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Memory Banks

Memory chips (DIPs, SIMMs, SIPPs, and DIMMs) are organized in banks on motherboards and memory cards. You should know the memory bank layout and position on the motherboard and memory cards.

You need to know the bank layout when adding memory to the system. In addition, memory diagnostics report error locations by byte and bit addresses, and you must use these numbers to locate which bank in your system contains the problem.

The banks usually correspond to the data bus capacity of the system's microprocessor. Table 6.18 shows the widths of individual banks based on the type of PC.

Table 6.18. Memory Bank Widths on Various Systems

Processor

Data Bus

Memory Bank Width

Memory Bank Width (Parity/ECC)

30-pin SIMMs per Bank

72-pin SIMMs per Bank

DIMMs per Bank

8088

8-bit

8 bits

9 bits

1

8086

16-bit

16 bits

18 bits

2

286

16-bit

16 bits

18 bits

2

386SX, SL, SLC

16-bit

16 bits

18 bits

2

486SLC, SLC2

16-bit

16 bits

18 bits

2

386DX

32-bit

32 bits

36 bits

4

1

486SX, DX, DX2, DX4, 5x86

32-bit

32 bits

36 bits

4

1

x86 and x86-64 running single-channel mode

64-bit

64 bits

72 bits

1

x86 and x86-64 running dual-channel mode

64-bit

128 bits

144 bits

2

x86 and x86-64 running tri-channel mode

64-bit

192 bits

216 bits

3

DIMMs are ideal for Pentium (and higher) systems because the 64-bit width of the DIMM exactly matches the 64-bit width of the Pentium processor data bus. Therefore, each DIMM represents an individual bank, and these can be added or removed one at a time. Many recent systems have been designed to use matched pairs or triples of memory modules for faster performance. So-called "dual-channel" and "tri-channel" designs treat two or three matched modules as a single bank of memory.

The physical orientation and numbering of the SIMMs or DIMMs used on a motherboard are arbitrary and determined by the board's designers, so documentation covering your system or card comes in handy. You can determine the layout of a motherboard or an adapter card through testing, but that takes time and might be difficult, particularly after you have a problem with a system.

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