Pc memory: A physical device

Computer memory is any physical device capable of storing information temporarily, like RAM (random access memory), or permanently, like ROM (read-only memory). Memory devices utilize integrated circuits and are employed by operating systems, software, and hardware.

PC's became increasingly memory-hungry in recent years, and therefore the minimum desirable memory seems to travel up year by year. Now, 8 MB of memory is not any longer an outsized amount, it's the minimum required in most situations.

Types of Memory

ROM (read-only memory)

Permanent memory that can't be reprogrammed easily or can't be reprogrammed in the least which retains it's contents when the system is turned off. the essential input or output system software (BIOS) for a PC is installed in ROM.

DRAM (dynamic random-access memory)

Used for the most system memory during a PC RAM is a memory which will be modified easily by the pc. Each memory location in DRAM functions sort of a little battery that's charged and discharged to store data. the fees in DRAM fade with time and a memory-refresh operation is required to take care of data in memory. DRAM isn't as fast as SRAM, but it's considerably less costly and usually is employed for system memory for that reason.

SRAM (static random-access memory)

A type of RAM that doesn't require a refresh operation. Memory locations in SRAM work like switches that applied in retain their settings as long as power. SRAM is quicker than DRAM but is employed sparingly because SRAM costs considerably more. SRAM caches High-performance systems often use to buffer I/O to slower DRAM memory. Much of the performance of a high-speed system relates to the way during which the designers have used SRAM.

When you add memory, it is vital to get memory that matches the speed requirements of your system. All memory types have a speed rating that's laid out in nanoseconds (ns). Speeds range from 40 ns (fastest and most expensive) to about 100 ns (much too slow for many new PCS). Most memory you encounter will have a speed rating within the range of 70 ns. Your system won't run faster if you purchase memory that's faster than it requires. you would possibly want to get memory, however, that's slightly faster than the minimum. Memory performance varies, and a few manufacturers hold tighter tolerances than others. Buying faster memory ensures that the memory you purchase will meet or exceed your system's requirements.

Starting with the 80386, most CPU's have the potential of out-running most DRAM, and system designers have used a variety of techniques to hamper access. An early approach was to use wait states--"do nothing" operations that bogged down processing to a speed that the memory could deal with. Wait states are a significant drag on system performance, you ought to avoid systems that need them. the necessity for wait states was eliminated largely by the incorporation of cache memory into the system design.

Memory Expansion

The discussion about buses makes the purpose that the majority expansion buses operate at much slower speeds than faster CPU's can support. A Pentium processor can perform burst-data transfers rates as high as 528 Mbps and is capable of outrunning even a PCI bus, which is restricted to 60-66 MHz operation. For that reason, it's not practical to put in memory expansion cards with most bus types. With the exception of some proprietary memory-expansion designs, it's become common to put in all memory directly on the motherboard, where it is often more directly serviced by the microprocessor. 

* SIMM memory capacities typically range from 256 KB bits to eight MB bits.

* SIMM's are often used individually. In many cases, however, SIMM's are utilized in banks of two or four SIMM's. you want to review your system specifications to ascertain what you would like. On Pentium systems, SIMM's must be installed in matched pairs.

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