Main Memory Memory Organisation & Memory Signals Physical Structure
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Main Memory Refers to physical memory that is internal to the computer. The word main is used to distinguish it from external mass storage devices such as disk drives. Another term for main memory is RAM. The computer can manipulate only data that is in main memory. Therefore, every program you execute and every file you access must be copied from a storage device into main memory. The amount of main memory on a computer is crucial because it determines how many programs can be executed at one time and how much data can be readily available to a program. Because computers often have too little main memory to hold all the data they need, computer engineers invented a technique called swapping, in which portions of data are copied into main memory as they are needed. Swapping occurs when there is no room in memory for needed data. When one portion of data is copied into memory, an equal-sized portion is copied (swapped) out to make room. Now, most PCs come with a minimum of 32 megabytes of main memory. You can usually increase the amount of memory by inserting extra memory in the form of chips. Memory Organisation & Memory Signals Physical Structure of IC Memory Package & Memory Modules such as SIMM & DIMM. Also their layout Memory module is a broad term used to refer to a series of dynamic random access memory integrated circuits modules mounted on a printed circuit board and designed for use in personal computers, workstations and servers. SIMM A SIMM, or single in-line memory module, is a type of memory module containing random access memory used in computers from the early 1980s to the late 1990s. It differs from a dual in-line memory module (DIMM), the most predominant form of memory module today, in that the contacts on a SIMM are redundant on both sides of the module. SIMMs were standardised under the JEDEC JESD-21C standard. 30-pin SIMMs Standard sizes: 256 KB, 1 MB, 4 MB, 16MB. 30-pin SIMMS have 12 address lines, which can provide a total of 24 address bits. With an 8 bit data width, this leads to an absolute maximum capacity of 16 MB for both parity and non-parity modules (the additional redundancy bit chip usually does not contribute to the useful capacity). 72-pin SIMMs Standard sizes: 1 MB, 2 MB, 4 MB, 8 MB, 16 MB, 32 MB, 64 MB, 128 MB. With 12 address lines, which can provide a total of 24 address bits, two ranks of chips, and 32 bit data output, the absolute maximum capacity is 227 = 128 MB. DIMM A DIMM or dual in-line memory module, comprises a series of dynamic random-access memory integrated circuits. These modules are mounted on a printed circuit board and designed for use in personal computers, workstations and servers. DIMMs began to replace SIMMs (single in-line memory modules) as the predominant type of memory module as Intel P5-based Pentium processors began to gain market share. 72-pin SO-DIMM, used for FPM DRAM and EDO DRAM 100-pin DIMM, used for printer SDRAM 144-pin SO-DIMM, used for SDR SDRAM 168-pin DIMM, used for SDR SDRAM 172-pin MicroDIMM, used for DDR SDRAM 184-pin DIMM, used for DDR SDRAM 200-pin SO-DIMM, used for DDR SDRAM and DDR2 SDRAM 204-pin SO-DIMM, used for DDR3 SDRAM 214-pin MicroDIMM, used for DDR2 SDRAM 240-pin DIMM, used for DDR2 SDRAM, DDR3 SDRAM and FB-DIMM DRAM 244-pin MiniDIMM, used for DDR2 SDRAM Differences The main difference between SIMMs and DIMMs is that DIMMs have separate electrical contacts on each side of the module, while the contacts on SIMMs on both sides are redundant. Another difference is that standard SIMMs have a 32-bit data path, while standard DIMMs have a 64-bit data path. Since Intel's Pentium has (as do several other processors) a 64-bit bus width, it requires SIMMs installed in matched pairs in order to complete the data bus. The processor would then access the two SIMMs simultaneously. DIMMs were introduced to eliminate this practice. Memory Speed, Wait Signal, Refresh & Types of Memory Wait Signal A microprocessor clock cycle in which nothing at all occurs. A wait state is programmed into a computer system to allow other components, such as random-access memory (RAM), to catch up with the central processing unit (CPU). The number of wait states depends on the speed of the processor in relation to the speed of memory. Wait states can be eliminated- resulting in a zero wait states machine by using fast (but expensive) cache memory, interfaced memory, page-mode , or static chips. Types of Memory SRAM: Static random access memory uses multiple transistors, typically four to six, for each memory cell but doesn't have a capacitor in each cell. It is used primarily for cache. DRAM: Dynamic random access memory has memory cells with a paired transistor and capacitor requiring constant refreshing. FPM DRAM: Fast page mode dynamic random access memory was the original form of DRAM. It waits through the entire process of locating a bit of data by column and row and then reading the bit before it starts on the next bit. Maximum transfer rate to L2 cache is approximately 176 MBps. EDO DRAM: Extended data-out dynamic random access memory does not wait for all of the processing of the first bit before continuing to the next one. As soon as the address of the first bit is located, EDO DRAM begins looking for the next bit. It is about five percent faster than FPM. Maximum transfer rate to L2 cache is approximately 264 MBps. SDRAM: Synchronous dynamic random access memory takes advantage of the burst mode concept to greatly improve performance. It does this by staying on the row containing the requested bit and moving rapidly through the columns, reading each bit as it goes. The idea is that most of the time the data needed by the CPU will be in sequence. SDRAM is about five percent faster than EDO RAM and is the most common form in desktops today. Maximum transfer rate to L2 cache is approximately 528 MBps. DDR SDRAM: Double data rate synchronous dynamic RAM is just like SDRAM except that is has higher bandwidth, meaning greater speed. Maximum transfer rate to L2 cache is approximately 1,064 MBps (for DDR SDRAM 133 MHZ). RDRAM: Rambus dynamic random access memory is a radical departure from the previous DRAM architecture. Designed by Rambus, RDRAM uses a Rambus in-line memory module (RIMM), which is similar in size and pin configuration to a standard DIMM. What makes RDRAM so different is its use of a special high-speed data bus called the Rambus channel. RDRAM memory chips work in parallel to achieve a data rate of 800 MHz, or 1,600 MBps. Since they operate at such high speeds, they generate much more heat than other types of chips. To help dissipate the excess heat Rambus chips are fitted with a heat spreader, which looks like a long thin wafer. Just like there are smaller versions of DIMMs, there are also SO- RIMMs, designed for notebook computers. Credit Card Memory: Credit card memory is a proprietary self-contained DRAM memory module that plugs into a special slot for use in notebook computers. PCMCIA Memory Card: Another self-contained DRAM module for notebooks, cards of this type are not proprietary and should work with any notebook computer whose system bus matches the memory card's configuration. CMOS RAM: CMOS RAM is a term for the small amount of memory used by your computer and some other devices to remember things like hard disk settings -- see Why does my computer need a battery? for details. This memory uses a small battery to provide it with the power it needs to maintain the memory contents. VRAM: VideoRAM, also known as multiport dynamic random access memory (MPDRAM), is a type of RAM used specifically for video adapters or 3-D accelerators. The "multiport" part comes from the fact that VRAM normally has two independent access ports instead of one, allowing the CPU and graphics processor to access the RAM simultaneously. VRAM is located on the graphics card and comes in a variety of formats, many of which are proprietary. The amount of VRAM is a determining factor in the resolution and color depth of the display. VRAM is also used to hold graphics-specific information such as 3-D geometry data and texture maps. True multiport VRAM tends to be expensive, so today, many graphics cards use SGRAM (synchronous graphics RAM) instead. Performance is nearly the same, but SGRAM is cheaper. Memory Bank System & how it is loaded onto the motherboard A memory bank is a logical unit of storage in electronics, which is hardware dependent. In computer the memory bank may be determined by the memory access controller and the CPU along with physical organization of the hardware memory slots. Some computers have several identical memory banks of RAM, and use bank switching to switch between them. .