
Students Guide to Information
Technology
The memory map
I've said that memory on the PC is limited to 640K. This is not
strictly true - the CPU is able to access up to
1
Mb. However, the
area of memory above 640K (the top 360K) is reserved for system
tasks such as controlling the output to the monitor. The 640K is the
amount of memory reserved for programs and data, including the
operating system itself which takes up about 40K. (The exact
amount depends on the version on the operating system.)
It is in fact possible to add many Mbytes of expanded' memory.
The CPU is not able to access this directly, but what it can do is
swap a 64K chunk o ...