Random Access Memory (RAM)

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1 Best known form of computer memory. "random access" because you can access any memory cell directly if you know the row and column that intersect at that cell. CS1111 CS Prof J.P. Morrison UCC 33 Magnetic-core memory was the predominant form of random-access computer memory for 20 years (circa ). It used tiny magnetic toroids (rings), the cores, through which wires were threaded to write and read information. Each core represents one bit of information. The cores can be magnetized in two different ways (clockwise or counter clockwise) and the bit stored in a core is zero or one depending on that core's magnetization direction. The wires are arranged to allow an individual core to be set to either a "one" or a "zero", and for its magnetization to be changed, by sending appropriate electric current pulses through selected wires. The process of reading the core causes the core to be reset to a "zero", thus erasing it. This is called destructive readout. Such memory is often just called core memory, or, informally, core. Although core memory had been superseded by semiconductor memory by the end of the 1970s, memory is still occasionally called "core"; in particular, a file recording the contents of memory after a system error is usually called a core dump. Taken from wikipedia.org. 1

2 Modern memory has the same basic configuration as magnetic core memory although the rings have been replaced with electronic memory cells such as the D- Latch. The cells are arranged so that each row represents a memory location where a binary value is stored and the columns represent different bits of those memory locations. Memory is like a matrix where the number of rows identifies the number of memory locations in the memory and the number of columns identifies the number of bits in each memory location. CS1111 CS Prof J.P. Morrison UCC CS

3 41 A memory chip is an integrated circuit (IC) made of millions of transistors and capacitors. In 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 The transistor acts as a switch that lets the control circuitry on the memory chip read the capacitor or change its state. CS1111 CS Prof J.P. Morrison UCC 42 3

4 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. To do this, the memory controller reads the memory and then writes it right back. This refresh operation happens automatically thousands of times per second. 43 Static RAM uses a completely different technology. a form of switch (called a flip-flop) holds each bit of memory. A flip-flop for a memory cell needs four or six transistors and some wiring, but never has to be refreshed. This makes static RAM significantly faster than dynamic RAM. However, because it has more parts, a static memory cell takes a lot more space on a chip than a dynamic memory cell. Therefore you get less memory per chip, and that makes static RAM a lot more expensive. Static RAM is fast and expensive, and dynamic RAM is less expensive and slower. Therefore static RAM is used to create the CPU's cache, while dynamic RAM forms the larger system RAM space 44 4

5 Error Checking memory available today is highly reliable. Most systems simply have the memory controller check for errors at start-up and rely on that. Memory chips with built-in error-checking typically use a method known as parity to check for errors. Parity chips have an extra bit for every 8 bits of data. The way parity works is simple. Let's look at even parity first. The memory chip adds up the total number of 1s in each byte written to the memory. If the total number of 1s is odd, the parity bit is set to "1." If the total is even, the parity bit is set to "0. When the data is read from the memory the process is repeated and checked against the parity bit. Odd parity works the same way, but the parity bit is set to "1" when the total number of 1s in the byte are even. 46 Error Checking The problem with parity is that it discovers errors but does nothing to correct them. Computers in critical positions need a higher level of fault tolerance. High-end servers often have a form of error-checking known as error correction code (ECC). Like parity, ECC uses additional bits to monitor the data in each byte. The difference is that ECC uses several bits for error checking. ECC memory uses a special algorithm not only to detect single bit errors, but to correct them as well. ECC memory will also detect instances when more than one bit of data in a byte fails. Such failures are very rare, and they are not correctable, even with ECC. The majority of computers sold today use non-parity memory chips. These chips do not provide any type of built-in error checking but instead rely on the memory controller for error detection. 47 5

6 How Much Do You Need? you can never have enough RAM. Next to the CPU itself, RAM is the most important factor in computer performance. If you don't have enough, adding RAM can make more of a difference than getting a new CPU! If your system responds slowly or accesses the hard drive constantly, then you need to add more RAM. For Windows 95/98, you needed a minimum of 32 MB, Windows NT/2000 needed at least 64 MB Windows XP: needed 128MB Windows 8: 1 GB (32-bit); 2 GB (64-bit) Windows 10: 4GB Linux works happily on a system with only 4 MB of RAM. If you plan to add X-Windows you'll probably want 64 MB. Ubuntu: 1GB min You may also need more RAM if your computer acts as a server. 50 6