What is a RAM Memory
Random-access memory (RAM) is a series of small cards or modules plugged into slots on the motherboard. The CPU can request any data in RAM. It is then located, opened, and delivered to the CPU for processing in a few billionths of a second. Since all the contents of RAM are erased when you turn off the computer, RAM is the temporary or volatile storage location for the computer.
Figure: A RAM Memory Module.
Similar to a microprocessor, a memory chip is an integrated circuit (IC) made of millions of transistors and capacitors. In the most common form of computer memory, dynamic random access memory (DRAM), a transistor and a capacitor are paired to create a memory cell, which represents a single bit of data. The capacitor holds the bit of information – a 0 or a 1. The transistor acts as a switch that lets the control circuitry on the memory chip read the capacitor or change its state.
|HEAT SINKS, FANS||PC POWER SUPPLY||MOTHERBOARD|
|Graphics, Sound and Network Card||What is Cache Memory|
A capacitor is like a small bucket that is able to store electrons. To store a 1 in the memory cell, the bucket is filled with electrons. To store a 0, it is emptied. The problem with the capacitor’s bucket is that it has a leak. In a matter of a few milliseconds, a full bucket becomes empty. Therefore, for dynamic memory to work, either the CPU or the memory controller has to come along and recharge all of the capacitors holding a 1 before they discharge. The capacitors must be energized every 15ms or so (hundreds of times per second) to maintain their charge. To do this, the memory controller reads the memory and then writes it right back. This is refresh operation happens automatically and is how dynamic RAM got its name. Dynamic RAM has to be dynamically refreshed all of the time or it forgets what it is holding. The downside of all of this refreshing compared to SRAM is that it takes time and slows down the memory.
Figure: (a) Empty Memory Cells (a 0), (b) voltage applied to Column and Row lines (c) 4 cells charged
Memory cells are etched onto a silicon wafer in an array of columns (bit lines) and rows (word lines). The intersection of a bit line and a word line constitutes the address of the memory cell.
DRAM works by sending a charge (an electrical signal called a strobe) through the appropriate column (CAS) to activate the transistor at each bit in the column. When writing, the row lines contain the state the capacitor should take on. When reading, the sense-amplifier determines the level of charge in the capacitor. If it is more than 50 percent, it reads it as a 1; otherwise, it reads it as a 0. The counter tracks the refresh sequence based on which rows have been accessed in what order. The length of time necessary to do all this is so short that it is expressed in nanoseconds (billionths of a second). A memory chip rating of 70ns means that it takes 70 nanoseconds to completely read and recharge each cell.
Memory cells alone would be worthless without some way to get information in and out of them. So the memory cells have a whole support infrastructure of other specialized circuits. These circuits perform functions such as:
- Identifying each row and column (row address select and column address select)
- Keeping track of the refresh sequence (counter)
- Reading and restoring the signal from a cell (sense amplifier)
- Telling a cell whether it should take a charge or not (write enable)
Other functions of the memory controller include a series of tasks that include identifying the type, speed and amount of memory and checking for errors.
- When you open an application such as Excel, it is loaded into RAM. To conserve RAM usage, many applications load only the essential parts of the program initially and then load other pieces as needed.
- After an application is loaded, any files that are opened for use in that application are loaded into RAM.
- When you save a file and close the application, the file is written to the specified storage device, and then it and the application are purged from RAM.
How Much RAM do you need?
The amount of RAM actually sitting on memory modules in your computer is your computer’s physical memory. The memory that your operating system uses is referred to as kernel memory. To determine how much RAM your computer needs, look at the memory requirements for each program and add them up.
- You need RAM for the operating system, application software, and data. If your system responds slowly or accesses the hard drive constantly, then you need to add more RAM
|Application||Minimum RAM Required|
|Windows 7||1000 MB|
|Microsoft Office Professional 2007||256 MB|
|Internet Explorer 8||128 MB|
|Adobe Photoshop Elements||512 MB|
|Total RAM required to run all programs simultaneously||2,152 MB or 2.15 GB|
To save data more permanently, you need to save it to the hard drive or to another permanent storage device such as a CD or flash drive.
Read-only memory (ROM) holds all the instructions the computer needs when it is powered on. The data does not get erased when the power is turned off.
Metrics for RAM speed:
System RAM speed is controlled by bus width and bus speed. Bus width refers to the number of bits that can be sent to the CPU simultaneously, and bus speed refers to the number of times a group of bits can be sent each second. A bus cycle occurs every time data travels from memory to the CPU. For example, a 100-MHz 32-bit bus is theoretically capable of sending 4 bytes of data to the CPU 100 million times per second, while a 66-MHz 16-bit bus can send 2 bytes of data 66 million times per second. If you do the sums, you’ll find that simply changing the bus width from 16
bits to 32 bits and the speed from 66 MHz to 100 MHz in our example allows for three times as much data (400 million bytes versus 132 million bytes) to pass through to the CPU every second for processing.
This post contains the content of book Computer Hardware_ Hardware Components and Internal PC Connection