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Memory Upgrades

In recent times, as well as becoming much cheaper, RAM has also become more complicated. There are currently a proliferation of different varieties, shapes and voltages. The first step in planning an upgrade is therefore to determine what memory modules are already fitted.

Often the memory module as a whole will have a part number, and the memory chips that are mounted on the module will have different part number(s). Of these, the latter is by far the more important. Memory chips tend to have 2 or 3 lines of text on them that include a part number, speed, and date code. Most part numbers start with a two or three character abbreviation that identifies the manufacturer, such as HM (Hitachi), M5M (Mitsubishi), TMS (Texas Instruments) or MT (Micron Technology). The numbers (and sometimes letters) that follow describe the memory configuration of the chip, for example HM514400 is a 1Mx4 configuration.

After the part number, there is usually a A, B, C, or D. This is how some manufacturers the revision of the memory, with A being the oldest and D being the most recent. In many cases, there will be an additional letter that codes the package type of the memory, e.g. HM514400AS. In this example, S stands for SOJ-type packaging.

Speed of the memory is an important aspect of identification. A 70ns chip may be encoded at the end of the part number, e.g. HM514400AS7. In this case, the 7 stands for 70ns. Sometimes there is a dash before the speed marking and at other times the speed is printed on a line above or below the part number. If the speed is printed on a separate line, a dash usually precedes the speed number. For most common memory chips, speed ranges from 50ns to 200ns. The trailing zero is commonly left off, so that -6, -7, -8, -10 or -12, represents 60ns, 70ns, 80ns, 100ns and 120ns respectively.

On most chips, there is a date code printed above or below the part number. The date code indicates when the chip was made, most typically in a year and week format (such as 9438 for the thirty-eighth week of 1994). Often, the first digit of the decade will be omitted, so that 438 may also represent the thirty-eighth week of 1994.

Obviously, if there aren’t any empty sockets, existing memory modules will have to be replaced with ones of greater capacity. The best way to ensure compatibility is to add more memory of the same specification, so new memory should match the old on the following points:

  • Physical format: It’s preferable to stick to the same module format. This format also determines how many modules should be fit: a 486-based PC accepts 72-pin SIMMs individually, while 30-pin SIMMs must be installed in sets of four. A Pentium will accept DIMMs individually, but 72-pin SIMMs have to be fit in sets of two. When installing a set of SIMMs its important to ensure they’re all alike and have the same capacity.
  • Parity or non-parity: Parity memory has 3,6,9,12 or 18 chips on each SIMM, non-parity has 2,4,8 or 16. The existing RAM should be used as a guide – fitting parity RAM to a board that doesn’t use it may result in it grinding to a halt, while adding non-parity to parity RAM is likely to render parity checking inoperative.
  • Number of chips: A few really fussy motherboards prefer a particular number of chips per SIMM. They are mainly 486 boards and they might not be happy with a mix of three-chip 30-pin SIMMs with nine-chip 30-pin SIMMs, or a mix of single-sided 72-pin SIMMs with double-sided 72-pin SIMMs.
  • Voltage: Most memory these days is 5V. If the motherboard manual claims it also supports 3.3V memory, the jumper setting should be checked. If 3.3V is installed, any upgrade should also use 3.3V.
  • Type: The three main types of memory are: standard page mode (aka fast page mode or FPM), extended data out (EDO) and synchronous (SDRAM). A system will normally report (at bootup or in the BIOS) if it is using EDO or SDRAM – if it doesn’t the chances are that it’s FPM. It’s safest not to mix memory types, although there are often occasions where this is possible.
  • Speed: The speed is usually the last two digits of the chip part number, often 60ns or 70ns. The exception is SDRAM where the speed is measured in megahertz corresponding to the maximum host bus speed it can cope with. It is advisable to stick to the same speed as fitted.

Matching the installed memory on all six points practically guarantees compatibility. It is still advisable, however, to check the motherboard manual on allowed DRAM configurations and there are three things to look out for. Before installing really big SIMMs (32MB and upwards) it is important to check that the motherboard supports them. For motherboards with a lot of SIMM sockets (over eight 30-pin or four 72-pin) it’s necessary to check there aren’t any restrictions on using too many double-sided SIMMs. Last, if the motherboard manual mentions memory interleaving, extra care must be taken in checking the intended configuration as boards with interleaved memory are usually more fussy.

There may be occasions when it is not possible to keep the memory consistent and different modules to those already installed have to be fit. First, there may be no free sockets of the occupied kind but only ones of a different type (for instance boards with both 30-pin and 72-pin SIMM sockets, or Triton VX boards with both SIMMs and DIMMs). Here, it is important to consult the motherboard manual, as any board mixing different socket-types is non-standard and likely to have strange restrictions on socket population. For instance, Triton VX boards with both SIMMs and DIMMs often require that a SIMM bank be vacated, or be populated with single-sided SIMMs only in order to be able to use the DIMM socket.

Alternatively, the motherboard may support memory of a higher performance than already fitted. Some chipsets will happily mix memory technologies. Triton VX- and HX-based boards allow different banks to be populated with different memory types (FPM, EDO or SDRAM) and run each bank independently at its optimum speed. Triton FX allows a mix of FPM and EDO within the same bank, but both will perform like FPM. However, with less well-known chipsets, mixing RAM types is a dangerous ploy and should be avoided.

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