Information Technology Reference
In-Depth Information
4.
As initial processing proceeds, the CPU needs to retrieve one
instruction after another, but the CPU does not remember
where these instructions are found. Thus, at each step, the
CPU asks the highspeed cache for the next instructions. If the
instructions are not in cache, the CPU consults main memory;
if the instructions are not there either, then another section
(page) of the program is loaded from virtual memory into
main memory.
5.
Once initial windows are drawn on the screen, the wordpro
cessing package identifies the document to be edited and real
izes that the first part of the document should be retrieved for
display in an editing window. To retrieve this part of the doc
ument, the computer first copies part of the document file to
virtual memory. This version of the document stored in vir
tual memory may become a working copy of the original doc
ument.
6.
The computer then copies the needed section of the working
copy from virtual memory into main memory, and pieces are
brought into the CPU and cache as the display is needed.
Suppose now that the user wants to edit text at the bottom of
the document, so he or she uses the arrow keys to move methodi
cally down the document to the end. At this stage, processing
changes from the initial display to the constant redisplay of addi
tional document material, based on instructions that correspond to
the arrow keys. Here are the main steps for this work:
7.
Use of the arrow keys probably involves different instructions
from the opening of windows, so the CPU needs the arrow
handling part of the program. Upon checking cache and main
memory, the computer realizes this material is needed from
virtual memory. This part of the program then is copied, first
to main memory (likely as one intermediatesized section) and
then to cache (in small pieces) as needed. If space in main
memory is limited, then the instructions for the arrow han
dling may replace the startup instructions that are no longer
needed.
8.
As the program gets to new parts of the document, they, too,
will move to virtual memory, main memory, and cache as
needed for the display. While the virtual memory may retain
