set MCNT = 90 to act as an inner counter for the STA MCNT \ following loop JSR TWIST \ Call TWIST three times to pitch by a smallish angle in JSR TWIST \ a positive direction (i.e. 3 x 3.6 degree, or 10.8 JSR TWIST \ degrees) \ The following loop iterates 90 times while HIMCNT is 1 \ and 256 times while HIMCNT is 0, to give a total of \ 346 iterations .FLYL4 JSR LL9 \ Call LL9 to draw the Cobra on-screen JSR MVEIT \ Call MVEIT to move the Cobra slowly forward DEC MCNT \ Decrement the inner counter in MCNT BNE FLYL4 \ Loop back to FLYL4 until the inner counter is 0 DEC HIMCNT \ Decrement the outer counter in HIMCNT BPL FLYL4 \ Loop back to FLYL4 until the outer counter is negative JSR ZZAAP \ Call ZZAAP to draw a vertical laser line from the \ Cobra LDA #0 \ Call the NOISE routine with A = 0 to make the sound JSR NOISE \ of a laser firing LDY #10 \ Wait for 10/50 of a second (0.2 seconds) JSR DELAY LDA #44 \ Set the Cobra's roll counter to 44 to make it roll in STA INWK+29 \ a positive direction (clockwise), for just under a \ half roll (44 * 1/16 radians = 2.75 radians = 158 \ degrees) LDA #8 \ Set the Cobra's speed to 8 STA INWK+27 LDA #&87 \ Set the Cobra's pitch counter to -7 to make it pitch STA INWK+30 \ slightly in a negative direction (pull up), so it \ starts slying gently towards the top of the screen JSR STORE \ Call STORE to copy the ship data block at INWK back to \ the K% workspace at INF, so the Cobra becomes the \ second ship in the K% block LDA #%10000000 \ Set bit 7 of byte #31 of the first ship in the K% TSB K%+31 \ block, which is the Elite logo, so this flags the logo \ as having been killed (the TSB instruction applies the \ accumulator to the memory location using an OR) JSR EXNO3 \ Make an explosion sound JSR ZZAAP \ Call ZZAAP to redraw the vertical laser line from the \ Cobra, which removes it from the screen .FLYL5 \ We now want to draw the logo exploding, so we first \ need to copy the logo's ship data block from K% to \ INWK LDX #NI%-1 \ Set a counter in X so we can loop through the NI% \ bytes in the ship data block .DML3 LDA K%,X \ Copy the X-th byte of the first ship data block in K% STA INWK,X \ to the X-th byte of INWK DEX \ Decrement the loop counter BPL DML3 \ Loop back for the next byte, until we have copied the \ last byte from K% to INWK INX \ Increment X back to 0 JSR GINF \ Call GINF to fetch the address of the ship data block \ for the ship in slot 0 (the logo) and store it in INF LDA XX21-2+2*LGO \ Set XX0(1 0) to point to the ship blueprint for the STA XX0 \ Elite logo LDA XX21-1+2*LGO STA XX0+1 LDA #LGO \ Set the ship type to the Elite logo STA TYPE INC INWK \ Increment x_lo to move the logo a little to the right JSR LL9 \ Call LL9 to draw the now-exploding logo on-screen JSR STORE \ Call STORE to copy the ship data block at INWK back to \ the K% workspace at INF JSR PBFL \ Call PBFL to send the contents of the pixel buffer to \ the I/O processor for plotting on-screen LDA INWK+31 \ Test whether bits 5 and 7 of the logo's byte #31 are AND #%10100000 \ both set ("ship is exploding" and "ship has been CMP #%10100000 \ killed") and store the resulting flags on the stack PHP \ We now want to animate the Cobra flying into the \ distance while rolling clockwise, so we first need to \ copy the logo's ship data block from K% to INWK LDX #NI%-1 \ Set a counter in X so we can loop through the NI% \ bytes in the ship data block .DML4 LDA K%+NI%,X \ Copy the X-th byte of the second ship data block in K% STA INWK,X \ to the X-th byte of INWK DEX \ Decrement the loop counter BPL DML4 \ Loop back for the next byte, until we have copied the \ last byte from K% to INWK LDX #1 \ Call GINF to fetch the address of the ship data block JSR GINF \ for the ship in slot 1 (the Cobra) and store it in INF LDA XX21-2+2*CYL \ Set XX0(1 0) to point to the ship blueprint for a STA XX0 \ Cobra Mk III LDA XX21-1+2*CYL STA XX0+1 LDA #CYL \ Set the ship type to a Cobra Mk III STA TYPE JSR MVEIT \ Call MVEIT to move the Cobra so it pitches, rolls and \ flies on JSR LL9 \ Call LL9 to draw the Cobra on-screen JSR STORE \ Call STORE to copy the ship data block at INWK back to \ the K% workspace PLP \ Restore the flags that we stored on the stack above BNE FLYL5 \ If it is not the case that bits 5 and 7 of the logo's \ byte #31 are both set, jump back to FLYL5 to keep \ drawing the exploding logo and the Cobra \ The logo has now finished exploding, as bits 5 and 7 \ of the logo's byte #31 are now both set ("ship is \ exploding" and "ship has been killed"). By this time \ the Cobra will have reached the upper part of the \ screen, where we leave it while we do the next bit of \ scrolling text LDA #14 \ Set DELTA to 14, so our current speed (i.e. that of STA DELTA \ the camera) is 14 STZ DELT4 \ Set DELT4 = 0 LSR A \ Set DELT4+1 = 14 / 4 ROR DELT4 \ LSR A \ so DELT4(1 0) therefore contains 14 * 64 ROR DELT4 STA DELT4+1 IF _EXECUTIVE LDX #LO(executive) \ Set (Y X) to the address of executive, which contains LDY #HI(executive) \ the text: "THE EXECUTIVE VERSION" JSR SLIDE \ Call SLIDE to display the Star Wars scroll text ENDIF LDX #LO(byian) \ Set (Y X) to the address of byian, which contains the LDY #HI(byian) \ text: "BY IAN BELL AND DAVID BRABEN" JSR SLIDE \ Call SLIDE to display the Star Wars scroll text \ We now want the Cobra to start flying again, so we \ first need to copy the logo's ship data block from K% \ to INWK LDX #NI%-1 \ Set a counter in X so we can loop through the NI% \ bytes in the ship data block .DML5 LDA K%+NI%,X \ Copy the X-th byte of the second ship data block in K% STA INWK,X \ to the X-th byte of INWK DEX \ Decrement the loop counter BPL DML5 \ Loop back for the next byte, until we have copied the \ last byte from K% to INWK .FLYL6 JSR STARS1 \ Call STARS1 to process the stardust for the front view JSR MVEIT \ Call MVEIT to move the Cobra so it pitches, rolls and \ flies onwards JSR LL9 \ Call LL9 to draw the Cobra on-screen LDA INWK+8 \ Keep looping back to FLYL6 until z_sign is negative, BPL FLYL6 \ i.e. the Cobra has been overtaken by the camera as the \ camera moves forward, and it's disappeared off the top \ of the screen LDX #LO(true3) \ Set (Y X) to the address of true3, which contains the LDY #HI(true3) \ text: "THE GALAXY IS IN TURMOIL, THE NAVY FAR AWAY AS \ THE EMPIRE CRUMBLES" (or, in the Executive version: \ "CONGRATULATIONS ON OBTAINING A COPY OF THIS ELUSIVE \ PRODUCT") JSR SLIDE \ Call SLIDE to display the Star Wars scroll text JSR RES2 \ Reset a number of flight variables and workspaces LDA #&E0 \ Set nosev_z_hi = -1 (as &E0 is a negative unit vector STA INWK+14 \ length), so the ship points out of the screen, towards \ us STZ DELTA \ Set DELTA to 0, so our current speed (i.e. that of the \ camera) is 0 STZ ALPHA \ Set ALPHA and ALP1 to 0, so our roll angle (i.e. that STZ ALP1 \ of the camera) is 0 LDX #15 \ Set the ship's speed to 15 STX INWK+27 LDX #5 \ Set the ship's z_hi to 5, so it's in the distance STX INWK+7 LDA #ADA \ Set the ship type to an Adder STA TYPE JSR NWSHP \ Add a new Adder to the local bubble (in this case, the \ demo screen) .FLYL7 JSR MVEIT \ Call MVEIT to move the Adder so it flies towards the \ camera JSR LL9 \ Call LL9 to draw the Adder on-screen LDA INWK+7 \ Keep looping back to FLYL7 until z_hi is zero BNE FLYL7 LDA #3 \ Set the Adder's roll counter to 3 to make it roll in STA INWK+29 \ a positive direction (clockwise) STA INWK+30 \ Set the Adder's pitch counter to 3 to make it pitch in \ a positive direction (dive) LDA INWK+8 \ Keep looping back to FLYL7 until z_sign is negative BPL FLYL7 \ i.e. the Adder has flown past the camera JSR SCAN \ Call SCAN to remove the adder from the scanner (by \ redrawing it) JSR RES2 \ Reset a number of flight variables and workspaces LDA #CYAN2 \ Set the scanner colour for the Cobra Mk III to its STA scacol+CYL \ default colour of cyan, so Cobras will appear on the \ scanner once again JMP DEATH2 \ Jump to DEATH2 to reset most of the game and restart \ from the title screen