READ81s is a small but high integrated utility for reading 1581 disks on a PCs 3.5" floppy drive.  The old 1581 drive for C64/C128 homecomputers uses a 800KB floppy format which normally is completely unreadable by a PC drive. DOS and even BIOS routines are useless to access a drive with a 1581 disk inside. So READ81s contains not only code for tracking the 1581 file format but also has every code implemented to directly access a PCs disk controller. This is the reason why you shouldn't use the program in multitasking environment - every access of the disk controller from any other task would surely crash the computer.  As a result of this direct access it is possible to read any data from original 1581 disks without additional hardware.