PC Architecture. A book by Michael B. Karbo

In 1987, Compaq hit on the idea of separating the system bus from the I/O bus, so that the two buses
could work at different clock frequencies. By letting the CPU and RAM work on their own bus,
independent of the I/O devices, their speeds could be increased.
In Fig. 119, the CPU and RAM are connected to a common bus, called the system bus, where in reality
the CPU’s clock frequency determines the working speed. Thus the RAM has the same speed as the
CPU; for example, 12, 16 or 25 MHz.
Figure 119. With this architecture, the I/O bus is separate from the system bus (80386).
The I/O devices (graphics card, hard disk, etc.) were separated from the system bus and placed on a
separate low speed bus. This was because they couldn’t keep up with the clock frequencies of the new
CPU versions.
The connection between the two buses is managed by a controller, which functions as a “bridge”
between the two paths. This was the forerunner of the multibus architecture which all motherboards
use today.
Clock doubling
With the introduction of the 80486, the CPU clock frequency could be increased so much that the RAM
could no longer keep up. Intel therefore began to use clock doubling in the 80486 processor.
The RAM available at the time couldn’t keep up with the 66 MHz speed at which an 80486 could work.
The solution was to give the CPU two working speeds.
● An external clock frequency
● An internal clock frequency
Inside the processor, the clock frequency of the system bus is multiplied by a factor of 2, doubling the
working speed.