PC Hardware A Beginner’s Guide

(^142) PC Hardware: A Beginner’s Guide
Deep, Wide, and Fast Memories
SIMMs and DIMMs and memory chips have special markings to indicate their bus width
anddatacapacities.Ifthesemarkingsarenotdirectlyonthemoduleorchip,youcandefi-
nitely find this information in the technical specifications for your memory on its manu-
facturer’s Web site.
The information marked on the memory is theDWS(depth, width, and speed) nota-
tion. The DWS, which looks something like 16x64-60, indicates the overall size of the
memory on the module, but you do have to calculate it. For example, the marking of
16x64-60 does not mean 16 times 64 minus 60. This is the DWS notation for a DIMM that
has 16 million bits on each of its 64 bits of width and has a data speed of 60ns. The “x”
meansby, which is another way to saytimes, as in 16 megabitsby64 bits. Think of it like a
big matrix with rows and columns, which is how memory is organized anyway. The
module in the example has 64 rows of 16 million bits each.
MemorydepthsonSIMMsandDIMMsrangefrom1to32millionbits(Mb).Thereare
exceptions, especially in older and smaller SIMMs, which have 256 and 512 kilobits (Kb)
depths, but these are the exception.
A memory module’s width is always in bits and is usually 8 or 9 bits on 30-pin SIMMs,
32 bits on 256 or 512Kb SIMMs, 32 or 36 bits on 72-pin SIMMs, and 64 or 72 bits on 168-pin
DIMMs. The different widths, such as the 32 or 36 bits on a 72-pin SIMM, reflect memory
modules without parity (6, 32, or 64 bits) and those with parity systems (9, 36, or 72 bits).
The number of bits available to store data on a memory module is calculated as the
memory depth times the memory width. For example, a DIMM with a 16x64-60 notation
has just over 1 billion bits (1,024,000,000) of memory or 128 million bytes, calculated by
dividing the number of bits by 8 (there are 8 bits to a byte). Table 7-5 shows the memory
size for many SIMM and DIMM modules.
Memory Module D x W Memory Size (MB)
30-pin SIMM
(without parity)
1 x 2 1
1 x 8 1
2 x 8 2
4 x 8 4
16 x 8 16
30-pin SIMM
(parity)
1 x 3 1
Table 7-5. Storage Capacities for Common SIMM and DIMM Modules