Disk-directory system code within a user process

The disk/directory system implemented a two level virtual machine on the ECS system. The two levels were the disk system and the directory system. The disk system implemented the concept of disk files, which the directory system in turn used to implement directories. The original purpose for this division was to modularize the implementation, and make debugging easier by isolating the problems. A later intention was to combine the two layers once the system was debugged, but this point was never reached. The disk system consisted of two subprocesses within each user process, and a number of ECS system processes, invisible to the user. (The user process tree is described in chapter 14, ``A Short Tour of a User Process''.) One of the two subprocesses within the user process was intended to handle F-returns on disk file read and write requests, that is, reads and writes directed to portions of files not currently in ECS. This subprocess was placed in the tree so that it would be in the full path of any subprocess causing such an F-return. Thus it had direct access to the core of the requesting subprocess. Because the entire full path had to be capable of residing in central memory, any subprocess in the full path of a user subprocess reduced the maximum possible size of that user subprocess. Therefore, much of the disk system code was placed in a second subprocess which sat off to one side of the users full path.