Skip to navigation

⌂ ∞ Elite on the BBC Micro and NES

Keyboard: DKS1

[Elite-A, Parasite]

       Name: DKS1                                                    [Show more]
       Type: Subroutine
   Category: Keyboard
    Summary: Scan the keyboard or joystick for a flight key by sending a scan_y
             command to the I/O processor

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * DK4_FLIGHT calls DKS1
             * DKJ1 calls DKS1
             * DOKEY_FLIGHT calls DKS1




 Scan the keyboard for the flight key given in register Y, where Y is the
 offset into the KYTB table above (so we can scan for Space by setting Y to
 2, for example). If the key is pressed, set the corresponding byte in the
 key logger at KL to a negative value (i.e. with bit 7 set).



 Arguments:

   Y                    The offset into the KYTB table above of the key that we
                        want to scan on the keyboard



 Other entry points:

   b_quit               Contains an RTS



.DKS1

 LDA #&96               \ Send command &96 to the I/O processor:
 JSR tube_write         \
                        \   =scan_y(key_offset, delta_14b)
                        \
                        \ which will update the roll or pitch dashboard
                        \ indicator to the specified value

 TYA                    \ Send the first parameter to the I/O processor:
 JSR tube_write         \
                        \   * key_offset = Y

 LDA BSTK               \ Send the second parameter to the I/O processor:
 JSR tube_write         \
                        \   * delta14b = BTSK

 JSR tube_read          \ Set A to the response from the I/O processor, which
                        \ will be the key number, but with bit 7 clear if the
                        \ key is not being pressed, or bit 7 set if it is being
                        \ pressed

 BPL b_quit             \ If the response is positive (i.e. bit 7 is clear) then
                        \ the key is not being pressed, so skip the following
                        \ instruction

 STA KL,Y               \ The response is negative, so store this in the Y-th
                        \ byte of the key logger at KL to indicate that the key
                        \ is being pressed

.b_quit

 RTS                    \ Return from the subroutine