Sound: NO3

       Name: NO3                                                     [Show more]
       Type: Subroutine
   Category: Sound
    Summary: Make a sound from a prepared sound block
    Context: See this subroutine in context in the source code
 Variations: See code variations for this subroutine in the different versions
 References: This subroutine is called as follows:
             * EXNO calls NO3


 Make a sound from a prepared sound block in XX16 (if sound is enabled). See
 routine NOS1 for details of preparing the XX16 sound block.


.NO3

 LDX DNOIZ              \ Set X to the DNOIZ configuration setting

 BNE ECMOF-1            \ If DNOIZ is non-zero, then sound is disabled, so
                        \ return from the subroutine (as ECMOF-1 contains an
                        \ RTS)

 LDA XX16               \ Set X = to bit 0 of the first SOUND parameter, so
 AND #&01               \ that's the channel number (as the channel is either
 TAX                    \ 0 or 1)

 LDY XX16+8             \ We stored the sound number (0, 8, 16 etc.) in XX16+8
                        \ back in NOS1, so fetch it into Y

 LDA SFX2,Y             \ Fetch this sound's byte from SFX2 into A
                        \
                        \ (This seems wrong. Y is a multiple of 8 (0, 8 ... 72)
                        \ rather than the actual sound number (0-9), and there
                        \ are only 10 bytes at SFX2, so this doesn't feel
                        \ correct - surely Y should be divided by 8 before
                        \ fetching the relevant SFX2 byte?)

 CMP SFXPR,X            \ If the new sound's SFX2 byte is less than the current
 BCC ECMOF-1            \ channel's SFXPR value, return from the subroutine as
                        \ the current sound has a higher priority than the new
                        \ one (as ECMOF-1 contains an RTS)

 STA SFXPR,X            \ Otherwise the new sound is a higher priority sound, so
                        \ store the new sound's SFX2 byte as the channel's new
                        \ SFXPR priority

 AND #%00001111         \ And store the low nibble of the SFX2 byte in the
 STA SFXDU,X            \ channel's new SFXDU duration

 LDX #LO(XX16)          \ Otherwise set (Y X) to point to the sound block in
 LDY #HI(XX16)          \ XX16

 LDA #7                 \ Call OSWORD 7 to makes the sound, as described in the
 JMP OSWORD             \ documentation for variable SFX, and return from the
                        \ subroutine using a tail call