PCBs are associated with each job running on the system. They tell the system all the variables, what they currently are, and which resources are allocated to it. The system saves this information during round robin timeslices, so it can restart jobs.
Each PCB uses about four frames beyond the ABS. More is linked as necessary. Since the workspace is a series of linked frames that get attached to the user as he logs on, it follows that available frames are required in order to log on. There is always the danger that the overflow table is exhausted, and no one, not even the console, is able to log on, rendering the system useless. A reserve overflow system is used to wrap up processes and recover disk space.
Sysbase
Following the D3 system frames (locks, ABS, and User Workspaces) are the rest of the file system. This area is called Sysbase or Base of the D3 file system.
File System
The D3 file system uses the remainder of the disk. This may consist of multiple collections of related files called accounts, or part of the pool of available frames called overflow.
Maxfid
The maximum limit of the frame addresses (essentially, the end of the D3 file system) is kept in a special value called maxfid, or maximum FID. This is the largest frame address available on D3. The number is dependent upon how much disk space a system has, and what already has been allocated. It is possible to allocate additional disk space to a system, assuming more is available, though this requires that the system be shut down and rebooted.
Physical Limitations
The D3 data structure resides either in a separate partition from the host and is accessed by D3 or as a file system under control of the host.
For UNIX: There is a file that holds a table of filenames where the D3 file system resides. Currently there are four table entries available, so disk capacity is limited to the handling capacity of the host operating system for those four table entries. On UNIX, the limit is 4,000 GB.
D3 System Pictorial Diagram
See Also