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Main memory
- Stores information usually in electrical form
- Can store data and programs
- The computer needs to bring information to memory in order to use it. It cannot use information directly from a floppy or a hard drive.
- Ordinary computer memory can access information in about 50 nanoseconds (10 -9 seconds).
- Memory can be compared with a set of mail boxes:
- Each memory position has a number, the memory address.
- Inside each memory position, there is a value (only one value at a time) the memory contents.
- Memory address do not change, but contents can be changed at any time.
- The CPU can request:
- the contents of a given memory address
- to write a given value to a memory address (changing the previous content)
The figure below illustrates a set of 40 memory locations numbered 00 to 39 (one row at a time).
Contents of position 12 is 5
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | |
| 0 | 15 | 3 | 22 | 2 | 1 | 0 | 0 | 0 | 0 | 2 |
| 1 | 2 | 3 | 5 | 7 | 9 | 9 | 3 | 2 | 1 | 1 |
| 2 | 1 | 1 | 1 | 2 | 1 | 1 | 1 | 1 | 2 | 2 |
| 3 | 4 | 9 | 0 | 567 | 45 | 78 | 32 | 21 | 2 | 5 |