CPU Glossary

accumulator-a register within the CPU. This register will be one of the operands in an arithmetic or logical operation.

address-a binary pattern used to select a location in memory. In this text, addresses are 16 bits wide.

addressing modes-techniques used by software or hardware in calculating an address.

ALU-arithmetic logic unit; the section of the CPU that performs calculations for arithmetic and logical operations.

assembler-a program that translates mnemonic code into machinelevel code.

asynchronous-an event or operation not controlled by the master clock.

base address-the location of the first datum in a data table.

bit-a binary digit.

BCD-binary coded decimal; a number system in which each group of 4 bits (referred to as decades) represents a single decimal digit.

breakpoint-a place in a program where execution is temporarily stopped. Once a breakpoint is reached, analysis of the program can be performed.

buffer-a circuit that can be used to isolate one part of the computer from another. Buffers often provide extra current capabilities not found in the circuit driving the inputs. A buffer can also be used as a temporary storage register.

bus-a set of lines used to move information from one part of the computer to another. Typically, a bus is divided into three functional groups: data, address, and control.

bus contention-two or more circuits trying to place information on a common line at the same time.

byte-a group of 8 bits.

CPU-central processing unit; a section of a computer responsible for execution of programs. This section manipulates the data, generates control signals, and stores results. In modern computers these functions have been integrated into a single IC. The 8080, 8085, and Z80 are examples of CPUs.

clock cycle-the basic unit of processor activity.

compiler-a program that translates highlevel code into machine code.

controller-a circuit that acts as an interface between the computer and a device such as a disk drive. The controller will pass information back and forth between the computer and the device and will generate specialized control signals needed to operate the device.

DAC-digitaltoanalog converter.

deviceselect pulse-a control pulse created by logically combining a port address with an IN or OUT control signal.

DMA-direct memory access; a process in which circuits other than the CPU can read from or write to memory without processor intervention.

displacement-the amount by which a location differs from a reference point. When using data tables, the reference point is usually the base address.

duplex-twoway data transfers. Half duplex is twoway communication, with each end of the system taking turns transmitting. Full duplex is twoway communication that occurs simultaneously.

dynamic memory-a memory in which information is stored capacitively. This type of memory requires periodic refresh to replenish capacitor charges.

EEPROM-electrically erasable programmable read only memory.

EPROM-erasable by ultraviolet light.

firmware-a software routine that has been stored in a read only memory and thus is available at power up without the need for reloading the code.

flag-a flipflop used to indicate the status of an operation. For example, the zero flag will indicate if an operation results in zero if it is set.

flowchart-a pictorial diagram of the logical sequence of a program.

handshaking-the exchange of control and status information between two circuits. Handshaking is used to coordinate the transfer of data between circuits.

hardware- the physical parts of a system.

highlevel language-a computer language such as FORTRAN or COBOL. Such languages are Englishlike, requiring a compiler to translate them into machine code.

ICE-in circuit emulator.

index register-a specialpurpose register used by a processor when performing indexed addressing. The value in the index register is usually the reference location to which a displacement will be added.

I/O-input/output; the process by which the computer communicates with the outside world.

instruction-a computer command.

instruction cycle-the time and activities associated with the performance of an instruction.

instruction fetch-a machine cycle used by the processor to obtain instructions from memory.

instruction register-the part of the CPU that stores the instruction while it is being decoded.

instruction set-the group of instructions that is recognized by a processor.

interrupt-an asynchronous request by a circuit asking for the processor's attention. If acknowledged, the processor will temporarily stop what it was doing and interact with the circuit that generated the interrupt.

interrupt service routine-a section of code written to handle the tasks associated with an interrupt request.

interrupt vector-a special code that identifies the circuit requesting an interrupt.

invisible subtraction-a subtraction in which the answer is not stored back in the accumulator. The results of the subtraction can, however, affect the flags, making it possible to make decisions based on the result.

isolated I/O-a method of I/O that differentiates between memory and I/O ports. In this text, the only register that can be used with isolated I/O is the accumulator.

linear decoding-a method of addressing a circuit using a single address line.

logical construct-a method of organizing how a task will be performed. When used in programming, constructs partition the code into sections in which each section has but one entry point and one exit point.

machine cycle-a set of clock states grouped together to perform a data transfer.

machine language-a computer language that is directly executable by a computer without the need for translation by a compiler or an assembler. Although the computer works on binary patterns, the program can usually be entered in octal or hexadecimal.

masking-a process in which an operation can be performed on a single bit.

memory cell-a circuit in memory that represents a single bit.

memory map-documentation that lists or shows the function of each location in memory.

memory mapped I/O-a system of I/O in which each I/O location is treated as if it were memory.

microprocessor-an integrated circuit in which CPU, ALU, and control functions are combined.

mnemonic-an Englishlike representation of an instruction.

monitor program-a program, usually stored in ROM, that is responsible for basic computer operations.

nesting-the technique of placing a loop or subroutine within another loop or subroutine.

nibble-4 bits.

nonmaskable interrupt-an interrupt that cannot be turned off.

operating system-a program similar to a monitor program, but more sophisticated. Not only does it handle basic computer functions, it is capable of interacting with peripherals such as disk drives and printers.

up code-a binary pattern that when loaded into the instruction register will cause the CPU to perform tasks associated with the instruction.

page-256 bytes of memory.

parallel-a bus system in which information is moved in multibit units simultaneously.

polling-a process in which the status of devices attached to a bus system is periodically sampled.

port-an interface circuit capable of receiving from or placing information on the bus.

priority-the level of importance of an event. Most often, interrupts are assigned priorities.

program counter-a 16bit register that places addresses on the bus to retrieve information stored within a program.

programmable interface chip-an IC in which the specific tasks to be performed are not hardwired into the chip, but selectable through the use of command words.

PROM-programmable read only memory.

random access-each location within memory can be directly accessed without the need to sequence through prior locations. Direct access is probably more appropriate, as this access method is not really random.

read-to transfer information into the CPU.

relative addressing-an addressing mode that calculates a new address based on the position of an instruction within a program.

sequential access-a method of access in which all prior records or locations must be traversed before reaching the desired location or record.

signature analysis-a troubleshooting technique in which a stream of serial data is converted into a binary pattern that can then be compared to a pattern known to represent a functional circuit.

simplex-oneway communication.

single step-a process in which a program can be performed one instruction or one machine cycle at a time.

stack-an area of memory used to implement a data structure that follows the last in, first out method of access. In most cases, the stack is used by the processor to keep track of subroutine calls and returns.

stack pointer-a specialpurpose register that tracks the location of the last entry in the stack.

static memory-memory in which the basic cell circuit consists of a flipflop.

synchronous-an event or operation that takes place in step with a master clock.

throughput-the amount of information processed in a given time.

USART-universal synchronous asynchronous receiver transmitter; a specialpurpose IC used in data communications.

volatile-memory in which data is lost when power is removed.

word-16 bits.

write-data transfer out of the CPU.

Microprocessor Glossary