Hardware Reference
In-Depth Information
Figure 4.41. The AMR slot and CNR slot compared to PCI slots. When the AMR slot is used,
the PCI slot paired with it cannot be used.
Several hidden buses exist on modern motherboards—buses that don't manifest themselves in visible
slots or connectors. I'm talking about buses designed to interface chipset components, such as the
Direct Media Interface (DMI), the HyperTransport interface, and the LPC bus. DMI is basically a
dedicated four-lane (4-bit-wide) PCI Express connection allowing for 1GBps (version 1.0) or
2GBps (version 2.0) each direction simultaneously.
The most recent chipsets from major third-party vendors also bypass the PCI bus with direct high-
speed connections, the most common of which is HyperTransport, between chipset components.
See “
Traditional North/South Bridge Architecture
,”
p.
185
.
In a similar fashion, the LPC bus is a 4-bit bus that has a maximum bandwidth of 16.67MBps; it was
designed as an economical onboard replacement for the ISA bus. In systems that use LPC, it typically
connects Super I/O chip or motherboard ROM BIOS components to the main chipset. LPC is faster
than ISA yet uses far fewer pins and enables ISA to be eliminated from the board entirely.
The system chipset is the conductor that controls the orchestra of system components, enabling each to
have its turn on its respective buses.
Table 4.56
shows the widths, speeds, data cycles, and overall
bandwidth of virtually all PC buses in use today.
Table 4.56. Bandwidth (in MBps) and Detailed Comparison of Most PC Buses and Interfaces
