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RAM / ROM / Cache Memory
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 .
RAM(Random Access Memory)
Random Access Memory (RAM) is the main 'working' memory used by the computer. When the
operating system loads from disk when you first switch on the computer, it is copied into RAM. The original
IBM PC could only use up to 640 KB of memory (just over half a megabyte), whereas a modern computer can
effectively house as much RAM as you can afford to buy.
Modern types of writable RAM generally store a bit of data in either the state of a flip-flop, as in SRAM
(static RAM), or as a charge in a capacitor (or transistor gate), as in DRAM (dynamic RAM), EPROM,
EEPROM and Flash. Some types have circuitry to detect and/or correct random faults called memory errors in
the stored data, using parity bits or error correction codes. RAM of the read-only type, ROM, instead uses a
metal mask to permanently enable/disable selected transistors, instead of storing a charge in them. Of special
consideration is SIMM and DIMM memory modules.
SRAM and DRAM are volatile, other forms of computer storage, such as disks and magnetic tapes,
have been used as persistent storage in traditional computers. Many newer products instead rely on flash
memory to maintain data when not in use, such as PDAs or small music players. Certain personal computers,
such as many rugged computers and netbooks, have also replaced magnetic disks with flash drives. With
flash memory, only the NOR type is capable of true random access, allowing direct code execution, and is
therefore often used instead of ROM; the lower cost NAND type is commonly used for bulk storage in memory
cards and solid-state drives.
SDRAM (Synchronous DRAM)
Almost all systems used to ship with 3.3 volt, 168-pin SDRAM DIMMs. SDRAM is not an extension of
older EDO DRAM but a new type of DRAM altogether. SDRAM started out running at 66 MHz, while older fast
page mode DRAM and EDO max out at 50 MHz. SDRAM is able to scale to 133 MHz (PC133) officially, and
unofficially up to 180MHz or higher. As processors get faster, new generations of memory such as DDR and
RDRAM are required to get proper performance.
DDR (Double Data Rate SDRAM)
DDR basically doubles the rate of data transfer of standard SDRAM by transferring data on the up
and down tick of a clock cycle. DDR memory operating at 333MHz actually operates at 166MHz * 2 (aka
PC333 / PC2700) or 133MHz*2 (PC266 / PC2100). DDR is a 2.5 volt technology that uses 184 pins in its
DIMMs. It is incompatible with SDRAM physically, but uses a similar parallel bus, making it easier to
implement than RDRAM, which is a different technology.
Difference between static and dynamic RAM
Dynamic RAM is the most common type of memory in use today. Inside a dynamic RAM chip, each memory
cell holds one bit of information and is made up of two parts: a transistor and a capacitor. These are, of
course, extremely small transistors and capacitors so that millions of them can fit on a single memory chip.
The capacitor holds the bit of information -- a 0 or a 1 (see How Bits and Bytes Work for information on bits).
The transistor acts as a switch that lets the control circuitry on the memory chip read the capacitor or change
its state.
Static RAM uses a completely different technology. In static RAM, a form of flip-flop holds each bit of memory
(see How Boolean Gates Work for detail on flip-flops). A flip-flop for a memory cell takes 4 or 6 transistors
along with 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.
So static RAM is fast and expensive, and dynamic RAM is less expensive and slower. Therefore
static RAM is used to create the CPU's speed-sensitive cache, while dynamic RAM forms the larger system
RAM space.
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Read-only memory (ROM)
Read Only Memory (ROM) as the name suggests is a special type of memory chip which holds
software which can be read but not written to. A good example is the ROM-BIOS chip, which contains readonly software. Often network cards and video cards also contain ROM chips.
ROM is a class of storage media used in computers and other electronic devices. Data stored in ROM
cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware
(software that is very closely tied to specific hardware, and unlikely to need frequent updates).
In its strictest sense, ROM refers only to mask ROM (the oldest type of solid state ROM), which is
fabricated with the desired data permanently stored in it, and thus can never be modified. However, more
modern types such as EPROM and flash EEPROM can be erased and re-programmed multiple times; they
are still described as "read-only memory"(ROM) because the reprogramming process is generally infrequent,
comparatively slow, and often does not permit random access writes to individual memory locations. Despite
the simplicity of mask ROM, economies of scale and field-programmability often make reprogrammable
technologies more flexible and inexpensive, so mask ROM is rarely used in new products as of 2007.
Cache Memory
Cache Memory is fast memory that serves as a buffer between the processor and main memory. The
cache holds data that was recently used by the processor and saves a trip all the way back to slower main
memory. The memory structure of PCs is often thought of as just main memory, but it's really a five or six level
structure:
The first two levels of memory are contained in the processor itself, consisting of the processor's small
internal memory, or registers, and L1 cache, which is the first level of cache, usually contained in the
processor.
The third level of memory is the L2 cache, usually contained on the motherboard. However, the
Celeron chip from Intel actually contains 128K of L2 cache within the form factor of the chip. More and more
chip makers are planning to put this cache on board the processor itself. The benefit is that it will then run at
the same speed as the processor, and cost less to put on the chip than to set up a bus and logic externally
from the processor.
The fourth level, is being referred to as L3 cache. This cache used to be the L2 cache on the
motherboard, but now that some processors include L1 and L2 cache on the chip, it becomes L3 cache.
Usually, it runs slower than the processor, but faster than main memory.
The fifth level (or fourth if you have no "L3 cache") of memory is the main memory itself.
The sixth level is a piece of the hard disk used by the Operating System, usually called virtual
memory. Most operating systems use this when they run out of main memory, but some use it in other ways
as well.
This six-tiered structure is designed to efficiently speed data to the processor when it needs it, and
also to allow the operating system to function when levels of main memory are low.
ROM-BIOS :• The 'Read Only Memory Basic Input Output System' chip is a chip located on the computer's
system (mother) board, which contains software. This software performs a variety of tasks. When you first
switch on the computer the ROM-BIOS software performs a self-diagnostic to check that the computer is
working OK. This software then loads your operating system from the disk into the RAM.
Flash BIOS :• Most modern computers are actually supplied with a flash BIOS rather than a ROMBIOS. This
chip contains exactly the same type of in-built software, but has the advantage that the software on the chip
can be upgraded. This upgrade is achieved by simply running a small program supplied by the computer
manufacturer.
video (graphics) memory:-• The picture which you see on your screen is a form of data and this data has to
be stored somewhere. The on-screen pictures are held in special memory chips called video memory chips;
these chips are usually located on the video card. A modern computer will be supplied with several
Megabytes of video memory.
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