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Elite on the BBC Micro and NES

Version analysis of NOS1

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

Code variations between these versions are shown below.

Name: NOS1 Type: Subroutine Category: Sound Summary: Prepare a sound block
Copy four sound bytes from SFX into XX16, interspersing them with null bytes, with Y indicating the sound number to copy (from the values in the sound table at SFX). So, for example, if we call this routine with A = 40 (long, low beep), the following bytes will be set in XX16 to XX16+7: &13 &00 &F4 &00 &0C &00 &08 &00 This block will be passed to OSWORD 7 to make the sound, which expects the four sound attributes as 16-bit big-endian values - in other words, with the low byte first. So the above block would pass the values &0013, &00F4, &000C and &0008 to the SOUND statement when used with OSWORD 7, or: SOUND &13, &F4, &0C, &08 as the high bytes are always zero.
Arguments: A The sound number to copy from SFX to XX16, which is always a multiple of 8

Code variation 1 of 2Specific to an individual platform

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

STA XX16+8 \ Store the sound number in XX16+8, so we can retrieve \ it in the NO3 routine
 LSR A                  \ Divide A by 2, and also clear the C flag, as bit 0 of
                        \ A is always zero (as A is a multiple of 8)

 ADC #3                 \ Set Y = A + 3, so Y now points to the last byte of
 TAY                    \ four within the block of four-byte values

 LDX #7                 \ We want to copy four bytes, spread out into an 8-byte
                        \ block, so set a counter in Y to cover 8 bytes


 LDA #0                 \ Set the X-th byte of XX16 to 0

 DEX                    \ Decrement the destination byte pointer

 LDA SFX,Y              \ Set the X-th byte of XX16 to the value from SFX+Y

 DEY                    \ Decrement the source byte pointer again

 DEX                    \ Decrement the destination byte pointer again

 BPL NOL1               \ Loop back for the next source byte

Code variation 2 of 2A variation in the labels only

This variation is blank in the Electron version.

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

RTS \ Return from the subroutine
\ Fall through into KYTB to return from the subroutine, \ as the first byte of KYTB is an RTS