UPX v5.0.2 by The UPX Team is an executable compressor for several operating systems including ms-dos. It is very useful notably on PC XT where disk space was limited, whatever 5,25 or 3,5 disks or on hard disks. At the time of my Amstrad PC 1512, I was using Lzexe by Fabrice Bellard (who also owned an Amstrad PC 1512 DD), Diet by Teddy Matsumoto, PKLite by Phil Katz (PKWare) then later UPX.