esl, really?
ProDOS was released to address shortcomings in the earlier Apple operating system (called simply DOS), which was beginning to show its age. DOS only had built-in support for 5.25" floppy disks and required patches to use peripheral devices such as hard disk drives and non-Disk II floppy disk drives, including 3.5" floppy drives. ProDOS added a standard method of accessing ROM-based drivers on expansion cards for disk devices, expanded the maximum volume size from about 400 kilobytes to 32 megabytes, introduced support for hierarchical subdirectories (a vital feature for organizing a hard disk's storage space), and supported RAM disks on machines with 128kB or more of memory. ProDOS addressed problems with handling hardware interrupts, and included a well-defined and documented programming and expansion interface, which DOS had always lacked.