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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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