As a verb, to alter some aspect of a program or an operating system through manipulation of its code rather than through operation of the program itself – for example, to switch the default system font in the Apple Macintosh operating system from Chicago to Helvetica. As a noun, a sloppy job or the act of altering the code in a program, usually without taking the time to find an elegant solution.
A logic circuit used in a computer to add binary digits (bits). A half adder accepts two digital inputs (bits) and produces two outputs, a sum and a carry bit, as shown in the table. Although it can produce a carry, a half adder cannot accept a carry bit from a previous addiction. The addition of two inputs and a carry is the function of a full adder. One half adder and one or more full adders are combined to enable computers to add 4 or more bits at a time.
Half a word. Commonly, a computer’s word size is either 2 or 4 bytes, and its half-words are, accordingly, 1 or 2 bytes.
A code used to detect and correct errors in individual bits of transmitted data. The Hamming code adds three check (verification) bits to the end of each four bits of data. Each check bit is a calculated value representing some combination of three of the four data bits. When recalculated by the receiving device, these check bits can be used to determine whether each of the four data bits was received correctly, and they can, in some circumstances, be used to correct erroneous bits.
An error caused by a hardware failure or by accessing incompatible hardware; also, any error that prevents a program from continuing an operation.
A search algorithm that uses hashing to find an element of a list. Hash searches are highly efficient because the hashing enables direct (or perhaps almost direct) access to the target element.
Hard Disk Assembly. The sealed part (consists of a spindal motor, voice coil actuator, platters, read/write heads, air filter, and etc) of the drive is known as the HDA.
Hard disk heads are extremely small electromagnets that read and write binary data onto the spinning platters. Hard disk usually has more than one platter with one head on each sides.
A hard disk failure in which a read/write head collides with the surface of the platter on which information is stored. In a hard disk, the read/write heads float less than a hair’s breadth over the surface of the platter as it spins. When a head crash occurs, a read/write head comes into contact with the platter, cutting a groove in its magnetic coating. Such a crash can be caused by mechanical failure or by heavy shaking of the disk drive. It generally causes loss of information and can cause even more damage when the head encounters loose debris. The result of a head crash can be disastrous, particularly if the crash occurs on a directory track, which would effectively destroy the data that indicates where all files are located on the disk, rendering the disk unreadable.
A portion of memory reserved for a program to use for the temporary storage of data structures whose existence or size cannot be determined until the program is running. The program can request free memory from the heap to hold such elements, use it as necessary, and later free the memory. Programming languages such as C and Pascal include functions and procedures for requesting and freeing memory from the heap. Unlike another reserved portion of memory called the stack, the heap is allocated in differently sized blocks according to the needs of the program. These blocks may come from various portions of the heap – wherever a block of sufficient size happens to be. As the program continues running, the heap becomes fragmented, and a process known as heap compaction might be needed to merge small blocks into larger units and enable memory to be used more efficiently. In sorting, a complete binary tree with the property that the value of any node is not exceeded by the value of either of its children.
Number of cycle per second. 1 KHz equals to 1000 Hz. 1 MHz equals to 1000 KHz. 1 GHz equals to 1000 MHz.
A file that is not shown in a normal listing of the files contained in a directory. Files are hidden to protect them from change or deletion. MS-DOS, for example, contains two hidden files, IO.SYS and MSDOS.SYS (IBMBIO.COM and IBMOS.COM in IBM releases of MS-DOS); both contain critical portions of the operating system. On the Apple Macintosh, hidden files are also called invisible files.
In a 2-byte group of bits, numbered 0 through 15, the byte containing the most significant digits, bits 8 through 15.
The main computer in a system of computers or terminals connected by communications links.
The position in a mouse pointer that marks the exact screen location that will be affected by a mouse action such as a button press. Regardless of its shape, a mouse pointer’s hotspot is only a single pixel in size and represents only a small portion of the graphical mouse pointer shape – for example, the screen position at the tip of a pointer shaped like an arrow, the position at the tip of the pointing finger in a hand.
A computer that contains both digital and analog circuits. Digital circuits use discrete on/off signals to represent the 0’s and 1’s of binary numbers; they are found in almost all computers, including personal computers. Analog circuits use continuously variable physical quantities (such as voltage or current) to represent values; they are found in some specialized computers used in engineering and other fields.