Chapter 7: Computer Memory^141
Filling Up Memory Banks
It is entirely possible that a PC with installed memory chips or modules will fail during the
bootprocessbecauseitcandetectnomemoryonthesystem.Thisisbecause,unlessthefirst
memory bank is completely filled with memory chips or modules, as the case may be, the
PC simply ignores it. If the first memory bank (usually designated as Bank 0) is not com-
pletely filled, the PC will not boot because it does not detect any memory at all. Virtually all
motherboards (see Chapter 1) include one or more memory banks that are numbered
beginning with either 0 or 1.
Every memory module is marked with itsmemory bit width, or the number of bits it
transfers to the data bus at one time. A module’s memory bit width is used to determine
how many modules must be installed in a memory bank to match the system’s bus width.
A 30-pin SIMM has an 8-bit width; a 72-pin SIMM has a 32-bit width; and a 168-pin
DIMM has a 64-bit width. On a system with a 32-bit bus, the memory banks must have
four of the 30-pin SIMMs (4 times 8 bits equals 32 bits) or one 72-pin SIMM (32 bits). A
32-bit system cannot install even one DIMM because its 64-bit memory bit width is too
wide for the data bus. Table 7-4 lists the combinations of SIMMs and DIMMs that can be
used for different data bus widths. Theoretically, eight 30-pin SIMMs could be used to fill
a 64-bit memory bank. However, because of the physical space this would require, most
newer systems do not support the 8-bit SIMM.
There are special adapter cards, called SIMM converters, that can be used to install
30-pin SIMMs on a motherboard with only 72-pin sockets. A SIMM converter plugs into a
72-pin socket and features two or more sockets into which 30-pin sockets can be installed.
However, even with a SIMM converter, you still have to get enough memory installed to
match the data bus width.
Those memory modules that support parity or error-correcting code (ECC), expand
the memory bus by one additional bit. In general, parity and ECC systems add 1 extra bit
for each 8-bits in the bus width, which increases an 8-bit SIMM’s width to 9 bits and a
32-bit SIMM with parity to a data width of 36 bits. These bits do not affect the system data
bus because they are not sent out.
Bus Width 8-bit Bus 16-bit Bus 32-bit Bus 64-bit Bus
30-pin SIMM 1 2 4 -
72-pin SIMM - - 1 2
168-pin DIMM - - - 1Table 7-4. Matching Data Bus Widths to Memory Modules