Routines
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41420: Play Note
Used by the routines at PlayMusic_WaitKey and PlayMusic.
Plays a note of a piece of music, using the table at NoteTable. Each entry in the table has three bytes: the lengths of the inner and outer delay loops, which set the pitch, and the number of cycles.
Input
A The next byte of the music
DE The address of the byte of the music
Output
DE The address of the next byte of the music
PlayNote 41420 AND %00111111 Use bits 0-5, the number of the note.
41422 JR Z,PlayRest Jump to PlayRest if it is a rest.
Look up the delays and the number of cycles in the table at NoteTable.
41424 LD L,A Set HL to the note number.
41425 LD H,0
41427 ADD HL,HL Double it.
41428 CALL AddToAddress Add the note number to make three times the number.
41431 LD BC,41510 Point BC at NoteTable.
41434 ADD HL,BC Find the entry of the note.
41435 LD B,(HL) Fetch the length of the inner delay loop.
41436 INC HL Move onto the next byte.
41437 LD C,(HL) Fetch the length of the outer delay loop.
41438 INC HL Move onto the next byte.
41439 LD L,(HL) Fetch the number of cycles.
41440 LD H,0 Clear H.
Bits 6 and 7 of the note are the length, less one.
41442 LD A,(DE) Fetch the byte of the music again.
41443 RLCA Rotate bits 6 and 7 of the note down...
41444 RLCA ...to bits 0 and 1.
41445 AND %00000011 Keep them, but the result is not used.
41447 XOR A Set the multiplier to zero...
41448 INC A ...and then to one.
Multiply the number of cycles by the length of the note.
PlayNote_Multiply 41449 PUSH DE Save the address of the music.
41450 LD E,L Copy the low byte of the number of cycles into E.
41451 LD D,H Copy the high byte into D.
PlayNote_MultiplyLoop 41452 DEC A Decrease the multiplier by one.
41453 JR Z,PlayNote_Multiplied Jump to PlayNote_Multiplied if it is zero.
41455 ADD HL,DE Add the number of cycles again.
41456 JR PlayNote_MultiplyLoop Jump back to count the multiplier down.
PlayNote_Multiplied 41458 POP DE Restore the address of the music.
Each pass of this loop makes one cycle of the note, so the length of the note is the number of cycles.
PlayNote_Cycle 41459 PUSH BC Save the lengths of the delay loops.
41460 XOR A Clear the speaker bit.
41461 OUT (254),A Write to the port.
PlayNote_LowDelay 41463 DJNZ PlayNote_LowDelay Wait for the inner delay loop.
41465 DEC C Decrease the outer loop count by one.
41466 JR NZ,PlayNote_LowDelay Jump back until the outer delay loop has finished.
41468 POP BC Restore the lengths of the delay loops.
41469 PUSH BC Save them again.
41470 LD A,16 Set the speaker bit.
41472 OUT (254),A Write to the port.
PlayNote_HighDelay 41474 DJNZ PlayNote_HighDelay Wait for the inner delay loop.
41476 DEC C Decrease the outer loop count by one.
41477 JR NZ,PlayNote_HighDelay Jump back until the outer delay loop has finished.
41479 POP BC Restore the lengths of the delay loops.
41480 DEC HL Decrease the number of cycles by one.
41481 LD A,H Fetch the high byte of the number of cycles left.
41482 OR L Combine it with the low byte, to test whether it is zero.
41483 JR NZ,PlayNote_Cycle Jump back for the next cycle until the note has finished.
41485 INC DE Move onto the next byte of the music.
41486 RET Return.
View the equivalent code in;
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