RAM

The level of satisfaction you have with your PC depends on the amount of RAM it has. Even if the

processor is the fastest or the hard drive the best and quickest around, you just won’t get the performance you’re looking for without the right type and amount of RAM. So, it’s important for you to

understand the various types of RAM starting from the earlier EDO to the current SDRAM and the future RDRAM. This will help you decide which RAM to buy for your PC.

Memory Types

For a computer to work, the CPU must take program instructions and exchange data directly

with memory. As a consequence, memory must keep pace with the CPU (or make the CPU wait

for it to catch up). Now that processors are so incredibly fast (and getting faster), traditional

memory architectures are being replaced by specialized memory devices that have been tailored

to serve specific functions in the PC. As one upgrades and repair various systems, one will

undoubtedly encounter some of the following memory designations

DRAM (Dynamic Random-Access Memory)

This remains the most recognized and common form of computer memory. DRAM achieves a

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. Unfortunately, DRAM contents must be

refreshed every few milliseconds or the contents of each bit location will decay. DRAM

performance is also limited because of relatively long access times. Today, many video boards

are using DRAM SIMMs to supply video memory.

SRAM (Static Random-Access Memory)

The SRAM is also a classic memory design—it is even older than DRAM. SRAM does not

require regular refresh operations, and can be made to operate at access speeds that are much

faster than DRAM. However, SRAM uses six transistors (or more) to hold a single bit. This

reduces the density of SRAM and increases power demands (which is why SRAM was never

adopted for general PC use in the first place). Still, the high speed of SRAM has earned it a place

as the PC’s L2 (or external) cache.

SRAM Types -Three types of cache

• Level 1 (L1)

– Primary or internal cache

– Built directly into the processor chip

– Small capacity

• Level 2 (L2)

– External cache

– Much larger capacity, but slower

• Level 3 ( L3)

-     It control data transfer between RAM and Processor

Internal Cache(L1)

The Pentium has two separate internal caches,they are code

cache and data cache.Each code and data has the size of

8KB.The cahe controllerand the cache memory are embedded in

the CPU chip itself.The cache mirrors the inrformation the

normal RAM by keeping a copy of data and code from different

memory location.

External Cache (L2)

System based on Pentium can benefit from secondary processor

cache(L2)which usually consist of up to 512KBor more extremely

fast .When the CPU fetches the data that are not already available

in its internal cache, it has to wait till data are provided from

DRAM.If the data already in the secondary processor the CPU can

go ahead with its processing work without wait state.

DRAM Types

• FPM

• EDO

• SDRAM

• RDRAM

• DDR

FPM DRAM (Fast-Page Mode DRAM)

This is a popular twist on conventional DRAM. Typical DRAM access is accomplished in a fashion similar to reading from a book—a memory “page” is accessed first, then the contents of that “page” can be located. The problem is that every access requires the DRAM to re-locate the “page.” Fast-page mode operation overcomes this delay by allowing the CPU to access multiple pieces of data on the same “page” without having to “re-locate” the “page” every time—as long as the subsequent read or write cycle is on the previously located “page,” the FPDRAM can access the specific location on that

“page” directly.

 EDO RAM (Extended Data Output RAM)

EDO RAM is a 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. Because a 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 (and most current

motherboards) currently support EDO. EDO RAM can be used in non- EDO motherboards, but there will be no performance improvement

 SDRAM (Synchronous or Synchronized DRAM

Typical memory can only transfer data during certain portions of a clock cycle. The SDRAM modifies memory operation so that outputs can be valid at any point in the clock cycle. By itself, this is not really significant, but SDRAM also provides a “pipeline burst” mode that allows a second access to begin before the current access is complete. This “continuous” memory access offers effective access speeds as fast as 10 ns, and can transfer data at up to 100MB/s. SDRAM is becoming quite popular on current motherboard designs, and is supported by the Intel VX chipset, and VIA 580VP, 590VP, and 680VP chipsets. Like BEDO, SDRAM can transfer data in a 5-1-1-1 pattern, but it can support motherboard speeds up to 100MHz, which is ideal for the 75MHz and 82MHz motherboards now becoming so vital for Pentium II systems. Check out the following references for more information on SDRAM.

RDRAM (Rambus DRAM)

Most of the memory alternatives so far have been variations of the same basic architecture. Rambus, Inc. (joint developers of EDRAM) 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 channel in 256- byte blocks. With a dual 250MHz clock, the Rambus Channel can transfer data based on the timing of both clocks—this results in data-transfer rates approaching 500MB/s (roughly equivalent to 2-ns access time). The problem with RDRAM is that a Rambus Channel would require an extensive re-design to the current PC memory architecture—a move that

most PC makers strenuously resist. As a result, one is most likely to see RDRAM in high-end, specialized computing systems. Still, as memory struggles to match the microprocessor, PC makers might yet embrace the Rambus approach for commercial systems.

DRAM pacakges

• SIMM

• DIMM

• RIMM

Memory Management

The following are the type memory that we are going to discuss in detail:

• Conventional memory

• Upper memory Area

• High Memory Area

• Extended Memory

• Expanded Memory

• Video RAM memory

• Adapter ROM and special purpose RAM

• Motherboard ROM BIOS

Base or Conventional Memory

DOS can read and write the entire mega byte, but can manage the loading of program only in the

portion of first mega byte of RAM space called conventional Memory. Refer to figure 1.

The 8088 processor and its equivalent will be able to address one megabytes of memory with 20-bit

address lines. Out of one mega bytes of memory, 640 KB is used for DOS operating system and

applications, and the remaining 384 KB is reserved for addressing BIOS ROM of motherboard, video adapter board, NIC card, video RAM and etc

Upper Memory Area (UMA)

The address space 384K above conventional memory is called Upper Memory Area.

This memory has the addresses from A0000 through FFFFF. See figure 1.

The first two 64 K blocks of UMA (address range from A0000-BFFFF) after conventional memory are

reserved for video RAM. This space is used for storing video information like text and graphic data, in video

RAM of Display Adapter before it is sent to the monitor screen. The next two 64 K blocks (address range

from C0000-DFFFF) of UMA are reserved for the software programs, or adapter BIOS that is stored in

ROM chip on the adapter plugged into the system slots. Most VGA compatible video adapters use the first

32 K Of this area for their onboard BIOS. The rest can be used for any other adapter plugged into the

slots.

The last two 64 K blocks (address range from E0000-FFFFF) of UMA are used for addressing the

motherboard BIOS program stored in EPROM. This also contains POST (Power On self Test) bootstrap

loader program, which initiate boot process from floppy or hard disk, CD-ROM drive and or from other

drives. The last motherboard BIOS is allotted the address of 64 K (from E0000 – FFFFF).

It is possible to load device drivers and memory resident programs in the unused space of UMA, which

frees up the conventional memory otherwise they would occupy. The amount of free space available in

UMA depends on the adapters plugged into Upper Memory Block (UMB).the system slots. The free space available in UMA.

Extended Memory

The memory beyond first mega bytes that can be accessed through the protected mode of the 80286 and higher

processors is generally called extended memory. Up to 16 Mega bytes of memory in 286 system and 4 Gigabytes

in systems having processor 386 and above. Refer to figure.1

Only protected mode applications can use the extended memory in 286 machines. But with 386 and later

microprocessors virtual 8086 mode allows software to split extended memory into one megabyte or smaller ranges

that each act like base memory of an individual PC.

Running several programs at the same time in virtual real mode, which is termed multitasking requires software

that can manage each program from crashing into one another. Operating systems like Unix and Novell Netware,

can directly access and load their OS Software into the extended memory.

XMS Memory

The Extended Memory Specification specifies how program would use the extended Memory. Several programs

that use XMS memory can operate together under DOS on the same system, switching the processor into and out

of protected mode to access the memory. XMS rules prevent one program from accessing memory that another

program has in use.

Extended memory can be made to conform the XMS specification by installing a device driver in the CONFIG.SYS

High Memory Area (HMA) and the A 20 Lines

The high memory area is an area of 64 K –16 bytes right on top of the first mega Bytes which can be accessed

by 286 and higher processors in real mode whereas 8088 processor cannot access this area. In fact processors

should not access beyond the first mega bytes in real mode.

This is because processors (286 and above) having more than 20 address lines. This happens due to the

(defect) bug present in these processors. That means when the segment and offset addresses are added

together the resultant address spans the extended memory. The extended memory starts from 100000 (Hex).

FFFF0 segment address

FFFF offset address

----------

10FFEF total

----------

If the 21st address line(A 20) of these processors is disabled these CPUs cannot access the HMA area. This can

be done by using unused pins of 8042 keyboard controller on the motherboard to enable and disable A 20

address line with the help of HIMEM.SYS.

Now the bug in these processors turned to be an advantage (feature) by using HMA area for loading DOS kernel

in real mode. This is accomplished simply by first loading an XMS driver such as (HIMEM.SYS).

file. The most common XMS driver is HIMEM.SYS Which is included with latest versions of DOS and Windows

Expanded Memory

Expanded memory is used to meet the demand of the applications, which require more memory

space to run in PC and PC-XT. Expanded memory is located in an adapter board termed as

Memory Expansion Board, which cannot be directly addressable, by processors like conventional

memory and extended memory.

Instead it can only be accessed through small 64 K window established in the unused area of the

Upper Memory Area (usually from D0000-DFFFF). This segment (Window) is divided into 4 pages

of 16 K each. When all the pages are filled one page is swapped into memory in memory

expansion board, the page, which becomes empty can be used for loading required data for

processing from the memory board. Using segment and offset concept, memory manager can

address up to several megabytes in the memory board.

Lotus, Intel and Microsoft developed driver program (LIM EMS) to exploit several mega bytes (32

MB) available in the card by the applications. It is only the way to run large applications, in

systems using an 8088 processor.

Only data can normally be placed in this segment because it is above the area of contiguous

memory (640 K) that DOS can use. For example, a program cannot run while it is swapped out

and therefore not visible by the processor.

ROM BIOS and MEMORY Conflicts

If two adapters have overlapping ROM or RAM address, usually neither board operates properly. Each

board functions if one removes or disable the other on, but they do not work together. With many adapters,

one can change the actual memory location to be used with jumpers, switches, or driver software, which

might be necessary to allow two boards to coexist in one system.

Additionally one must ensure that adapter boards do not use the same IRQ (Interrupt Request Line), DMA

channel (Direct Memory Access), or I/O Port address that lead to conflicts.

ROM Shadowing

In a high-end system reading ROM BIOS and adapter BIOS by either 8-bit or 16 bit at a time will bring down

the performance (bottle neck) of the system. And also the speed of ROM available is very slow (150 to

200ns) comparing to System memory (DRAM/SDRAM with 60,40 ns or less). In 386 and higher systems,

by moving the programming code from slow ROM chips into 32-bit system memory, processors can access

the same code from system memory through 32 or 64 bit bus width and with speedier access.

The shadowing is accomplished by using powerful Memory Management Unit in the 386 and higher

processors. With appropriate instructions, the MMU can take a copy of the ROM code place it in RAM and

enable the RAM chips that it appears to the system at exactly the same addresses where it was originally

located. This actually disables the ROM chips, which are essentially shut down. The system RAM where ROM program resides is fully write protected.

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