MEMORY TYPES

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					     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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