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[ The PC Guide | Systems and Components Reference Guide | The Processor | Processor Architecture and Operation | External Processor Interfaces and Operation ]

Data Bus Size and Bandwidth for Specific Processors

This table illustrates memory data bus size, speed and bandwidth for various processors. You can see that while in recent years raw processors speeds have increased a great deal, memory bus speeds have remained somewhat stagnant, and memory system bandwidth has been essentially unchanged since the introduction of the Pentium running on a 66 MHz system bus in 1994. Look here to see full details of all the characteristics for any specific processor.

There are three different tables, reflecting the three general speed ranges used by data buses in PCs over the last 15 years. In each table the processor family is listed along with the data bus width. Then a column is provided showing the bus bandwidth in MB/sec corresponding to each of the bus speeds normally used by the processor. Remember that processor clock multipliers mean that many 486 and later CPUs run at a multiple of the system bus speed.

Note: Many people (incorrectly) deduce the "size" of the processor from the width of the data bus. For example, people see the 64-bit data bus width on the Pentium and conclude that "the Pentium is a 64 bit processor". In fact, the internal register size is what determines the "size" of a processor (and based on this definition every processor introduced in the last 10 years has been 32-bit).

First and second generation processors:

Processor Family

Width (bits)

4.77 MHz

6 MHz

8 MHz

10 MHz

12 MHz

16 MHz

20 MHz

8088

8

4.5

--

7.6

--

--

--

--

8086

16

9.1

--

15.3

19.1

--

--

--

80286

16

--

11.4

15.3

19.1

22.9

30.5

38.1

Third and fourth generation processors (plus Pentium OverDrive for 486):

Processor Family

Width (bits)

16 MHz

20 MHz

25 MHz

33 MHz

40 MHz

50 MHz

80386DX

32

63.6

76.3

95.4

127.2

152.6

--

80386SX

16

31.8

38.1

47.7

63.6

--

--

80486DX

32

--

--

95.4

127.2

--

190.7

80486SX

32

63.6

76.3

95.4

127.2

--

--

80486DX2

32

--

--

95.4

127.2

152.6

--

80486DX4

32

--

--

95.4

127.2

152.6

--

AMD 5x86

32

--

--

--

127.2

--

--

Cyrix 5x86

32

--

--

--

127.2

152.6

--

Pentium OverDrive (for 80486s)

32

--

--

95.4

127.2

--

--

Fifth and sixth generation processors (except Pentium OverDrive for 486):

Processor Family

Width (bits)

50 MHz

55 MHz

60 MHz

66 MHz

75 MHz

Pentium

64

381.5

--

457.8

508.6

--

Pentium OverDrive (for Pentiums)

64

381.5

--

457.8

508.6

--

Pentium with MMX

64

381.5

--

457.8

508.6

--

Pentium with MMX OverDrive

64

381.5

--

457.8

508.6

--

6x86

64

381.5

419.6

457.8

508.6

572.2

K5

64

381.5

--

457.8

508.6

--

Pentium Pro

64

--

--

457.8

508.6

--

Pentium II

64

--

--

--

508.6

--

K6

64

--

--

--

508.6

--

6x86MX

64

--

--

457.8

508.6

572.2

Note: You may be somewhat confused by the bandwidth numbers I have listed in the table above. For example, shouldn't the bandwidth of a Pentium running on a 66 MHz bus be 64/8*66.6=533.3 MB/sec? This is how most people and even companies write it, but this is not technically correct, because of the old problem of different definitions of what "M" stands for. The "M" in "MHz" is 1,000,000 (10^6), but the "M" in "MBytes/second" is 1,048,576 (2^20). So the bandwidth of the Pentium is more properly stated as 64/8*66.6*1,000,000/1,048,576=508.6 MBytes/second.

Next: Processor / Memory Address Bus


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