So to do music and other things during loading requires precise timing. The C64 effectively had interrupt driven loading music and the serial chip handled clocking in of the data bits and signalled an interrupt when the byte of data was ready.
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I think only Firebird games had loading music on cassette. The music was composed by Melvyn Wright ([http://www.melright.com www.melright.com]). The music data was embeded into the data on cassette. The data was read as a group of 3 bits and 8 data bits. The 3 bits determined if the 8 data bits were for data or for music.
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This list is not complete, please add more. Will upload YM of the loading music soon.
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Booty
Comet Game (The)
Harvey Headbanger
Thrust
Spiky Harold[[Media:Spiky_harold-loader.ym]]<br>
=== Disc ===