Routines
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C63D: Play the menu's tune at DE, once, until a key is pressed
Used by the routine at MENU.
The menu (MENU) calls this every time round its loop. TUNE_PLAYED remembers that the tune has been played, so it plays only the first time; a new game clears it with the rest of the variables, so it plays again when the menu comes back. Before each note the whole keyboard is read, and any key stops the tune at once. The same routine as Alien 8's and Pentagram's.
Input
DE the tune
PLAY_TUNE_ONCE C63D LD HL,TUNE_PLAYED played already: done
C640 LD A,(HL)
C641 AND A
C642 RET NZ
C643 SET 0,(HL) not again
PLAY_TUNE_ONCE_0 C645 XOR A A = 0 reads every half-row at once: any key stops it
C646 CALL READ_KEYS
C649 JR Z,PLAY_TUNE_ONCE_1
C64B RET
PLAY_TUNE_ONCE_1 C64C LD A,(DE) $FF ends the tune; play a note, and look at the keys again
C64D CP $FF
C64F JR Z,PLAY_TUNE_0
C651 CALL PLAY_NOTE
C654 JR PLAY_TUNE_ONCE_0
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