The
organizations of page and page interleave memory was described in chapter 4 as
a way of increasing the speed of systems when using slow memory devices. The
only problem with interleaving is that is requires more than one bank of memory
upgrades must be planned with this in mind. In other words, you can expand
memory by a single bank but you have to expand in pairs if you want to take
advantage of the speed increase provided by interleave or page interleave. This
also explains the very common restriction on fitting the same type of device to
pairs of banks. If a pair of banks is being used to provide alternate memory
locations then each has to supply the same amount of memory!
For example,
if a 386SX machine can support a maximum of eight 256Kx9 Bit or 1Mx9Bit SIMMs
(i.e. it has four banks) then there is a very real problem in deciding how a
2MByte machine should be configured. A 2Mbyte machine could be realized by
using eight 256Kx9Bit SIMMs or a pair of 1MByte SIMMs. Using the eight 256Kbyte
SIMMs would produce a faster machine because the multiple banks of memory could
be used in interleave mode to reduce wait states. Unfortunately, this
configuration makes it impossible to increase the memory without starting
again. The alternative arrangement of using two 1MByte SIMMs has the advantage
is that, being a single physical bank of memory, interleaving cannot be used
and so the system will run slower. In nearly all cases it is batter to accept
the slower machine that results from using a single bank with larger capacity
memory modules. The reason is that memory upgrade is usually inevitable and so
it makes sense to prepare for the future at the expense of present performance.
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