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Cisco UCS C240 M4 Server Installation and Service Guide
OL-32474-01
Chapter 3 Maintaining the Server
Installing or Replacing Server Components
Resolving Insufficient 16-Bit I/O Space
The system has only 64 KB of legacy 16-bit I/O resources available. This 64 KB of I/O space is divided
between the CPUs in the system because the PCIe controller is integrated into the CPUs. This server
BIOS has the capability to dynamically detect the 16-bit I/O resource requirement for each CPU and then
balance the 16-bit I/O resource allocation between the CPUs during the PCI bus enumeration phase of
the BIOS POST.
When a large number of PCIe cards are installed in the system, the system BIOS might not have
sufficient I/O space for some PCIe devices. If the system BIOS is not able to allocate the required I/O
resources for any PCIe devices, the following symptoms have been observed:
• The system might get stuck in an infinite reset loop.
• The BIOS might appear to hang while initializing PCIe devices.
• The PCIe option ROMs might take excessive time to complete, which appears to lock up the system.
• PCIe boot devices might not be accessible from the BIOS.
• PCIe option ROMs might report initialization errors. These errors are seen before the BIOS passes
control to the operating system.
• The keyboard might not work.
To work around this problem, rebalance the 16-bit I/O load using the following methods:
1. Physically remove any unused PCIe cards.
2. If the system has one or more Cisco virtual interface cards (VICs) installed, disable the PXE boot
on the VICs that are not required for the system boot configuration by using the Network Adapters
page in Cisco IMC Web UI to free up some 16-bit I/O resources. Each VIC uses a minimum 16 KB
of 16-bit I/O resource, so disabling PXE boot on Cisco VICs would free up some 16-bit I/O
resources that can be used for other PCIe cards that are installed in the system.
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