Computers in Spaceflight: The NASA Experience

[377-392] Appendix I: Glossary Of Computer Terms
Accumulator ---The register in the central processing unit of a computer used to store the current results of calculations.
Algorithm ---A step-by-step solution to a problem that is the basis for writing the code that will enable the computer to solve it.
Analog Circuit ---An electrical circuit that models the behavior of a real object or force, providing a nondigital means of calculation.
Analog Computer ---A machine that computes by modeling objects and forces using either mechanical or electrical means.
Assembly Language ---A low-level programming language for computers that express algorithtms in statements consisting of mnemonics representing actions and numbers representing addresses. For example, the statement "ADD A,7FFF" tells a computer to add the contents of location 7FFF (a hexadecimal number) to the contents of the accumulator and leave the result in the accumulator. The specific mnemonics for assembly languages may be different for different processors but are closely related. Programs written in assembly languages, although conservative of machine resources and quite fast, are much more difficult to write and maintain than those written in high-level languages such as FORTRAN, Pascal, and HAL/S.
Asynchronous ---Occurrences happening at no set time. Asynchronous interrupts mean that signals to a computer to start a specified process may come at any time.
Backup Program ---A computer program shorter and with less functionality than the primary program that performs only critical functions in case the primary program or the hardware in which it resides fails.
Bandwidth ---The amount of information that can be transferred in a discrete amount of time. The higher the bandwidth, the more information.
Batch Processing ---A method of executing programs on a computer that reserves resources for the use of a particular program and releases them upon completion. Older batch systems could process only one program at a time.
Binary Object Code ---The result of an assembler processing an assembly language program or a compiler processing a high level language program. It consists of binary numbers in the machine language of the specific computer on which the program is to run.
Bit ---A binary digit, representing either a one or a zero.
Breadboard ---A prototype of a computer or other electronic device built by the design group to test the device before it is packaged for production.
Bubble Memory ---A type of nonvolatile computer memory using materials that can retain a specific magnetic polarity when electrical power is cut off. The polarity determines whether a one or zero is being stored.
Buffer ---A cache of memory used to store information temporarily during transfer operations. It is usually used to adjust for differences in operating speed between devices.
Buffer Register ---A register used to temporarily store information in transit to another device.
Bug ---Common term for an error in a computer program or hardware.
Bus ---An interconnection device that can be used to speed up information transfer (as when a bus made up of multiple wires carries the bits of an entire computer word in parallel) and to act as a connector for multiple devices (as when several devices that do not need to transmit simultaneously time-share the use of the bus for intercommunication). Also used to refer to heavy-duty electrical power cables or bars supplying power to many devices.
Byte ---A collection of bits, commonly eight.
Central Processor ---The portion of a computer that contains the control circuits and does the actual calculations.
CMOS ---Complimentary metal-oxide silicon circuits, characterized by low-power requirements, tolerance of wide variation in voltages, and susceptibility to damage from discharges of static electricity.
CMOS Processor---A microcomputer built of CMOS circuits.
Coding---The act of writing a program for a computer.
Compiler---A computer program that accepts statements of a high-level language as input and generates machine code that will execute those statements as output.
Condition Code---A message several bits in length used to communicate the physical status of one device to another device.
Contiguous Memory Locations---Addresses in a machine memory located adjacent to one another.
Core Memory---A type of computer memory constructed of a series of two-dimensional planes containing networks of wires with ferrite rings called "cores" at their intersections. The magnetic polarity of the cores can be changed by electrical pulses. Each core stores 1 bit of information. Core memory is nonvolatile; when power is cut off, it does not lose information. Destructive-readout core memory loses the information stored in a core when the core is read, so a temporary register must be used to intermediately store the information before writing it back to its original location as it is simultaneously sent to other parts of the computer. Nondestructive-readout core memory can be read without the information being changed.
Core Rope---A type of core memory that stores entire computer words rather than individual bits. Each core in a core rope is permanently charged to represent a "one." A number of wires equal to the number of bits in a word is weaved through the cores. When a bit within a word is to represent a one, its wire is connected to a core. Bits representing zeroes are not connected. Thus, by selecting the correct core and sensing which wires represent ones and which zeroes, the word can be reconstructed. More than one word can be attached to a core by adding more wires to the rope. Core rope, once constructed, can only be read.
Core Storage---Another name for core memory.
Core Transistor Logic---Circuits made up of discrete transistors used to form the control unit in the central processor of a computer.
Cycle Time---The length of time it takes for a computer to do a fundamental operation, such as reading a word from memory into the central processor. Some instructions, such as multiply, take several cycles.
Data Flow Diagram---A software design tool that uses circles to represent operations and arrows to represent data movement. It is used to determine the ordering of processes and input and output requirements.
Data Formatters---Hardware or software that takes raw data from devices and puts it into a uniform format for transmission, usually adding some special error detection bits.
Data Word---A computer word containing only data, not instructions.
Demultiplexer---A device that receives data transmitted on a bus and routes it to the correct device.
Digital Circuit---A circuit constructed to handle discrete units of information that can represent ones and zeroes.
Digital Computer---A calculating device using digital circuits, usually consisting of a central processing unit, memory, and input and output devices.
Diode Transistor Logic Integrated Circuit---A type of miniaturized digital circuit used to construct logic units in the central processor of a computer.
Direct Addressing---Using the absolute address of a memory location to access data within it. For example, in a hypothetical machine with a word size of 4 bits, up to 16 memory locations can be directly addressed, simply by matching them one for one with the 16 numbers 4 bits can represent. Thus, memory location 1011 is the twelfth location in the memory. Since it is often necessary to have more memory than the number of locations a single word can represent, indirect addressing schemes must be devised. The 4 bit computer can indirectly represent a memory location by using two words; one word can indicate which bank of l6 words to access, the second word can indicate which of the 16 addresses in that bank to read.
Direct Memory Access---Reading or writing to a memory location in a computer without passing the information through the central processing unit for disposition.
Discrete Component---A component containing a single entity, such as a transistor, as opposed to containing many entities, such as an integrated circuit with thousands of transistors.
Disk Drive---A type of mass storage device in which bits are represented by magnetized areas on a plane, or disk, covered with a suitable material of the same type used for magnetic tape. A disk drive may have one or many disks.
Double Precision---Using two computer words to represent a number instead of one.
Drum Memory---A type of mass storage device in which the material (similar to material used to make magnetic tape) that contains the information is placed on a rotating drum.
Emulator---A device that can be programmed to replicate the logic and functions of another device and operate at the same speed.
Erasable Memory---Memory in which information can be overwritten by new information.
Event Word---A word of information containing code to activate devices or functions.
File---A collection of related information, such as a computer program or imaging data, which can be thought of as a unit.
Firmware---Software stored in read-only memory devices used to control logic flow in a computer. Changing the firmware changes the nature of the computer.
Fixed Memory---Memory that can only be read.
Fixed Point---A method of representing numbers in a computer in which the decimal point is permanently fixed. Therefore, numbers used in calculations must be properly scaled relative to the location of their decimal point or the results will be meaningless. Such scaling is usually left to the programmer.
Flat Packs---Collections of integrated circuits packaged in modules for use in a computer.
Flip-Flop---A logic device that can change from containing a one to a zero and vice-versa depending on inputs. Hip-flops are often used in the central processing unit of a computer.
Floating Point---A method of storing numbers in a computer in which the location of the decimal point is stored with the values of the individual places.
Flowchart---A method of program design in which algorithms are represented by specific two-dimensional shapes and connecting arrows. Each shape represents a specific logical act. For example, a diamond indicates a true/false decision.
Full Word---All the bits of a computer's word size.
Gate---A logic device. For example, an "AND Gate" returns the result of a Boolean AND operation on its inputs.
General-Purpose Register---A register in the central processing unit of a computer not assigned to a specific task but that can be dynamically required to act as an accumulator, program counter, or index register.
Half Word---One half of the bits of a computer's word size.
Hard-Wired Logic Circuits---Logic implemented in hardware, as opposed to implementation in software.
Hard-Wiring---Permanently representing logic in hardware.
Hard Logic---Logic permanently represented in hardware.
Hardware---Physical components of a computer system or other device, such as memories, registers, and control logic circuits.
Hexadecimal---Base 16. One-digit numbers include 0 through 9 and A through F.
High-Level Language---A language in which algorithms can be represented in a series of structured, formal statements using selected easily recognizable words from a natural language. For example, "IF VALVE_POSITION = 2 THEN SET FUEL_FLAG TO TRUE" is a high-level language statement.
Image Processing---Using computers to operate on the digital information that represents images to enhance its value for specific purposes. Most images are represented by collections of 8-bit "gray scale" values, which contain a number ranging from 0 to 255 indicating the level of darkness in one picture element, or dot, in an image. Image processing works on these 8-bit values to increase contrast, translate oblique images to vertical images, and emphasize certain colors.
Imaging---The process of acquiring images using vidicon tubes and digital circuits.
Index Register---A register in the central processing unit of a computer that contains the value of the memory bank currently being accessed.
Instruction Set---The list of instructions that a computer can execute. It varies from a few to several hundred depending on the computer.
Instruction Word---A word in a computer containing the bits representing an instruction and an address on which the instruction is to operate.
Integrated Circuit---An electronic circuit containing hundreds, thousands, or millions of components, such as transistors, and used for a specific purpose, such as logic or memory.
Interactive Processing---Executing computer programs so that the user can actively send information to the program and receive information from it while the program is running.
Interface---The connection between two devices for the exchange of data.
Interface Table---A collection of information containing instructions for connecting devices so that data can be exchanged.
Interpreter---A computer program that executes statements written in a high-level language one at a time.
Interrupt Stack---Storage of interrupts so that they can be handled in a last-in first-out fashion.
Interrupt-Driven System---A computer that is programmed to execute processes on demand, the demand taking the form of signals sent from other devices or itself that cause processes of lower priority to halt execution and be replaced by processes of higher priority. If the interrupt is of a lower priority than the current process, it is saved for later execution.
Kilobyte---One thousand twenty-four (1,024) 8-bit bytes. Abbreviated "Kb" or, more commonly, "K."
Listing---The content of a computer program, often used to refer to the printed result of sending a program through a compiler.
Logic Channels---Hardware that represents logic and through which data flows for processing.
Logic Circuit Board---A board containing electrical connections into which circuits are plugged representing the logic of a device.
Logic Gate---See Gate.
Machine Code---The representation of instructions as a series of bits, which cause the computer to execute the specified actions. Machine code is idiosyncratic to a particular type of computer.
Machine Cycle---See Cycle Time.
Machine Time---The amount of time a computer takes to execute a program or function.
Macro---A subroutine in assembly language that can be invoked by name.
Magnetic Tape---A mass storage device in which bits are represented in areas on a magnetic surface.
Main Memory---The memory of a computer used for both reading and writing operations, and of a faster type than secondary storage devices, such as magnetic tape or disk. Main memory is often made from core or semiconductor devices.
Mainframe Computer---A large, fast computer system capable of supporting hundreds of individual users, usually with a long word size, millions of words of main memory, and many peripherals.
Medium-Scale Integration---Integration of several thousand transistors or other devices on a single chip. Abbreviated "MSI."
Megabyte---One million 8-bit bytes. Abbreviated "Mb" or, more commonly, "M."
Microprocessor---A small computer built of integrated circuits, often on a single chip. Usually a microprocessor will support a single user or function.
Microcode---The programs used to create firmware.
Microsecond---One millionth of a second.
Millisecond---One thousandth of a second.
Minicomputer---A computer sized between a microprocessor and a mainframe computer, capable of supporting from one to several dozen users or tasks.
Mnemonics---Short groups of letters representing instructions in an assembly language. The mnemonic for "decrement the number in the specified register by one and branch to another address if the number is zero" is "DBZ."
Modularization---A technique for creating large computer programs based on the principle of "divide and conquer." Each module of a large program performs one task, can be entered at only one point and exited at only one point. For example, the "BOOST THROTTLING TASK" module of the Shuttle on-board software handles the throttling of the main engines during the ascent phase of a mission. It is scheduled to execute many times each second. By isolating the function to this one module, it can be tested more easily and also reused in software loads for many Shuttle flights.
Multiplexer---A device that controls the time-sharing of a bus so that many devices can send information over the same interconnection.
Multitasking---A method of using computer resources so that more than one program can be in the process of execution at one time. The operating system of the computer will do calculations for one program while another is using the printer, for instance.
Nanosecond---One billionth of a second.
Networking---The process of interconnecting several computers together so that they can share data and programs.
Noise---Stray electromagnetic signals that may or may not interfere with data transmission and calculations. Noise may be generated locally, as when devices that leak electromagnetic radiation are placed next to one another, or from radiation fields in space.
NOR Gate---A type of logic gate that executes a Boolean OR operation on its inputs and then complements the result (reverses the value) before outputting it.
Object Code---See Binary Object Code.
Octal---Representation of numbers in base eight. Octal digits range from 0 through 7.
One's Complement---A method of storing binary numbers in which each bit in a word is complemented (reversed in value). The one's complement of 101 is 010.
Operation Code---That part of an instruction word that contains the bits that represent the specific mnemonic to be executed.
Parallel Data---Data transmitted in several bits at once.
Parameter---Data made available as input or output to a module or procedure. In general, the current value of specific information, such as fuel remaining, angle of flight, or re-entry time.
Parity---A method of ensuring accurate data transfer. The number of ones or zeroes in a specific computer word is kept either even or odd by the addition of a changeable "parity bit." If the device is using even parity based on the number of ones, or if the number of ones in the word is odd, then the parity bit is set to one. When the transfer to another device is complete, that device examines all incoming words for even parity. If it detects odd parity, it requests a retransmission of the data that failed the parity test.
Parity Bit---See Parity.
Peripheral Device---Hardware associated with a computer used for input, output, or memory functions, such as disk drives, printers, terminals, and card readers.
Pixel---Short for picture element. One dot of a digital image.
Plated-Wire Memory---A type of nonvolatile computer memory using areas of wires plated with material that can be magnetically polarized to store bits. Its function and advantages are similar to core memory.
Primary Memory---See Main Memory.
Primary Storage---See Main Memory.
Procedural Language---A computer language that can represent algorithms, such as FORTRAN, Pascal, or Ada.
Processor---Alternative term for computer.
Propagation Time---The amount of time it takes for a signal to get from one part of a device to another, or to another device.
Pseudocode---A program design tool using structured English to represent algorithms. It has the advantage of being easily understandable and independent of the syntax of a particular computer language.
Radiation-Hardened Chips---Integrated circuits that have been protected from the effects of radiation, either by shielding, decreasing the density of components, or both.
Random Access Memory---A computer memory in which data can be written to or read from any location directly.
Read---The process of moving information from a storage device to some other place.
Read Only Memory---A type of computer memory that can only be read from, not written to, such as core rope.
Real-Time Processing---A type of processing in which the computer accepts or initiates continuous asynchronous inputs and outputs.
Redundant Circuits---Sections of hardware replicated to increase reliability.
Register---A storage location containing a set of bits. Registers in memory contain data words or instruction words, registers in the central processing unit of a computer contain data, instructions, the bank of memory currently being accessed, the location of the next program step, and intermediate results of calculations.
Secondary Memory---Mass storage such as tapes or disks, usually slower than main memory.
Secondary Storage---See Secondary Memory.
Semiconductor---Name for the electronic devices made of semiconducting metals such as silicon. A transistor is a semiconductor.
Sense lines---The wires in a core or core rope memory that sense the change in polarity during a read operation on the core and transmit the value of the core to a register.
Sequence---A hardware device that commands other devices according to a fixed time or event-initiated sequence.
Serial Access---Transmission of information one bit at a time.
Serial Data---See Serial Access.
Sign Bit---A bit of a computer word reserved for indicating the sign of a number.
Signed Two's Complement---Storage of the value of a number in two's complement form with an associated sign bit.
Simulator---A device or software that replicates the functions of another device, though not necessarily at the same speed or in exactly the same way.
Single Precision---Representing the value of numbers using one computer word.
Soft Logic---The representation of logic in software.
Software---Part of a computer or other device that is the representation of algorithms of functions and problem solutions and that is easily changeable.
Software Engineering---The creation of software according to engineering principles; emphasizing proper specification, design, development, accuracy, and reliability.
Solid-State Computer---A computer built wholly out of solid-state devices such as transistors and integrated circuits as opposed to vacuum tubes.
Spike---A portion of the storage medium on a magnetic storage device such as a tape or disk ruined by an excessive electrical discharge or other event, preventing that area from being used for reading or writing.
State Vector---The current position of a spacecraft in three dimensions plus time.
Status Variable---A parameter indicating the current state of data processed by a module, either reliable or corrupted. For example, if a calculation within a module cannot be done because of insufficient or damaged data, a status variable can be set to a specified value and sent to the main program indicating that a failure has occurred and, often, what type of failure.
Stored Command Processor---One of the virtual machines of the Galileo spacecraft command and data computers that executes stored commands.
Structured Macros---A pseudo-high-level language made by naming routines written in assembly language after statements in a typical high-level language. These routines can then be invoked by name. Thus, an IF-THEN statement can be represented by an assembly language macro which accepts as its parameters the information to be tested and what to do after the result of the operation is known.
Subroutine---A software module that is part of a larger program, often repeated many times during the execution of the program.
Subroutine Linkages---Code that collects and connects subroutines together for execution.
Sumword--The result of adding the current values of specified commands together. Used to check against sumwords of other computers operating redundantly on the same processes.
Superminicomputer---A minicomputer with the speed and accuracy characteristics of a mainframe.
Telemetry---Signals sent from a spacecraft to the earth containing data gathered or generated by experiments and flight hardware.
Time-Sharing System---A method of allocating computer resources so that a number of processes can be executing simultaneously.
Time-Slice Method---A time-sharing or multiprocessing method in which each currently executing process is given a discrete length of time to use machine resources. At the end of that time, its execution is temporarily suspended and the next process in line is activated for a discrete length of time. Eventually, all current processes are serviced and the original interrupted program can pick up where it left off, beginning the cycle again.
Transfer Register---A register used for temporary storage of data, such as data read from a core memory, before it is sent to the device that requested it.
Transistor-Transistor Logic---A type of integrated circuit for representing logic in hardware.
Two's Complement---A method of storing binary numbers that first complements (reverses) each bit, and then adds one to the result. For example, the two's complement of 101 is 011. This is useful for "subtracting by adding." To subtract a two from a five, the five (101) is added to the one's complement of two (110), with the carry past the left-most bit discarded. The result is 011, or three.
Uplink---Sending signals from the ground to a spacecraft.
Uplink Telemetry---See Uplink.
User Friendliness---A term relating to the degree of ease in the use of a computer system.
Very Large-Scale Integration---Combining millions of components on a single chip. Also "VLSI."
Virtual Machine---A system of managing machine resources so that many users have the impression that each has the full attention of the computer when in actuality it is rapidly servicing the processing needs of all of them. See Virtual Memory.
Virtual Memory---A system of memory management in which small segments of a program or data are brought from secondary mass storage into main memory on demand. For example, if a programmer is examining a very large program on an interactive terminal, the code for one page of the program is in real memory, with the rest on a disk drive. If the programmer moves to a different portion of the code, the computer automatically retrieves the correct segment of the code from secondary storage and places it in main memory. In this way, many users can be serviced with a main memory much smaller than secondary storage, each having the impression that large amounts of main memory are available to them. The speed of moving information from main memory to secondary storage is such that it is not usually noticeable.
Voter Circuit---A circuit in a device that has multiple identical logic paths, which compares the results of calculations and outputs the value that the majority of the input circuits carried. In triple modular redundant circuits, three logic paths are examined by voter circuits at frequent intervals in the machine.
Word---A single unit of information in a computer, made up of a number of bits. Small microprocessors often have 8-bit words; large mainframe computers, up to 64-bit words.
Write---The act of placing data in a storage device.

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