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⌂ ∞ ▽ Elite on the BBC Micro and NES

Start and end: TITLE

[6502 Second Processor version]

       Name: TITLE                                                   [Show more]
       Type: Subroutine
   Category: Start and end
    Summary: Display a title screen with a rotating ship and prompt

    Context: See this subroutine in context in the source code
 Variations: See code variations for this subroutine in the different versions
 References: This subroutine is called as follows:
             * BR1 (Part 1 of 2) calls TITLE
             * BR1 (Part 2 of 2) calls TITLE




 Display the title screen, with a rotating ship and a text token at the bottom
 of the screen.



 Arguments:

   A                    The number of the extended token to show below the
                        rotating ship (see variable TKN1 for details of
                        recursive tokens)

   X                    The type of the ship to show (see variable XX21 for a
                        list of ship types)



 Returns:

   X                    If a key is being pressed, X contains the internal key
                        number, otherwise it contains 0



.TITLE

 PHA                    \ Store the token number on the stack for later

 STX TYPE               \ Store the ship type in location TYPE

 JSR RESET              \ Reset our ship so we can use it for the rotating
                        \ title ship

 JSR ZEKTRAN            \ Reset the key logger buffer that gets returned from
                        \ the I/O processor

 LDA #32                \ Send a #SETVDU19 32 command to the I/O processor to
 JSR DOVDU19            \ set the mode 1 palette to yellow (colour 1), white
                        \ (colour 2) and cyan (colour 3)

 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

 LDA #RED               \ Send a #SETCOL RED command to the I/O processor to
 JSR DOCOL              \ switch to colour 2, which is white in the title screen

 STZ QQ11               \ Set QQ11 to 0, so from here on we are using a space
                        \ view

 LDA #96                \ Set nosev_z hi = 96 (96 is the value of unity in the
 STA INWK+14            \ rotation vector)

 STA INWK+7             \ Set z_hi, the high byte of the ship's z-coordinate,
                        \ to 96, which is the distance at which the rotating
                        \ ship starts out before coming towards us

 LDX #127               \ Set roll counter = 127, so don't dampen the roll and
 STX INWK+29            \ make the roll direction clockwise

 STX INWK+30            \ Set pitch counter = 127, so don't dampen the pitch and
                        \ set the pitch direction to dive

 INX                    \ Set QQ17 to 128 (so bit 7 is set) to switch to
 STX QQ17               \ Sentence Case, with the next letter printing in upper
                        \ case

 LDA TYPE               \ Set up a new ship, using the ship type in TYPE
 JSR NWSHP

 LDA #6                 \ Move the text cursor to column 6
 JSR DOXC

 LDA #30                \ Print recursive token 144 ("---- E L I T E ----")
 JSR plf                \ followed by a newline

 LDA #10                \ Print a line feed to move the text cursor down a line
 JSR TT26

 LDA #6                 \ Move the text cursor to column 6 again
 JSR DOXC

 LDA PATG               \ If PATG = 0, skip the following two lines, which
 BEQ awe                \ print the author credits (PATG can be toggled by
                        \ pausing the game and pressing "X")

 LDA #13                \ Print extended token 13 ("BY D.BRABEN & I.BELL")
 JSR DETOK

.awe

 LDA brkd               \ If brkd = 0, jump to BRBR2 to skip the following, as
 BEQ BRBR2              \ we do not have a system error message to display

 INC brkd               \ Increment the brkd counter

 LDA #7                 \ Move the text cursor to column 7
 JSR DOXC

 LDA #10                \ Move the text cursor to row 10
 JSR DOYC

                        \ The following loop prints out the null-terminated
                        \ message pointed to by (&FD &FE), which is the MOS
                        \ error message pointer - so this prints the error
                        \ message on the next line

 LDY #0                 \ Set Y = 0 to act as a character counter

 JSR OSWRCH             \ Print the character in A (which contains a line feed
                        \ on the first loop iteration), and then any non-zero
                        \ characters we fetch from the error message

 INY                    \ Increment the loop counter

 LDA (&FD),Y            \ Fetch the Y-th byte of the block pointed to by
                        \ (&FD &FE), so that's the Y-th character of the message
                        \ pointed to by the MOS error message pointer

 BNE P%-6               \ If the fetched character is non-zero, loop back to the
                        \ JSR OSWRCH above to print it, and keep looping until
                        \ we fetch a zero (which marks the end of the message)

.BRBR2

 JSR CLYNS              \ Clear the bottom three text rows of the upper screen,
                        \ and move the text cursor to column 1 on row 21, i.e.
                        \ the start of the top row of the three bottom rows.
                        \ It also returns with Y = 0

 STY DELTA              \ Set DELTA = 0 (i.e. ship speed = 0)

 STY JSTK               \ Set JSTK = 0 (i.e. keyboard, not joystick)

 PLA                    \ Restore the recursive token number we stored on the
                        \ stack at the start of this subroutine

\JSR ex                 \ This instruction is commented out in the original
                        \ source (it would print the recursive token in A)

 JSR DETOK              \ Print the extended token in A

 LDA #7                 \ Move the text cursor to column 7
 JSR DOXC

 LDA #12                \ Print extended token 12 ("({single cap}C) ACORNSOFT
 JSR DETOK              \ 1984")

 LDA #12                \ Set CNT2 = 12 as the outer loop counter for the loop
 STA CNT2               \ starting at TLL2

 LDA #5                 \ Set the main loop counter in MCNT to 5, to act as the
 STA MCNT               \ inner loop counter for the loop starting at TLL2

.TLL2

 LDA INWK+7             \ If z_hi (the ship's distance) is 1, jump to TL1 to
 CMP #1                 \ skip the following decrement
 BEQ TL1

 DEC INWK+7             \ Decrement the ship's distance, to bring the ship
                        \ a bit closer to us

.TL1

 JSR MVEIT              \ Move the ship in space according to the orientation
                        \ vectors and the new value in z_hi

 LDX #128               \ Set z_lo = 128, so the closest the ship gets to us is
 STX INWK+6             \ z_hi = 1, z_lo = 128, or 256 + 128 = 384

 LDA MCNT               \ This value will be zero on one out of every four
 AND #3                 \ iterations, so for the other three, skip to nodesire
 BNE nodesire           \ so we only scan for key presses once every four loops

 STX NEEDKEY            \ Set NEEDKEY = 128, so the call to LL9 below draws the
                        \ ship and scans for key presses (LL9 resets NEEDKEY to
                        \ 0 so we have to reset NEEDKEY every four iterations
                        \ round the inner loop)

.nodesire

 STZ INWK               \ Set x_lo = 0, so the ship remains in the screen centre

 STZ INWK+3             \ Set y_lo = 0, so the ship remains in the screen centre

 JSR LL9                \ Call LL9 to display the ship

 LDA KTRAN+12           \ Fetch the key press state for the joystick 1 fire
                        \ button from the key logger buffer, which contains
                        \ the value of the 6522 System VIA input register IRB
                        \ (SHEILA &40)

 AND #%00010000         \ Bit 4 of IRB (PB4) is clear if joystick 1's fire
                        \ button is pressed, otherwise it is set, so AND'ing
                        \ the value of IRB with %10000 extracts this bit

 TAX                    \ Copy the joystick fire button state to X, though this
                        \ instruction has no effect, as the comparison flags are
                        \ already set by the AND, and the value of X is not used
                        \ anywhere

 BEQ TL2                \ If the joystick fire button is pressed, jump to TL2

 LDA KTRAN              \ Fetch the internal key number of the current key
                        \ press from the key logger buffer

 BNE TL3                \ If a key is being pressed, jump to TL3

 DEC MCNT               \ Decrement the inner loop counter

 BNE TLL2               \ Loop back to keep the ship rotating, until the inner
                        \ loop counter is zero

 DEC CNT2               \ Decrement the outer loop counter in CNT2

 BNE TLL2               \ Loop back to keep the ship rotating, until the outer
                        \ loop counter is zero

 JMP DEMON              \ Once we have iterated through CNT2 iterations of MCNT,
                        \ jump to DEMON to start the demo

.TL2

 DEC JSTK               \ Joystick fire button was pressed, so set JSTK to &FF
                        \ (it was set to 0 above), to disable keyboard and
                        \ enable joysticks

.TL3

 RTS                    \ Return from the subroutine