Version analysis of TITLE - Elite on the 6502

This code appears in the following versions (click to see it in the source code):

Cassette, Disc (docked), 6502 Second Processor, Master and Electron

Code variations between these versions are shown below.

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


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


 Arguments:

Code variation 1 of 35A variation in the comments only

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Cassette, Docked, Electron
6502SP, Master
   A                    The number of the recursive token to show below the
                        rotating ship (see variable QQ18 for details of
                        recursive tokens)
   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)

Code variation 2 of 35A variation in the comments only

This variation is blank in the Cassette, Disc (docked), 6502 Second Processor and Electron versions.

Master
   Y                    The distance to show the ship rotating, once it has
                        finished moving towards us

Code variation 3 of 35A variation in the comments only

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Cassette, Docked, 6502SP, Electron
Master
 Returns:

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

   X                    If a key is being pressed, X contains the ASCII code
                        of the key pressed


.TITLE

Code variation 4 of 35Related to the Master version

In the Master version, the Cobra Mk III shown on the first title page is further away than in the other versions, which is implemented by a new variable that contains the distance that the ship should be shown at.

See below for more variations related to this code.

This variation is blank in the Cassette, Disc (docked), 6502 Second Processor and Electron versions.

Master
 STY distaway           \ Store the ship distance in distaway

 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

Code variation 5 of 35Related to Elite's use of the Tube

This variation is blank in the Cassette, Disc (docked) and Electron versions.

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Master
6502SP
 JSR ZEKTRAN            \ Call ZEKTRAN to clear the key logger

 JSR ZINF               \ Call ZINF to reset the INWK ship workspace
 JSR ZEKTRAN            \ Reset the key logger buffer that gets returned from
                        \ the I/O processor

Code variation 6 of 35A variation in the comments only

This variation is blank in the Cassette, Disc (docked) and Electron versions.

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Master
6502SP
 LDA #32                \ Set the mode 1 palette to yellow (colour 1), white
 JSR DOVDU19            \ (colour 2) and cyan (colour 3)
 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)

Code variation 7 of 35Specific to an individual platform

The Master version has a unique view type for the title screen (13).

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Cassette, Docked, 6502SP, Electron
Master
 LDA #1                 \ Clear the top part of the screen, draw a border box,
 JSR TT66               \ and set the current view type in QQ11 to 1
 LDA #13                \ Clear the top part of the screen, draw a border box,
 JSR TT66               \ and set the current view type in QQ11 to 13 (rotating
                        \ ship view)

Code variation 8 of 35Related to the screen mode

This variation is blank in the Cassette, Disc (docked) and Electron versions.

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Master
6502SP
 LDA #RED               \ Switch to colour 2, which is white in the title screen
 STA COL
 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

Code variation 9 of 35Minor and very low-impact

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Cassette, Docked, Electron
Master
6502SP
 DEC QQ11               \ Decrement QQ11 to 0, so from here on we are using a
                        \ space view
 LDA #0                 \ Set QQ11 to 0, so from here on we are using a space
 STA QQ11               \ view
 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)

Code variation 10 of 35Other (e.g. bug fix, optimisation)

The disc version contains various bits of copy protection code injected into the loader, and the results get checked in the main title sequence to make sure nothing has been changed.

This variation is blank in the Cassette, 6502 Second Processor, Master and Electron versions.

Docked
 LDA &9F                \ As part of the copy protection, location &9F is set to
 CMP #219               \ 219 in the OSBmod routine in elite-loader3.asm. This
 BEQ tiwe               \ jumps to tiwe if the value is unchanged, otherwise it
                        \ crashes the game with the following (as presumably
                        \ the game code has been tampered with)

 LDA #&10               \ Modify the STA DELTA instruction in RES2 to &10 &FE,
 STA modify+2           \ which is a BPL P%-2 instruction, to create an infinite
 LDA #&FE               \ loop and hang the game
 STA modify+3

.tiwe

Code variation 11 of 35Related to the Master version

See variation 4 above for details.

This variation is blank in the Cassette, Disc (docked), 6502 Second Processor and Electron versions.

Master
 LDA #96                \ Set A = 96 as the distance that the ship starts at

Code variation 12 of 35A variation in the comments only

This variation is blank in the Disc (docked), 6502 Second Processor, Master and Electron versions.

Cassette
\LSR A                  \ This instruction is commented out in the original
                        \ source. It would halve the value of z_hi to 48, so the
                        \ ship would start off closer to the viewer

 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

Code variation 13 of 35Related to Elite's use of the Tube

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Cassette, Docked, Electron
Master
6502SP
 LDY #6                 \ Move the text cursor to column 6
 STY XC
 LDA #6                 \ Move the text cursor to column 6
 STA XC
 LDA #6                 \ Move the text cursor to column 6
 JSR DOXC

Code variation 14 of 35Specific to an individual platform

This variation is blank in the Disc (docked), 6502 Second Processor, Master and Electron versions.

Cassette
 JSR DELAY              \ Wait for 6/50 of a second (0.12 seconds)

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

Code variation 15 of 35Related to Elite's use of the Tube

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Cassette, Docked, Electron
Master
6502SP
 LDY #6                 \ Move the text cursor to column 6 again
 STY XC

 INC YC                 \ Move the text cursor down a row
 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
 STA XC
 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")

Code variation 16 of 35Minor and very low-impact

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Cassette, Electron
Docked, 6502SP, Master
 LDA #254               \ Print recursive token 94 ("BY D.BRABEN & I.BELL")
 JSR TT27
 LDA #13                \ Print extended token 13 ("BY D.BRABEN & I.BELL")
 JSR DETOK

.awe

Code variation 17 of 35Specific to an individual platform

This variation is blank in the Cassette, Disc (docked), 6502 Second Processor and Electron versions.

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

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

Code variation 18 of 35Specific to an individual platform

This variation is blank in the Cassette, Master and Electron versions.

Docked, 6502SP
 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

Code variation 19 of 35Related to the Master version

The Master version shows the "Load New Commander (Y/N)?" prompt on row 20, while the other versions show it one line lower, on row 21.

See below for more variations related to this code.

This variation is blank in the Cassette and Electron versions.

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Docked
Master
6502SP
 LDA #7                 \ Move the text cursor to column 7
 STA XC

 LDA #10                \ Move the text cursor to row 10
 STA YC
 LDA #20                \ Move the text cursor to row 20
 STA YC

 LDA #1                 \ Move the text cursor to column 1
 STA XC
 LDA #7                 \ Move the text cursor to column 7
 JSR DOXC

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

Code variation 20 of 35Specific to an individual platform

This variation is blank in the Cassette, Master and Electron versions.

Docked, 6502SP
                        \ 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

Code variation 21 of 35Related to the Master version

See variation 19 above for details.

This variation is blank in the Master version.

Cassette, Docked, 6502SP, Electron
 JSR CLYNS              \ Clear the bottom three text rows of the upper screen,
                        \ and move the text cursor to the first cleared row.
                        \ 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)

Code variation 22 of 35Related to the Master version

The Master version shows the copyright year on the title screen as 1986 rather than 1984.

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Cassette, Electron
Docked
Master
6502SP
 PLA                    \ Restore the recursive token number we stored on the
 JSR ex                 \ stack at the start of this subroutine, and print that
                        \ token

 LDA #148               \ Set A to recursive token 148

 LDX #7                 \ Move the text cursor to column 7
 STX XC

 JSR ex                 \ Print recursive token 148 ("(C) ACORNSOFT 1984")
 PLA                    \ Restore the recursive token number we stored on the
                        \ stack at the start of this subroutine

 JSR DETOK              \ Print the extended token in A

 LDA #12                \ Set A to extended token 12

 LDX #7                 \ Move the text cursor to column 7
 STX XC

 JSR DETOK              \ Print extended token 12 ("({single cap}C) ACORNSOFT
                        \ 1984")
 PLA                    \ Restore the recursive token number we stored on the
                        \ stack at the start of this subroutine

 JSR DETOK              \ Print the extended token in A

 LDA #7                 \ Move the text cursor to column 7
 STA XC

 LDA #12                \ Print extended token 12 ("({single cap}C) ACORNSOFT
 JSR DETOK              \ 1986")
 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")

Code variation 23 of 35Related to the 6502SP version

The 6502SP version adds two loop counters to the title screen so we can start the demo after a certain period on the title screen.

See below for more variations related to this code.

This variation is blank in the Cassette, Disc (docked) and Electron versions.

6502SP, Master
 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

Code variation 24 of 35Specific to an individual platform

This variation is blank in the Cassette, Disc (docked), 6502 Second Processor and Electron versions.

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

.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

Code variation 25 of 35Minor and very low-impact

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Cassette, Docked, Electron
Master
6502SP
 LDA #128               \ Set z_lo = 128, so the closest the ship gets to us is
 STA INWK+6             \ z_hi = 1, z_lo = 128, or 256 + 128 = 384
 LDX distaway           \ Set z_lo to the distance value we passed to the
 STX INWK+6             \ routine, so this is the closest the ship gets to us
 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

Code variation 26 of 35Related to the 6502SP version

The 6502SP version only scans for key presses every four iterations on the title screen (as opposed to every iteration in the other versions), so you have to hold down "Y", "N" or Space for noticeably longer to load a commander or start the game.

This variation is blank in the Cassette, Disc (docked), Master and Electron versions.

6502SP
 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

Code variation 27 of 35Minor and very low-impact

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Cassette, Docked, Electron
Master
6502SP
 ASL A                  \ Set A = 0

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

 STA INWK+3             \ Set y_lo = 0, so the ship remains in the screen centre
 LDA #0                 \ Set x_lo = 0, so the ship remains in the screen centre
 STA INWK

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

Code variation 28 of 35Specific to an individual platform

This variation is blank in the 6502 Second Processor version.

Cassette, Docked, Master, Electron
 DEC MCNT               \ Decrement the main loop counter

Code variation 29 of 35Related to the Electron version

As joysticks are not supported in the Electron version, it doesn't check for the joystick fire button being pressed during the "Press Fire Or Space,Commander." stage of the title screen.

See below for more variations related to this code.

This variation is blank in the Electron version.

Tap on a block to expand it, and tap it again to revert.

Cassette, Docked
Master
6502SP
 LDA VIA+&40            \ Read 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
IF _SNG47

 LDA VIA+&40            \ Read 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

ELIF _COMPACT

 JSR RDFIRE             \ Call RDFIRE to check whether the joystick's fire
                        \ button is being pressed

 BCS TL3                \ If the C flag is set then the joystick fire button
                        \ is being pressed, so jump to TL3

ENDIF
 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

Code variation 30 of 35Minor and very low-impact

This variation is blank in the Disc (docked), Master and Electron versions.

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Cassette
6502SP
\TAX                    \ This instruction is commented out in the original
                        \ source; it would have no effect, as the comparison
                        \ flags are already set by the AND, and the value of X
                        \ is not used anywhere
 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

Code variation 31 of 35Related to the Electron version

See variation 29 above for details.

This variation is blank in the Electron version.

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Cassette, Docked, 6502SP
Master
 BEQ TL2                \ If the joystick fire button is pressed, jump to TL2
IF _SNG47

 BEQ TL3                \ If the joystick fire button is pressed, jump to TL3

ENDIF

Code variation 32 of 35Related to Elite's use of the Tube

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Cassette, Docked, Master, Electron
6502SP
 JSR RDKEY              \ Scan the keyboard for a key press

 BEQ TLL2               \ If no key was pressed, loop back up to move/rotate
                        \ the ship and check again for a key press

 RTS                    \ Return from the subroutine
 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

Code variation 33 of 35Related to the 6502SP version

See variation 23 above for details.

This variation is blank in the Cassette, Disc (docked), Master and Electron versions.

6502SP
 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

Code variation 34 of 35Related to the Electron version

See variation 29 above for details.

This variation is blank in the Electron version.

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Cassette, Docked
Master
6502SP
.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

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

Code variation 35 of 35Specific to an individual platform

This variation is blank in the Electron version.

Cassette, Docked, 6502SP, Master
 RTS                    \ Return from the subroutine