MEMORY TYPES ROM ( READ ONLY MEMORY) PROM ( PROGRAMMABLE ROM) EPROM ( ERASABLE PROM) EEPROM (ELECTRICALLY EPROM) RAM ( RANDOM ACCESS MEMORY) MEMORY TYPES THE VARIOUS TYPES OF MEMORIES ARE, DRAM( DYNAMIC RAM) SDRAM (SYNCHRONOUS DRAM) SRAM ( STATIC RAM) VRAM ( VIDEO RAM) EDO RAM ( EXTENDED DATA OUTPUT RAM) RD RAM ( ROMBUS D RAM) WRAM (WINDOW RAM) DRAM ( DYNAMIC RANDOM ACCESS MEMORY) DRAM achieves good mix of speed and density while being relatively simple and inexpensive to produce. Only a single transistor and capacitor is needed to hold a bit. DRAM contents must be refreshed every few milliseconds or the contents of each bit location will decay. RAM performance is also limited because of relatively long access times . Todays many video boards are using DRAM SIMMs to supply video memory. TYPES OF SRAMs * Asynchronous * Synchronous burst * Pipe line burst SDRAM(SYNCHRONOUS OR SYNCHRONISED DRAM) Typical memory can only transfer data during certain portion of a clock cycle The SDRAM modifies memory operation so that outputs can be valid at any point n the clock cycle. SDRAM also provides a pipe line burst a mode that allows a second access before the current access is complete. This continuous memory access offers effective access speeds as fast as 10 ns and can transfer data upto 100 mb/s. ASYNCHRONOUS STATIC RAM (ASRAM) This is the traditional form of L2 cache, introduced with i386 systems. Its contents can be accessed much faster than DRAM. SYNCHRONOUS - BURST STATIC RAM (SBSRAM) This is the best type of L2 cache for intermediate speed motherboards with access times of 8.5 ns and 12 ns. The SBSRAM can provide synchronous bursts of cache information in 2-1-1-1 cycles . As motherboards pass 66 mhz SBSRAM loses its advantage to Pipelined Burst SRAM. PIPE LINED- BURST STATIC RAM (PB RAM) At 4.5 to 8 ns this is the fastest form of high performance cache now available for 75 Mhz + motherboards, PBSRAM requires an extra clock cycle for “lead off”, but then can sync with the motherboard clock across a wide range of motherboard frequencies. VRAM ( VIDEO RANDOM ACCESS MEMORY) DRAM has been the traditional choice for video memory, but the ever increasing demand for fast video information requires a more efficient means of transferring data to and from video memory. Video RAM achieves speed improvements by using a “dual data bus” scheme. Ordinary RAM uses a single data bus data enters of leaves the RAM through single set of signals. Video RA provides an “input” data bus and an “output” data bus,.this allows data to be read from video RAM at the same time new information is being written to it. EDO RAM ( EXTENDED DTA OUTPUT RAM) EDO RAM is relatively well established variation to DRAM, which extends the time that output data is valid- thus the word’s presence on the data bus is “extended” This is accomplished by modifying the DRAM’s output buffer, which prolongs the time where read data is valid. The data will remain valid until a motherboard signal is received to release it. This eases timing constraints on the memory and allows a 15 to 30 % improvement in memory performance with little real increase in cost. As new external signal is needed to operate EDO RAM the motherboard must use a chipset designed to accommodate EDO. Intel’s Triton chipset was one of the first to support EDO although now most chipsets currently support EDO,EDO RAM can be used in non-EDO motherboards, but there will be no performance improvement. RDRAM (RAMBUS DRAM) Rambus inc has created a new memory architecture called the Rambus Channel. A CPU or specialized IC is used as the master device and the RDRAMs are used as slave devices data is then sent back and forth across the Rambus channels in 256 bytes blocks.With a dual 250 mhz clock the Rambus channel can transfer data based on the timing of both clocks this results in data transfer rates approaching 500 mb/s. The problem with RDRAM is that a Rambus Channel would require an extensive redesign to the current PC memory architecture . As a result you are most likely to see RDRAM is high end specialized computing systems still as memory struggles to match the microprocessor , PC makers might yet embrace the Rambus approach for commercial systems. WRAM (WINDOWS RAM) WRAM used multiple bit arrays connected with an extensive internal bus and high speed registers that can transfer data almost continuously . Other specialized registers support attributes such as fore ground color, background color, write block control bits and true byte masking. Data transfer rate is up to 649 mb/s about 50 % faster than VRAM yet WRAM devices are cheaper than VARM. MEMORY PACKAGE STYLES AND STRUCTURES Memory is mounted in a package just like any other IC . The component memory packages can then be soldered to motherboard or attached to plug in structures such as SIMMs, DIMMS and memory cards. Only four packages styles are normally used for memory devices. DIP ( DULA IN LINE PACKAGE) This classic IC package is used for through hole mounting The advantage of DIP IC is their compatibility with IC sockets , which allows ICs to be inserted or removed as required. The long metal pins can bend and break if the IC is inserted or removed incorrectly. The overall size of the package demands extra space. DIP ICs were used in older PCs and older VGA/SVGA video boards. DIPs are still sometimes used on motherboards to provide cache RAM. SIP ( SINGLE IN LINE PACKAGE This type of IC package is rarely used today, there are simply not enough pins. It was used with memory devices in late model 286 and 386 systems which flirted with proprietary memory expansions. NEC using such devices in a 2 MB add on for their 386 and you needed to add that module before you added even more memory in the form of proprietary SIMMs. SIPs can be trouble some because they are difficult to find replacements for so expect replacements memory modules using then to cost a premium. ADD ON MEMORY DEVICES Manufactures provides a minimum amount of memory with the system, then selling more memory as an add on option, this keeps the cost of a basic machine down and increases profit through add on sales. There are three types of add on memory. 1. Propriety 2. SIMMs 3. DIMMs PROPRIETARY ADD- ON MODULES They opened the door for more than 1 MB of memory in system. More memory demand resulted in non standard memory modules. Each new motherboard came with a new add- on memory scheme. This invariably led to a great deal of confusion among PC users and makers alike. You will likely find proprietary memory modules in 286 and early 386 systems. SINGLE IN LINE MEMORY MODULE (SIMMs) A SIMMs is light small and contains a relatively large block of memory. The greatest advantage of a SIMM is a standardization. Using a standard pins layout a SIMM from one PC can be installed in any other PC. The 30 pin SIMM provides 8 data bits and generally holds up to 4 MB of RAM. The 30 pin SIMM proved its worth in i386 and early i486 systems but fell short when providing more memory to later model PCs. The 72 pin SIMM improved the 30 pin version by providing 32 data bits and it could hold up to 32 MB of RAM Each electrical contact on the SIMM is tied together between the front and back. SINGLE IN LINE MEMORY MODULE (SIMMs) If your motherboard contains only a single SIMM card proceed to the next step. If your motherboard contains more than one SIMM card refer to the diagram below to identify the SIMM card to be removed. The SIMM card to be removed is the one that will fall clear when the metal tabs are released. DUAL IN LINE MEMORY MODULES (DIMMs) DIMMs appear virtually identical to SIMMs, but they are larger. Each electrical contact on the SIMM keeps front and back contacts separate which is doubling the number of contacts available on the device . DIMM keeps the front and back contacts electrically separate . All DIMM versions provide 168 pins. DIMMs are appearing in high end 64 bit data bus PCs As PCs move from 64 to 128 bits over the next few years, DIMMs will likely replace SIMMS as the preferred memory expansion device. MEMORY ORGANISATION Memory organization is defined as process in which the entire primary memory e.g. RAM which is divided in number of compartments such as conventional, extended and replaced memory. CONVENTIONAL MEMORY Conventional memory is the traditional 640 KB assigned to the DOS memory Area . The original PCs used microprocessors that cold only address 1MB of memory . Out of that 1 MB , portions of the memory must be set aside for basic system functions BIOS code , video memory , interrupt vectors and BIOS data are only some of the areas that require memory. CONVENTIONAL MEMORY The remaining 640 kb became available to load and run your application in which can be any combination of executable code and data. The original PC only provided 512 kb for the DOS program area but computer designers quickly learned that another 128 kb could be added to the DOS area whole still retaining enough memory for overhead function so 512kb became 640 kb. Memory had to be added in a way that did not interfere with the conventional memory area. Every IBM compatible PC still provides a 640kb base memory range and most DOS application programs continue to fit within that limit to ensure backward compatibility to older systems. The drawbacks to the 8088 CPU were soon apparent. More memory has to be added to the computer for its evolution to continue. EXTENDED MEMORY The 80286 introduced in IBM's PC/AT was envisioned to overcome the 640 kb barrier by incorporating a protected mode of addressing. The 80286 can address upto 16MB of memory in protected mode and its successors can handle 4 GB of protected mode memory. Memory management software must be loaded in advance for the computer to access its extended memory. Microsoft's DOS 5.0 and higher versions provides an extended memory manager utility. DOS itself cannot use extend memory. You might fill the extended memory with data but the executable code comprising the program remains limited to the original 640 kb of base memory. EXPANDED MEMORY expanded memory is another popular technique used to overcome the traditions 640KB limit of real mode addressing. Expanded memory uses the same “physical RAM chips”. Instead of trying to address physical memory locations outside of the conventional memory range as extended memory does expanded memory block are switched into the base memory range where the CPU can access it in real mode. The original expanded memory specifications used 16 kb banks of memory which were mapped into a 64 kb range of real mode memory existing just above the video memory range. EXPANDED MEMORY CONTINUED Thus four blocks of expanded memory could be dealt with simultaneously in the real mode. Memory managers supply software only solutions for i386,i486 and Pentium based machines. EMS/LIM 4.0 is the latest version of the expanded memory standard which handles upto 32 mb of memory. An expanded memory manager allows the extended memory sitting in computer to emulate expanded memory. UPPER MEMORY AREA (UMA) The upper 384 kb of real mode memory is not available to DOS because it is dedicated to handing memory requirements of the physical computer system. This is called the high DOS memory range or upper memory area. However even the most advanced PCs don't use the entire 384 kb so there is often a substantial amount of unused memory existing in system real mode range. Late model CPU such as the Intel 386 & 486 can re map extended memory into the range unused by your system. Because this found memory space is not contiguous with your 640 kb DOS space.DOS applications programs cannot use space but small independent drivers and TSRs can be loaded and run from this UMA. The advantage of using high DOS memory is that more of the 640 kb DOS range remains available for your application program. Memory management program such as EMM86.Exe are needed to locate and re map these memory blocks.
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