Information Technology Reference
In-Depth Information
H.8.2 Priority inheritance boosting
One problem that can occur is when a low priority thread accesses resources which are re-
quired by a higher priority thread. For example, an RS-232 program could be loading data
into memory while another program requires to access the memory. One method which can
be used to overcome this is priority inheritance boosting. In this case, low priority threads
gets a boost so that they can quickly release resources. For example, suppose a system has
three threads: Thread A, Thread B and Thread C. If Thread A has the highest priority and it
requires a resource from Thread C then Thread C gets a boost in its priority. Thread A re-
mains blocked until Thread C releases the required resource. When it does release it then
Thread C goes back to its normal priority and Thread A then gets access to the resource.
H.9 Plug-and-play process
Plug-and-play allows the operating system to configure hardware as required. On system
start-up, the configuration manager scans the system hardware. When it finds a new plug-
and-play device it does the following:
•
Sets the device into configuration mode - this is achieved by using three I/O ports. Some
data (the initiation key) is written to one of the ports and enables the plug-and-play logic.
•
Isolate and identify each device - each device is isolated, one at a time. The method used
is to assign each device a unique number, which is a unique handle for the device. This
number is made from a device ID and a serial number.
•
Determine device specifications - each device sends its functionality to the operating
system, such as how many joysticks it supports, its audio functions, its networking
modes, and so on.
•
Allocate resources - the operating system then allocates resources to the device depend-
ing on its functionality and the plug-and-play device is informed of the allocated re-
sources (such as IRQs, I/O addresses, DMA channels, and so on). It also checks for con-
flicts on these resources.
•
Activate device - when the above have been completed the device is enabled. Only the
initiation key can re-initialise the device.
H.10 Windows NT architecture
Windows NT uses two modes:
•
User mode - this is a lower privileged mode than kernel mode. It has no direct access to
the hardware or to memory. It interfaces to the operating system through well-defined
API (application program interface) calls.
•
Kernel mode - this is a privileged mode of operation and allows all code direct access to
the hardware and memory, including memory allocated to user mode processes. Kernel
mode processes also have a higher priority over user mode processes.
