Version analysis of DK4 - Elite on the 6502

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

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

Code variations between these versions are shown below.

       Name: DK4
       Type: Subroutine
   Category: Keyboard
    Summary: Scan for pause, configuration and secondary flight keys
  Deep dive: The key logger


 Scan for pause and configuration keys, and if this is a space view, also scan
 for secondary flight controls.

 Specifically:

   * Scan for the pause button (COPY) and if it's pressed, pause the game and
     process any configuration key presses until the game is unpaused (DELETE)

   * If this is a space view, scan for secondary flight keys and update the
     relevant bytes in the key logger

Code variation 1 of 25A variation in the comments only

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

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

Docked
Flight
6502SP
 Other entry points:

   DK9                  Set the Bitstik configuration option to the value in A
 Other entry points:

   DK5                  Contains an RTS
 Other entry points:

   FREEZE               Rejoin the pause routine after processing a screen save


.DK4

Code variation 2 of 25Related to Elite's use of the Tube

This variation is blank in the Master version.

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Cassette, Flight, Docked, Electron
6502SP
 JSR RDKEY              \ Scan the keyboard for a key press and return the
                        \ internal key number in A and X (or 0 for no key press)
 LDX KTRAN              \ Fetch the internal key number of the current key
                        \ press from the key logger buffer

Code variation 3 of 25Specific to an individual platform

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Cassette, Flight, Docked, 6502SP
Master
Electron
 STX KL                 \ Store X in KL, byte #0 of the key logger

 CPX #&69               \ If COPY is not being pressed, jump to DK2 below,
 BNE DK2                \ otherwise let's process the configuration keys
 LDX KL                 \ Fetch the key pressed from byte #0 of the key logger

 CPX #&8B               \ If COPY is not being pressed, jump to DK2 below,
 BNE DK2                \ otherwise let's process the configuration keys
 STX KL                 \ Store X in KL, byte #0 of the key logger

 CPX #&38               \ If COPY is not being pressed, jump to DK2 below,
 BNE DK2                \ otherwise let's process the configuration keys

.FREEZE

                        \ COPY is being pressed, so we enter a loop that
                        \ listens for configuration keys, and we keep looping
                        \ until we detect a DELETE key press. This effectively
                        \ pauses the game when COPY is pressed, and unpauses
                        \ it when DELETE is pressed

Code variation 4 of 25Specific to an individual platform

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Cassette, Flight, Docked, 6502SP, Master
Electron
 JSR WSCAN              \ Call WSCAN to wait for the vertical sync, so the whole
                        \ screen gets drawn
 JSR DEL8               \ Call DEL8 to wait for 30 delay loops

Code variation 5 of 25Specific to an individual platform

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Cassette, Flight, Docked, 6502SP, Electron
Master
 JSR RDKEY              \ Scan the keyboard for a key press and return the
                        \ internal key number in A and X (or 0 for no key press)
 JSR RDKEY              \ Scan the keyboard for a key press and return the
                        \ ASCII code of the key pressed in X (or 0 for no key
                        \ press)

Code variation 6 of 25Specific to an individual platform

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Cassette, Flight, Docked, 6502SP, Electron
Master
 CPX #&51               \ If "S" is not being pressed, skip to DK6
 BNE DK6

 LDA #0                 \ "S" is being pressed, so set DNOIZ to 0 to turn the
 STA DNOIZ              \ sound on
 CPX #'Q'               \ If "Q" is not being pressed, skip to DK6
 BNE DK6

 LDX #&FF               \ "Q" is being pressed, so set DNOIZ to &FF to turn the
 STX DNOIZ              \ sound off

 LDX #'Q'               \ Set X to the ASCII for "Q" once again, so it doesn't
                        \ get changed by the above

.DK6

Code variation 7 of 25Specific to an individual platform

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Cassette, Flight, Docked, 6502SP, Electron
Master
 LDY #&40               \ We now want to loop through the keys that toggle
                        \ various settings. These have internal key numbers
                        \ between &40 (CAPS LOCK) and &46 ("K"), so we set up
                        \ the first key number in Y to act as a loop counter.
                        \ See subroutine DKS3 for more details on this
 LDY #0                 \ We now want to loop through the keys that toggle
                        \ various settings, so set a counter in Y to work our
                        \ way through them

.DKL4

 JSR DKS3               \ Call DKS3 to scan for the key given in Y, and toggle
                        \ the relevant setting if it is pressed

 INY                    \ Increment Y to point to the next toggle key

Code variation 8 of 25Related to the 6502SP version

The Executive version supports two new configuration keys when paused: "@" toggles infinite jump range, while ":" toggles speech.

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Cassette, Flight, Docked, Electron
Master
6502SP
 CPY #&47               \ The last toggle key is &46 (K), so check whether we
                        \ have just done that one
 CPY #9                 \ Check to see whether we have reached the last toggle
                        \ key
IF _SNG45 OR _SOURCE_DISC

 CPY #&47               \ The last toggle key is &46 (K), so check whether we
                        \ have just done that one

ELIF _EXECUTIVE

 CPY #&49               \ The last toggle key is &48 (:), so check whether we
                        \ have just done that one

ENDIF

 BNE DKL4               \ If not, loop back to check for the next toggle key

Code variation 9 of 25A variation in the labels only

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

Cassette, Flight, Docked, Electron
.DK55

Code variation 10 of 25Specific to an individual platform

This variation is blank in the Master version.

Cassette, Flight, Docked, 6502SP, Electron
 CPX #&10               \ If "Q" is not being pressed, skip to DK7
 BNE DK7

 STX DNOIZ              \ "Q" is being pressed, so set DNOIZ to X, which is
                        \ non-zero (&10), so this will turn the sound off

.DK7

 CPX #&70               \ If ESCAPE is not being pressed, skip over the next
 BNE P%+5               \ instruction

Code variation 11 of 25Related to the Master version

The Master version allows you to change the volume of the sound effects using the "<" and ">" keys when the game is paused.

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

Master
 LDA VOL                \ Fetch the current volume setting into A

 CPX #'.'               \ If "." is being pressed (i.e. the ">" key) then jump
 BEQ DOVOL1             \ to DOVOL1 to increase the volume

 CPX #','               \ If "," is not being pressed (i.e. the "<" key) then
 BNE DOVOL4             \ jump to DOVOL4 to skip the following

 DEC A                  \ The volume down key is being pressed, so decrement the
                        \ volume level in A

 EQUB &24               \ Skip the next instruction by turning it into &24 &1A,
                        \ or BIT &001A, which does nothing apart from affect the
                        \ flags

.DOVOL1

 INC A                  \ The volume up key is being pressed, so increment the
                        \ volume level in A

 TAY                    \ Copy the new volume level to Y

 AND #%11111000         \ If any of bits 3-7 are set, skip to DOVOL3 as we have
 BNE DOVOL3             \ either increased the volume past the maximum volume of
                        \ 7, or we have decreased it below 0 to -1, and in
                        \ neither case do we want to change the volume as we are
                        \ already at the maximum or minimum level

 STY VOL                \ Store the new volume level in VOL

.DOVOL3

 PHX                    \ Store X on the stack so we can retrieve it below after
                        \ making a beep

 JSR BEEP               \ Call the BEEP subroutine to make a short, high beep at
                        \ the new volume level

 LDY #10                \ Wait for 10/50 of a second (0.2 seconds)
 JSR DELAY

 PLX                    \ Restore the value of X we stored above

.DOVOL4

Code variation 12 of 25Minor and very low-impact

This variation is blank in the Master version.

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Cassette, Flight, 6502SP, Electron
Docked
 JMP DEATH2             \ ESCAPE is being pressed, so jump to DEATH2 to end
                        \ the game
 JMP BR1                \ ESCAPE is being pressed, so jump to BR1 to end the
                        \ game

Code variation 13 of 25Specific to an individual platform

This variation is blank in the Cassette and Electron versions.

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

Flight, Docked, 6502SP
Master
 CPX #&64               \ If "B" is not being pressed, skip to nobit
 BNE nobit
 CPX #'B'               \ If "B" is not being pressed, skip to DOVOL2
 BNE DOVOL2

Code variation 14 of 25Related to an enhanced feature

The enhanced versions support a new Bitstik configuration option, "B", which toggles the Bitstik when the game is paused.

This variation is blank in the Cassette and Electron versions.

Flight, Docked, 6502SP, Master
 LDA BSTK               \ Toggle the value of BSTK between 0 and &FF
 EOR #&FF
 STA BSTK

 STA JSTK               \ Configure JSTK to the same value, so when the Bitstik
                        \ is enabled, so is the joystick

 STA JSTE               \ Configure JSTE to the same value, so when the Bitstik
                        \ is enabled, the joystick is configured with reversed
                        \ channels

Code variation 15 of 25Related to the Master version

The Master version makes two beeps when the Bitstik is configured and one when it is disabled, while the disc and 6502SP versions remain totally silent and give no clue as to whether you just turned the Bitstik on or off.

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

Master
 BPL P%+5               \ If we just toggled the Bitstik off (i.e. to 0, which
                        \ is positive), then skip the first of these two
                        \ instructions, so we get two beeps for on and one beep
                        \ for off

 JSR BELL               \ We just enabled the Bitstik, so give two standard
                        \ system beeps (this being the first)

 JSR BELL               \ Make another system beep

Code variation 16 of 25A variation in the labels only

This variation is blank in the Cassette and Electron versions.

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

Flight, Docked, 6502SP
Master
.nobit
.DOVOL2

Code variation 17 of 25Related to the 6502SP version

The 6502SP version lets you take screenshots, by pressing "D" when the game is paused.

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

6502SP
 CPX #&32               \ If "D" is being pressed, jump to savscr to save a
 BEQ savscr             \ screenshot

Code variation 18 of 25Specific to an individual platform

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

Master
 CPX #'S'               \ If "S" is not being pressed, jump to DK7
 BNE DK7

 LDA #0                 \ "S" is being pressed, so set DNOIZ to 0 to turn the
 STA DNOIZ              \ sound on

.DK7

 CPX #&1B               \ If ESCAPE is not being pressed, skip over the next
 BNE P%+5               \ instruction

 JMP DEATH2             \ ESCAPE is being pressed, so jump to DEATH2 to end
                        \ the game

Code variation 19 of 25Specific to an individual platform

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

Cassette, Flight, Docked, 6502SP, Electron
Master
 CPX #&59               \ If DELETE is not being pressed, we are still paused,
 BNE FREEZE             \ so loop back up to keep listening for configuration
                        \ keys, otherwise fall through into the rest of the
                        \ key detection code, which unpauses the game
 CPX #&7F               \ If DELETE is not being pressed, we are still paused,
 BNE FREEZE             \ so loop back up to keep listening for configuration
                        \ keys, otherwise fall through into the rest of the
                        \ key detection code, which unpauses the game

.DK2

Code variation 20 of 25Minor and very low-impact

This variation is blank in the Master version.

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

Cassette, Flight, Electron
Docked, 6502SP
 LDA QQ11               \ If the current view is non-zero (i.e. not a space
 BNE DK5                \ view), return from the subroutine (as DK5 contains
                        \ an RTS)
 LDA QQ11               \ If the current view is non-zero (i.e. not a space
 BNE out                \ view), return from the subroutine (as out contains
                        \ an RTS)

Code variation 21 of 25Related to an enhanced feature

The key logger scans for an additional secondary flight key, "P", which disables the docking computer.

This variation is blank in the Master version.

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

Cassette, Electron
Flight, Docked, 6502SP
 LDY #15                \ This is a space view, so now we want to check for all
                        \ the secondary flight keys. The internal key numbers
                        \ are in the keyboard table KYTB from KYTB+8 to
                        \ KYTB+15, and their key logger locations are from KL+8
                        \ to KL+15. So set a decreasing counter in Y for the
                        \ index, starting at 15, so we can loop through them
 LDY #16                \ This is a space view, so now we want to check for all
                        \ the secondary flight keys. The internal key numbers
                        \ are in the keyboard table KYTB from KYTB+8 to
                        \ KYTB+16, and their key logger locations are from KL+8
                        \ to KL+16. So set a decreasing counter in Y for the
                        \ index, starting at 16, so we can loop through them

Code variation 22 of 25Specific to an individual platform

This variation is blank in the Master version.

Cassette, Flight, Docked, 6502SP, Electron
 LDA #&FF               \ Set A to &FF so we can store this in the keyboard
                        \ logger for keys that are being pressed

Code variation 23 of 25Specific to an individual platform

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

Cassette, Flight, 6502SP, Electron
.DKL1

 LDX KYTB,Y             \ Get the internal key number of the Y-th flight key
                        \ the KYTB keyboard table

 CPX KL                 \ We stored the key that's being pressed in KL above,
                        \ so check to see if the Y-th flight key is being
                        \ pressed

 BNE DK1                \ If it is not being pressed, skip to DK1 below

 STA KL,Y               \ The Y-th flight key is being pressed, so set that
                        \ key's location in the key logger to &FF

.DK1

 DEY                    \ Decrement the loop counter

 CPY #7                 \ Have we just done the last key?

 BNE DKL1               \ If not, loop back to process the next key

Code variation 24 of 25A variation in the labels only

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

Cassette, Flight, Docked, Electron
.DK5

 RTS                    \ Return from the subroutine

Code variation 25 of 25Specific to an individual platform

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

Docked
.DK9

 STA BSTK               \ DK9 is called from DOKEY using a BEQ, so we know A is
                        \ 0, so this disables the Bitstik and switches to
                        \ keyboard or joystick

 BEQ DK4                \ Jump back to DK4 in DOKEY (this BEQ is effectively a
                        \ JMP as A is always zero)