19-06-2012, 03:50 PM
Introduction to RAM
Introduction to RAM.ppt (Size: 593 KB / Downloads: 182)
Random-access memory, or RAM, provides large quantities of temporary storage in a computer system.
Remember the basic capabilities of a memory:
It should be able to store a value.
You should be able to read the value that was saved.
You should be able to change the stored value.
A RAM is similar, except that it can store many values.
An address will specify which memory value we’re interested in.
Each value can be a multiple-bit word (e.g., 32 bits).
We’ll refine the memory properties as follows:
Typical memory sizes
Some typical memory capacities:
PCs usually come with 128-256MB RAM.
Many operating systems implement virtual memory, which makes the memory seem larger than it really is.
Most systems allow up to 32-bit addresses. This works out to 232, or about four billion, different possible addresses.
With a data size of one byte, the result is apparently a 4GB memory!
The operating system uses hard disk space as a substitute for “real” memory.
Reading RAM
To read from this RAM, the controlling circuit must:
Enable the chip by ensuring CS = 1.
Select the read operation, by setting WR = 0.
Send the desired address to the ADRS input.
The contents of that address appear on OUT after a little while.
Writing RAM
To write to this RAM, you need to:
Enable the chip by setting CS = 1.
Select the write operation, by setting WR = 1.
Send the desired address to the ADRS input.
Send the word to store to the DATA input.
Dynamic memory
Dynamic memory is built with capacitors.
A stored charge on the capacitor represents a logical 1.
No charge represents a logic 0.
However, capacitors lose their charge after a few milliseconds. The memory requires constant refreshing to recharge the capacitors. (That’s what’s “dynamic” about it.)
Dynamic RAMs tend to be physically smaller than static RAMs.
A single bit of data can be stored with just one capacitor and one transistor, while static RAM cells typically require 4-6 transistors.
This means dynamic RAM is cheaper and denser—more bits can be stored in the same physical area.