Processors And Memory

Part 3 - Random Access Memory

This subsection deals with Random Access memory. Memory is basically a series of cells with an address. Each memory cell stores a small piece of information and is identified by a unique address location, much like an email address or street address, so that the processor knows exactly where the cell is and can easily access it. Computers use several different types of memory, each serving a different purpose in the overall operation of things.

RAM, often referred to as main memory, or Primary Memory or Read-Write Memory is a temporary form of memory that the computer uses as a work area. This type of memory is dynamic, meaning that it is constantly changing because of the activity of the CPU. When you shut off the power to the computer, RAM loses everything stored in it. RAM stores program instructions and related data for the CPU to access quickly without the need for extracting the data from a much slower form of memory, such as the Hard Disk.

Generally, The Closer the Memory is to the CPU's ALU and CU, the faster the memory will be. As a result, it will be the most expensive, and the most limited type of memory in terms of availability and size.

RAM - Random Access Memory Classes

Register Memory - This type of Memory is actually located inside the CPU. It tends to be small and measured in bits. The CPU tends to have a limited number of Registers, many of which are for instructions, and special purposes, while others are for general purposes. Registers are accessed duringg any CPU Cycle but they and extremely expensive.

Cache Memory - By Definition, it is the storage of data duplicating original values stored elsewhere. Cached data tends to be more quickly accessible than from its original location. The cache is also used to temporarly store instructions, and information computed to be reused for other calculations, speeding up the time it takes to perform many computing tasks.

• There are a number lof Cache levels, and Cache can be refered to as L1 Cache, L2, L3, etc. The Lower the number, the closer to the CPU's ALU and CU the cache Resides.
• The lower the number, also means the memory type is faster, and the quicker the data can be accessed. The amount of cache is also smaller in the Lower faster levels, and higher in the farther levels.
• When looking for information, the Control Unit will first look in L1 cache, then L2, etc.
• In old days, there was special programs to create Hard Drive Caches in Memory to speed up access. Today this is part of the Operating system, and is almost totally seamless to the user, though the terms Write Back Cache and Read Ahead Cache are encountered from timme to time.
• a Cache Hit occurs when the information being sought is found in Cache
• a Cache Miss occurs when the information is not found
• Much Older systems did not have Cache on the CPU, rather fairly expensive high speed cache chips were located on the motherboard, to speed up memory access by the CPU.
• Today Cache sizes range from Low KB for L1 cache, to High KB or MB for L3 Cache./li>
• Cache memory is also found in other devices, like Hard Disks Drives, and Controllers to again Increase the speed at which data is accessed.

Main Memory - This is the commonly known Computer RAM. There are various types of Momory Modules that you can have in your system, and they will be discussed in the next subsection.

Virtual Memory - is, in fact, not real memory, rather, it is usually part of Hard disk that has been set aside to be used like RAM, in order to increase the amount of Memory the system works with.

• Virtual memory is usually stored in a file called a <Pagefile or a Swapfile. Many modern Operating Systems support multiple page files, accross multiple Hard disks.
• Swapping or Paging is the term used when information is Transfered between Physical RAM and Virtuual RAM. A high amount of this can slow down applications significantly

RAM Disk - In Contrast, is the use of Physical Memory to create a Virtual, very High-speed Drive.

• RAM Disks tend to be Very high speed, however expensive compared to storaage devices, and, the data is not permanent
• In older systems where hard drives were very slow, or only Floppy drives were available, a Small RAM Disk could allow for the temporary storage of temporary files and/or configuration information to speed up applications.
• Today RAM Disks are used by Bootable CDs and Disks to store their temporary files
• In some cases it may be advisable to setup a RAM Disk to with programs that are extremely intensive on Data Read/Writes during processing, however there is no impact if that information was lost

Shadow Ram - Is a section of RAM that stores a COPY of ROM, because it is easier and faster to access. The CPU in turn accesses this area, instead of trying to access the Real ROM.

RAM - Random Access Memory Classes

DRAM - Dynamic RAM is a type of memory that uses capacitors and transistors to store electrical chrges that represent the data. The capacitors that make up this type of memory needs to be refreshed every millisceond and cannot be reached by the CPU during this time. The frequence that memory has bte be refreshed is known as the Refresh Rate There are many types of Dynamic RAM used over the years:

• Fast Page Mode DRAM - FPM - used a technology to keep areas of Memory open so that reading or writing to and from Ram was faster
• Video DRAM - VRAM - Old now obsolete Memory used on Grapbhics Cards and for video operations.
• Extended Data Out DRAM - EDO - A technology newer than FPM that could gain 5% speed Advantage. Burst EDO - BEDO DRAM increased speed by up to 50%
• Synchronous DRAM - SDRAM - The basic technology that most of todays current Memory uses. While other forms of DRAM have an asynchronous interface, meaning that it reacts immediately to changes in its control inputs, SDRAM has a synchronous interface, meaning that it waits for a clock pulse before responding to its control inputs- it synchonizes with the computer's system bus, and thus with the processor, providing more efficient data transfers. The types of SDRAM are as follows:
  • Single Data Rate - SDR SDRAM - The original Format of SDRam when introduced.
  • Double Data Rate - DDR SDRAM - Achieves twice the bandwidth by transfering data Twice per system bush clock cycle. DDR-2, DDR-3 and newer Ram technologies are based on DDR with slight modifications, and better efficiency and memory clock usage to provide even higher bandwidth.
  • GDDR* Graphics DDR - used in Graphics Cards

SRAM - Static RAM is a type of memory that retains the information stored in it as long as power is supplied to the computer, allowing it to refresh. SRAM chips can store only about one-fourth the information that a RAM dynamic RAM chip can store. It is also the technology behind Solid State Storage used in USB Keys, etc.

NVRAM - Non Volatile RAM is the technology used in Memory cards, and other Momory based storage devices - It is basically a form of Static RAM