Version analysis of TT217 - 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: TT217
       Type: Subroutine
   Category: Keyboard
    Summary: Scan the keyboard until a key is pressed


 Scan the keyboard until a key is pressed, and return the key's ASCII code.
 If, on entry, a key is already being held down, then wait until that key is
 released first (so this routine detects the first key down event following
 the subroutine call).


 Returns:

   X                    The ASCII code of the key that was pressed

   A                    Contains the same as X

   Y                    Y is preserved


 Other entry points:

   out                  Contains an RTS

Code variation 1 of 7A variation in the comments only

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

6502SP, Master
   t                    As TT217 but don't preserve Y, set it to YSAV instead


.TT217

 STY YSAV               \ Store Y in temporary storage, so we can restore it
                        \ later

.t

Code variation 2 of 7Specific to an individual platform

This variation is blank in the Electron version.

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

Cassette
Docked, 6502SP, Master
 JSR DELAY-5            \ Wait for 8/50 of a second (0.16 seconds) to implement
                        \ a simple keyboard debounce and prevent multiple key
                        \ presses being recorded
 LDY #2                 \ Wait for 2/50 of a second (0.04 seconds) to implement
 JSR DELAY              \ a simple keyboard debounce and prevent multiple key
                        \ presses being recorded

Code variation 3 of 7Specific to an individual platform

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

Cassette, Docked, 6502SP
Master
Electron
 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)

 BNE t                  \ If a key was already being held down when we entered
                        \ this routine, keep looping back up to t, until the
                        \ key is released

.t2
 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)

 BNE t                  \ If a key was already being held down when we entered
                        \ this routine, keep looping back up to t, until the
                        \ key is released

.t2
 DEC KEYB               \ Decrement KEYB, so it is now &FF, to indicate that we
                        \ are reading from the keyboard using an OS command

 JSR OSRDCH             \ Call OSRDCH to read a character from the keyboard

 INC KEYB               \ Increment KEYB back to 0 to indicate we are done
                        \ reading the keyboard

Code variation 4 of 7Specific to an individual platform

This variation is blank in the Electron version.

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

Cassette, Docked, 6502SP
Master
 JSR RDKEY              \ Any pre-existing key press is now gone, so we can
                        \ start scanning the keyboard again, returning the
                        \ internal key number in A and X (or 0 for no key press)
 JSR RDKEY              \ Any pre-existing key press is now gone, so we can
                        \ start scanning the keyboard again, returning the
                        \ ASCII code of the key pressed in X (or 0 for no key
                        \ press)

Code variation 5 of 7Specific to an individual platform

This variation is blank in the Electron version.

Cassette, Docked, 6502SP, Master
 BEQ t2                 \ Keep looping up to t2 until a key is pressed

Code variation 6 of 7Related to Elite's use of the Tube

This variation is blank in the Master and Electron versions.

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

Cassette, Docked
6502SP
 TAY                    \ Copy A to Y, so Y contains the internal key number
                        \ of the key pressed

 LDA (TRTB%),Y          \ The address in TRTB% points to the MOS key
                        \ translation table, which is used to translate
                        \ internal key numbers to ASCII, so this fetches the
                        \ key's ASCII code into A
 TAY                    \ Copy A to Y, so Y contains the internal key number
                        \ of the key pressed

 LDA TRANTABLE,Y        \ TRANTABLE points to the MOS key translation table,
                        \ which is used to translate internal key numbers to
                        \ ASCII, so this fetches the key's ASCII code into A

Code variation 7 of 7Specific to an individual platform

This variation is blank in the Electron version.

Cassette, Docked, 6502SP, Master
 LDY YSAV               \ Restore the original value of Y we stored above

 TAX                    \ Copy A into X

.out

 RTS                    \ Return from the subroutine