Reading CDDA sound levels
The CD2 System ROM uses the following code to reverse the bits:
;D1:Input, D0:Output reverse16bits: moveq #16-1,d7 moveq #0,d0 rev: lsr #1,d1 bcc zerobit bset d7,d0 zerobit: dbf d7,rev
The CD2 system ROM reads both registers multiple times during each frame (every 16 raster lines, by using the raster interrupt) to get an average value. The displayed vu-meters use a double buffer to avoid getting updated with the average of only the first few register reads (those that were made before the vu-meters were actually drawn to the screen).
This does NOT work on current emulators.
Source for AS and iso image: File:Vumeter1.zip