System R was designed and developed over a period of 1974-79 at IBM San Jose Research Center. It is a prototype and its purpose was to demonstrate that it is possible to build a Relational System that can be used in a real life environment to solve real life problems, with performance at least comparable to that of existing system.

Its two subsystems are

Ø Research Storage

Ø System Relational Data System.

The repeated execution of a loop of code while waiting for an event to occur is called busy-waiting. The CPU is not engaged in any real productive activity during this period, and the process does not progress toward completion.

It is a useful, memory-saving technique for multiprogrammed timesharing systems. A Reentrant Procedure is one in which multiple users can share a single copy of a program during the same period. Reentrancy has 2 key aspects: The program code cannot modify itself, and the local data for each user process must be stored separately. Thus, the permanent part is the code, and the temporary part is the pointer back to the calling program and local variables used by that program. Each execution instance is called activation. It executes the code in the permanent part, but has its own copy of local variables/parameters. The temporary part associated with each activation is the activation record. Generally, the activation record is kept on the stack.

The DOS CHKDSK (Check Disk) command is a command-line utility that has remained in use with Windows 3.x, 9x, NT, and 2000, and is used to recover lost allocation units from the hard drive. These lost units occur when an application terminates unexpectedly and causes the file management system to lose track of where some parts of the file are stored. The total file becomes segmented into undefined pieces that can still be by the utility but cannot be associated with a particular filename in the FAT. Over a period of lost units can pile up and occupy large amounts of disk space. To remove lost units from the drive, an /F modifier is added to the command so that the lost units will be converted into files that can be investigated, and removed if necessary. In some cases, the converted file is a usable data file that can be rebuilt for use with an application. The CHKDSK /F command is often used before running a drive defragmentation program.

Session tracking is a mechanism that servlets use to maintain state about a series requests from the same user across some period of time. The methods used for session tracking are:
a) User Authentication - occurs when a web server restricts access to some of its resources to only those clients that log in using a recognized username and password
b) Hidden form fields - fields are added to an HTML form that are not displayed in the client's browser. When the form containing the fields is submitted, the fields are sent back to the server
c) URL rewriting - every URL that the user clicks on is dynamically modified or rewritten to include extra information. The extra information can be in the form of extra path information, added parameters or some custom, server-specific URL change.
d) Cookies - a bit of information that is sent by a web server to a browser and which can later be read back from that browser.
e) Http Session- places a limit on the number of sessions that can exist in memory. This limit is set in the session.maxresidents property