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Elite on the BBC Micro

Keyboard: get_key [Elite-A, I/O processor]

Name: get_key [Show more] Type: Subroutine Category: Keyboard Summary: Implement the get_key command (wait for a key press)
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * tube_table calls get_key

This routine is run when the parasite sends a get_key command. It waits until a key is pressed, and returns the key's ASCII code to the parasite. If, on entry, a key is already being held down, then it waits until that key is released first, so this routine detects the first key down event following the receipt of the get_key command.
.get_key JSR WSCAN \ Call WSCAN twice to wait for two vertical syncs JSR WSCAN JSR RDKEY \ Scan the keyboard for a key press and return the \ internal key number in X (or 0 for no key press) BNE get_key \ If a key was already being held down when we entered \ this routine, keep looping back up to get_key, until \ the key is released .press JSR RDKEY \ Any pre-existing key press is now gone, so we can \ start scanning the keyboard again, returning the \ internal key number in X (or 0 for no key press) BEQ press \ Keep looping up to press until a key is pressed TAY \ Copy A to Y, so Y contains the internal key number \ of the key pressed LDA (key_tube),Y \ The address in key_tube points to the MOS key \ translation table in the I/O processor, which is used \ to translate internal key numbers to ASCII, so this \ fetches the key's ASCII code into A JMP tube_put \ Send A back to the parasite and return from the \ subroutine using a tail call