KarbosGuide.com. Module 2e1.

About RAM

Here you can read about:

  • What is RAM?
  • About RAM types

    On the following pages:

  • About SIMMs
  • DIMMs
  • PC100 RAM and further
  • Rambus
  • DDR
  • Next page
  • Previous page

  • What is RAM?

    This page should be read together with modules 2a, 2b, 2c, and 2d, which deal with system board, system bus, I/O bus and chip sets. When we talk about motherboard and chip sets, we cannot ignore RAM. Warning: RAM and RAM chips is a very complicated, technical subject area. I can in no way give a complete, comprehensive description of this subject.

    RAM is our working memory storage. All the data, which the PC uses and works with during operation, are stored here. Data are stored on drives, typically the hard drive. However, for the CPU to work with those data, they must be read into the working memory storage, which is made up of RAM chips. To examine RAM, we need to look at the following:

  • RAM types (FPM, EDO, ECC, and SD RAM)
  • RAM modules (SIMM and DIMM) in different versions
  • RAM and the system bus

    First, let us look back in time. Not too many years ago, Bill Gates said, that with 1 MB RAM, we had a memory capacity, which would never be fully utilized. That turned out to be untrue.

    Historical review

    Back in the 80s, PCs were equipped with RAM in quantities of 64 KB, 256 KB, 512 KB and finally 1 MB. Think of a home computer like Commodore 64. It had 64 KB RAM, and it worked fine.

    Around 1990, advanced operating systems, like Windows , appeared on the market, That started the RAM race. The PC needed more and more RAM. That worked fine with the 386 processor, which could address larger amount of RAM. The first Windows operated PCs could address 2 MB RAM, but 4 MB soon became the standard. The race has continued through the 90s, as RAM prices have dropped dramatically.

    Today. it would be foolish to consider less than 32 MB RAM in a PC. Many have much more. 128 MB is in no way too much for a "power user" with Windows 95/98, it is important with plenty of RAM. Click here to read about the swap file and RAM considerations. Windows 98 is a little better at handling memory, but still a lot af RAM is a good thing.

    RAM types

    The traditional RAM type is DRAM (dynamic RAM). The other type is SRAM (static RAM). SRAM continues to remember its content, while DRAM must be refreshed every few milli seconds. DRAM consists of micro capacitors, while SRAM consists of off/on switches. Therefore, SRAM can respond much faster than DRAM. SRAM can be made with a rise time as short as 4 ns. It is used in different versions in L2 cache RAM (for example pipe line BURST Cache SRAM).

    DRAM is by far the cheapest to build. Newer and faster DRAM types are developed continuously. Currently, there are at least four types:

  • FPM (Fast Page Mode)
  • ECC (Error Correcting Code)
  • EDO (Extended Data Output)
  • SDRAM (Synchronous Dynamic RAM)

    A brief explanation of DRAM types

    FPM was the traditional RAM for PCs, before the EDO was introduced. It is mounted in SIMM modules of 2, 4, 8, 16, or 32 MB. Typically, it is found in 60 ns or 70 ns versions. 60 ns is the fastest and the one to use. You cannot mix different speeds on the same Pentium motherboard.

    EDO (Extended Data Out) RAM is an improvement of FPM RAM. Data are read faster. EDO extends the time that output data is valid, which betters timing issues between the CPU and RAM and this way improves the performance.

    By switching from FPM to EDO, one could expect a performance improvement of 2 to 5 percent. EDO RAM was usually sold in 60 ns versions. A 50 ns version was available at higher cost.

    EDO has now been replaced by the even faster SDRAM.

    ECC RAM is a special error correcting RAM type. It is especially used in servers.

    SDRAM (synchronous DRAM)): The replacement for DRAM, FPM, and EDO RAM types. SDRAM "locks" (synchronizes) the memory access to the CPU clock. This way we get faster data transfer. While one portion of data is tranported to the CPU another can be being prepared for transfer.

    SDRAM comes only in 64 bit modules (long 168 pin DIMMs). SDRAM has a access time of only 6-12 ns. The performance improvement over EDO RAM was a mere 5 percent running at 66 MHz. At 100 and 133 MHz it proves better.

    DDR RAM is clock doubled version of SDRAM, which is replacing SDRAM during 2001-2002.

    RAMBUS (RDRAM) is a more futuristic RAM type. Intel and others had great expectations from this type, but it flopped in 2000-2001.

    8 or 9 bits per byte?

    Normally you figure 8 bits to one byte. For many years, a ninth bit has been added as parity bit in the RAM blocks to verify correct transmission. That way you have to transmit 9 bits, to store 8 bits in the old 30 pin RAM chips. And it takes 36 bits to store 32 bits in the larger 72 pin chips, which increases the cost of the RAM chip by about 12%.

    If your motherboard requires 36 bit modules, you must respect that. Fortunately, most system boards accepts 32 bit modules, so this creates no problems.

    RAM and motherboard

    You cannot freely install your desired RAM type. RAM is controlled by the chip set on the motherboard, so you must install a type, which matches your motherboard. Furthermore, RAM chips come in different sizes, which must match the system board.

    On modern system boards, RAM is installed on SIMM or DIMM modules. Before, small individual DRAMs were used. There was usually room for 36 small chips on the system board. That made it cumbersome to install new RAM. Then, someone figured out to install RAM chips on cards, which are easily installed. First came the SIPP modules. They had multiple pins, which fit in the motherboard. Since then came the SIMM modules. They are mounted on a card, which has an edge connector. They fit in sockets on the motherboard, and anyone can install them.

    RAM speeds

    RAM speed is measured in ns (nano seconds). The fewer ns, the faster is the RAM. Years ago, RAM came in 120, 100 and 80 ns. Today, we are talking about 60 ns and faster.

    It becomes complicated to describe the relationship between RAM speed and the ability of the system bus to utilize fast RAM. I will gloss over that. But here is a table which illustrates RAM speed, relative to clock speed:

    Clock speed
    Time per clock tick
    20 MHz
    50 ns
    25 MHz
    40 ns
    33 MHz
    30 ns
    50 MHz
    20 ns
    66 MHz
    15 ns
    100 MHz
    10 ns
    133 MHz
    6 ns

    Peak Bandwidth

    Here you see the maximal peak bandwidth of the three well known RAM types. The figures illustrates the absolutely maximal transfer from RAM to the L2-cache - in peaks, not as continuously transferred.

    RAM type Max. peak bandwidth
    FPM 176 MB/sec
    EDO 264 MB/sec
    SD 528 MB/sec

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    Learn more

    Also see module 3. An illustrated Guide to CPUs from 8086 to the Pentium-III.

    Read module 5a about expansion cards, where we evaluate the I/O buses from the port side.

    Read module 5b about AGP and module 5c about Firewire.

    Read module 7a about monitors, and 7b on graphics card.

    Read module 7c about sound cards, and 7d on digital sound and music.

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