Case 3

Name: Calvin John P Dalino

Section: BSIT-3day

Case Study #:3

Multiprogramming and time-sharing.

• In multiprogramming systems, the running task keeps running until it performs an operation that requires waiting for an external event (e.g. reading from a tape) or until the computer's scheduler forcibly swaps the running task out of the CPU. Multiprogramming systems are designed to maximize CPU usage.
• In time-sharing systems, the running task is required to relinquish the CPU, either voluntarily or by an external event such as a hardware interrupt. Time sharing systems are designed to allow several programs to execute apparently simultaneously. The expression 'time sharing' was usually used to designate computers shared by interactive users at terminals, such as IBM's TSO, and VM/CMS

a.) Name at least two of these problems.

1.) One user can copy another user's program / memory space. This could be very detrimental if, for example, an administrator was running a decryption protocol, and another user stole the decryption program and/or key. It was really irritated for the part of one person if his /her data will copy by another person. The privacy will no longer exist.
2) Resource usage may not be completely controlled, and could cause deadlock for certain users. For example, if user A had resource 1 and was waiting for resource 2, and user B had resource 2 and was waiting for resource 1, deadlock would occur and neither user would be able to make progress in their program, no matter how many time slots they were allocated.

b.) Can we ensure the same degree of security in a time-share machine as we have in a dedicated machine?

Yes it can be ensure because a time-shared central computer can be protected from access from unauthorized users by utilizing automatic tracing equipment included within telephone switching systems, in combination with an independently organized offsite security computer system. The user, seeking to access the central computer through a telephone system, automatically causes his telephone number to be generated within the telephone system and sent to the offsite security computer. Meanwhile, the user provides the time-shared, central computer with a password which is used to address a memory which includes file information on all authorized users. The file information which includes the telephone number of all authorized users, is made available in the offsite security computer for comparison with the telephone number corresponding to the calling user as provided by the telephone switching system. A match indicates that a user, in possession of preauthorized password, is calling from appropriate telephone station. In this case, access is permitted, otherwise access to the central computer will be denied. By using a user nonalterable telephone number, generated in the telephone switching system, falsification of an authorized users identify is prevented. In addition, tampering with the security procedure is prevented by organizing the offsite security computer in such a manner that the user has no access to the system program of the offsite security computer either directly or through the central computer.