Version analysis of NO3 - Elite on the BBC Micro and NES

This code appears in the following versions (click to see it in the source code):

Cassette, Disc (flight), Disc (docked), 6502 Second Processor and Electron

Code variations between these versions are shown below.

       Name: NO3
       Type: Subroutine
   Category: Sound
    Summary: Make a sound from a prepared sound block


 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

Code variation 1 of 2A variation in the labels only

Tap on a block to expand it, and tap it again to revert.

Cassette, Docked
Flight, 6502SP
Electron
 BNE NO1                \ If DNOIZ is non-zero, then sound is disabled, so
                        \ return from the subroutine (as NO1 contains an RTS)
 BNE KYTB               \ If DNOIZ is non-zero, then sound is disabled, so
                        \ return from the subroutine (as KYTB contains an RTS)
 BNE ECMOF-1            \ If DNOIZ is non-zero, then sound is disabled, so
                        \ return from the subroutine (as ECMOF-1 contains an
                        \ RTS)

Code variation 2 of 2Related to the Electron version

Because the Electron only has two sound channels (one noise and one tone), each sound is allocated a priority and a minimum duration, so higher priority sounds take predence over lower priority sounds, but only for their minimum duration.

This variation is blank in the Cassette, Disc (flight), Disc (docked) and 6502 Second Processor versions.

Electron
 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