uClinux

(you see Linux) n. A variant of the popular Linux kernel designed for use in systems without a memory management unit. Despite the absence of an MMU, uClinux provides complete and stable support for multitasking, along with a TCP/IP stack, support for popular filesystems such as ext2, NFS, and FAT, and the familiar Linux API.

FURTHER READING: http://www.uclinux.org

unidirectional

adj. A one-way flow of information, as in some communications channels.

EXAMPLE: The address bus coming out of a processor is generally unidirectional.

Contrast with bidirectional.

Unified Modeling Language

N. A standardized visual notation for communication about system specifications and design details. Abbreviated UML.

uninitialized data

n. Static variables that have no initial value assigned to them. Uninitialized data is generally located in the bss segment.

EXAMPLE: int x;

Contrast with initialized data.

Universal Serial Bus

n. A serial protocol and physical link that transmits all data differentially on a single pair of wires. Abbreviated USB. USB is an increasingly popular replacement for slower serial and parallel ports, especially for connecting to general-purpose computers. In addition to higher throughput, USB also offers the ability to power downstream devices and to connect a much larger number of devices.

The USB 1.0 standard specifies two kinds of cables and two variations of connectors. High-speed cables, for 12 Mbps communication, are better shielded than their less expensive 1.5 Mbps counterparts. Each cable has an "A" connector on one end and a "B" on the other. Since the two types are physically different it's impossible to install a cable incorrectly.

The interconnections of USB devices in a tiered star topology

up counter

n. A counter that counts up from zero toward its maximum value. [more]

Contrast with down counter.

user mode

n. A state in which a processor will not execute some of its instructions. For example, while in user mode, it might not be possible to disable interrupts. Contrast with kernel mode.