Topic 2 Computer Memory

Computer Memory Memory is a storage for keeping data & it has to work fast because it work directly to CPU Random Access Memory Random Access Memory (RAM) is a computer storage temporary holding of data (volatile). Random Access meaning data can be access any order without restriction opposite of RAM is magnetic tape. Memory is consist of transistor & capacitors to create memory cell which represents a single bit of data ,capacitor holds the bit of information a 0 or a 1 ,transistor acts as a switch that control circuitry on the memory chip read the capacitor or change its state. 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.This refresh operation is where dynamic RAM gets 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 is that it takes time and slows down the memory. Single in line memory modules(SIMM)(small size ) Size :3.5 *75inch(9*2cm),30 pin connector Remark :Install a pair to get maximum speed because the bus width is bigger than a single SIMM Latest SIMM size is 4.25*1 inch(11*2.5cm),72pin connector to increase RAM speed up to256MB. Dual in –line memory modules(DIMM) (use memory to increase speed) Size 5.4* 1 inch(14*2.5cm),168 pins or 184 pins ,8 MB to 1 GB Rambus in line memory modules(RIMM) is comparable to DIMM but use a special memory to increase speed. Small Outline dual in-line memory module(SODIMM) 16MB to 1GB to save space for MAC PC) Size “2*1 inch (5*2.5CM),144 pin to 200 pins,16MB to 1GB to save space in mac computer smallest is 144pins to 172pins

Read Only Memory(ROM) ROM is also known as firmware, data store is non-volatile, ROM used a diode to transfer a value of 1 ,if the value is 0 then no diode is used. ROM cannot be reprogram & it has to be throw if it’s is programme is incorrect, ROM is not expensive & used only a little of power ,Rom are normally used for contain programme to control device. Programmable Read Only Memory(PROM)

Instead of creating expensive & time –consuming chips ,developers create ROM which can be coded by anyone using a tools call programmer but it can only be programmed once. Each bit of PROM carry a fuse ,fuse allow current to flow through it & represent “1”,if “0” is needed fuse is blown & stop current to flow through & cannot be joint again. Erasable Programmable Read-Only Memory(EPROM) EPROM unlike ROM & PROM ,they can rewrite many times they use a special tools to emits frequency of ultraviolet light & configure using a programmer that provides between 10 to 13 volts depending type of EPROM used .During programming bits are set to”1” & change to “0” when required, to reuse the EPROM the data is first erased with ultra violet light ,before it is reprogrammed ,cared must be taken as expose to ultra violet light for too long the EPROM can be damaged. Electrically Erasable Programmable Read-Only Memory (EEPROM) EEPROM unlike EPROM which does not need any equipment & erased any data to rewrite, it use voltage to rewrite & erased but is slower because 1 byte are change at a time. Flash Memory Flash memory is much faster than EEPROM because it erased a block or entire chips to rewrite ,flash memory uses in -circuit wiring to apply electric field to the entire chip or section known as block(example :BIOS) Cache Memory Cache memory is to store recently used program or data ,when CPU request a programs or data Cache search for the data then it supplied to CPU at a very short time, if the information or data is not available it would get it from either main memory or hard disk., if the data or information is not available in the cache memory is a “missed” if available is a “hit” Cache memory can increased the speed of the PC but it’s expensive therefore we need only small amount of it. The speed of L2 cache is (30 nanoseconds)(30 billionths of second) which is 2 times faster than the (RAM),so does L2 cache much faster above them speed 233(MHZ)megahertz ,pentium3.5 times. Virtual Memory When PC RAM space is not enough to run many programs at once time virtual memory help to look for areas that have not been used recently & copy them onto the hard disk this free up space in RAM to load new applications, but if it keep copying to & from hard disk the computer will slow down. Other benefit of virtual memory it automatically copying without you notice it & you feel like you have unlimited RAM space. The area of the hard disk that store the RAM image is called a page file ,it hold pages of RAM on the hard disk & the Operating system moves data back & forth between the page file & RAM .On a window machine ,page file have a SWP extension(2-11)