Information Technology Reference
In-Depth Information
start that print job. When that is done, the operating system can go on to
the third request. Altogether, different users or programs may make re
quests to print, but the operating system has the final say on which request
is fulfilled and when. In the terminology of computing, a printer is consid
ered a computing resource, and the operating system is responsible for
controlling which user or task has access to what resource and when.
Memory Example
Consider again the printing example, but this time from the standpoint of main mem
ory. As we know from Chapter 1, working with each of the two document windows
will require that some parts of the programs and some data be stored in main mem
ory. Each document window corresponds to a running program, and each document
contains information; the CPU will get its instructions and data for each document
from main memory. For both programs to run and appear on the screen, parts of
main memory will have to be allocated to the first window, and parts to the second.
Again, such memory allocation requires coordination. Not only must the operat
ing system allocate different parts of memory to different tasks, but also these
allocations must be remembered and utilized so that the material for one docu
ment is not confused with that of the other. To avoid errors, such allocation re
quires careful bookkeeping and administration.
Bank/File Example
Consider the processing involved within a bank for a deposit or withdrawal.
Typically, account information is stored on a file—perhaps on a disk. To be spe
cific, the processing of the withdrawal of $50 from a checking account (initial
balance $600) can use these steps:
1.
Determine old balance ($600) in the account.
2.
Deduct the $50 withdrawal.
3.
Record the new balance ($550) in the account.
This outline works quite well until both a husband and wife are on business in
separate cities and each decides to withdraw $50 from different branches of the
