A BasicOverview of Commonly Encountered types of Random Access Memory (RAM)Basic RAM Overview:Why Random Access?How The Data In RAM Is AccessedCommon Types of RAMMain RAMParity DRAMEnhanced DRAM (EDRAM)ECC RAMJEDEC SDRAMPC100 SDRAMPC133 SDRAMEnhanced DDRRAMVideo RAMRAMDACMultiport Dynamic Random access memory (MPDRAM)Window RAMSynchronous Graphics RAM3D-RAMMultibank Dynamic RAMFerroelectric Random Access MemoryHow RAM Effectiveness is measuredThe RAM TableA Basic Overview of Commonly Encountered types of Random Access Memory (RAM) ECE332 By: Peter Haugen Ian Myers Bret Sadler John Whidden Figure 1: Memory Hierarchy (Simoncelli 2000)Basic RAM Overview: RAM (Random Access Memory) is the hardware location in a computer where the operating system, application programs, and data in current use are kept so that they can be quickly reached by the computer's processor. RAM is much faster to read from and write to than most other kinds of storage in a computer (the hard disk, floppy disk, and CD-ROM). However, the data in RAM stays there only as long as it has power. When you turn the computer off, RAM loses its data. When you turn your computer on again, your operating system and other files are once again loaded into RAM, usually from your hard disk. RAM can be compared to a person's short-term memory and the hard disk to the long-term memory. The short-term memory focuses on work at hand, but can only keep so many facts in view at one time. If short-term memory fills up, your brain sometimes is able to refresh it from facts stored in long-term memory. A computer also works this way. If RAM fills up, the processor needs to continually go to the hard disk to overlay old data in RAM with new, slowing down the computer's operation. (Giakamozis 1999) Why Random Access? RAM is called "random access" because any storage location can be accessed directly. Originally, the term distinguished regular core memory from offline memory, usually on magnetic tape in which an item of data could only be accessed by starting from the beginning of the tape and finding an address sequentially. Perhaps it should have been called "non-sequential memory" because RAM access is hardly random. (Giakamozis 1999) RAM is organized and controlled in a way that enables data to be stored and retrieved directly to specific locations. A term IBM has preferred is direct access storage or memory. (Giakamozis 1999) Note that other forms of storage such as the hard disk and CD-ROM are also accessed directly (or "randomly") but the term random access is not applied to these forms of storage. In addition to hard disk, floppy disk, and CD-ROM storage, another important form of storage is read-only memory (ROM), a more expensive kind of memory that retains data even when the computer is turned off. Every computer comes with a small amount of ROM that holds just enough programming (BIOS) so that the operating system can be loaded into RAM each time the computer is turned on. What RAM architecture Looks Like In general, RAM is much like an arrangement of cells in which each cell can hold a 0 or a 1. Each cell has a unique address that can be found by counting across columns and then counting down by row. To find the contents of a cell, the RAM controller sends the column/row address down a very thin electrical line etched into the chip. There is an address line for each row and each column in the set of cells. If data is being read, the bits that are read flow back on a separate data line. In describing a RAM chip or module, a notation such as 256Kx16 means 256 thousand columns of cells standing 16 rows deep.An 8-megabyte module of dynamic RAM contains 8 million capacitors and 8 million transistors and the paths that connect them. In the most common form of RAM, dynamic RAM, each cell has a charge or lack of charge held in something similar to an electrical capacitor. A transistor acts as a gate in determining whether the value in the capacitor can be read or written. In static RAM, instead of a capacitor-held charge, the transistor itself is a positional flip/flop switch, with one position meaning 1 and the other position meaning 0. Externally, RAM is a chip that comes embedded in a personal computer motherboard with a variable amount of additional modules plugged into motherboard sockets. To add memory to your computer, you simply add more RAM modules in a prescribed configuration. These are single in-line memory modules (SIMMs) or dual in-line memory modules (DIMMs). Since DIMMs have a 64-bit pin connection, they can replace two 36-bit (32-bits plus 4 parity bits) SIMMs when synchronous DRAM is used. Laptop and notebook computers contain smaller 32-bit DIMMs known as small outline DIMMs (SO DIMMs). How The Data In RAM Is Accessed Figure 2: RAM Interaction (Simoncelli 2000) When the processor or CPU gets the next instruction it is to perform, the instruction may contain the address of some memory or RAM location from which data is to be read (brought to the processor for further processing). This address is sent to the RAM controller. The RAM controller organizes the request and sends it down the appropriate address lines so that transistors along the lines open up the cells so that each capacitor value can be read. In DRAM a capacitor with a charge over a certain voltage level represents the binary value of 1 and a capacitor with less than that charge represents a 0. For dynamic RAM, before a capacitor is read, it must be power-refreshed to ensure that the value read is valid. Depending on the type of RAM, the entire line of data may beread that the specific address happens to be located at or, in some RAM types, a unit of data called a page is read. The data that is read is transmitted along the data lines to the processor's nearby data buffer known as level-1 cache and another copy may be held in level-2 cache. For video RAM, the process is similar to DRAM except that, in some forms of video RAM, while data is being written to video RAM by the processor, data can simultaneously be read from RAM by the video controller (for example, for refreshing the display image). Common Types of RAM Figure 3: Common Types of RAM (Simoncelli 2000) RAM is often divided into two main categories (1) main RAM, which stores every kind of data and makes it quickly accessible to a microprocessor and (2) video RAM, which stores data intended for your display screen, enabling images to get to your display faster. Main