OSBYTE &7A
Simply performs OSBYTE &79 with X=16 to scan the main keyboard skipping the modifier keys and DIP links.
On entry: no parameters On exit: X=&FF : for no key pressed X<&80 : internal keynumber of key pressed
Usage
OSBYTE &7A is used by ROMs on startup to test for a keypress to select themselves, usually on service call &03, in the following manner:
LDA #&7A:JSR OSBYTE :\ Check for keys pressed TXA:BMI Select :\ No key pressed, select CMP #mykeynum:BNE Exit :\ My selection key not pressed, exit LDA #&78:JSR OSBYTE :\ Write the key back to cancel it .Select \ Select myself .Exit
The mykeynum is the internal keycode for the keypress, which is the negative INKEY number EOR'd with &FF. For example, &32 for 'D', &55 for 'N'.
OSBYTE &78 to cancel the keypress can write any value with bit 7 set to 0. The keypress value tested from OSBYTE &7A will always be less than &80, so it is fine to use it as the entry value to OSBYTE &78.
On exit from OSBYTE &7A, Y is undefined, and typically contains &EE pointing to the third keyboard workspace location, and an invalid keynumber.[1]
Paged ROMs that use the above code incorrectly pass Y unchanged to OSBYTE &78 where it sets an undefined keynumber as the second active rollover key. This does not cause a spurious keypress in practice as the value passed is not a valid keypress (it is the 'Kanji' key on a Japanese keyboard), so on the BBC it is treated as 'no keypress'.
If you want your code to rigourously adhere to the API, you would use the following:
LDA #&7A:JSR OSBYTE :\ Check for keys pressed TXA:BMI Select :\ No key pressed, select EOR #mykeynum:BNE Exit :\ My selection key not pressed, exit TAY:LDA #&78:JSR OSBYTE :\ Write the key back to cancel it .Select \ Select myself .Exit
See also
References
- ↑ Stardot forum post by Tom Seddon, 11th December 2024.
Jgharston 21:17, 26 May 2009 (UTC)
