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Glossary

This glossary describes some of the terms used in this manual. Where terms can have several meanings, the meaning presented here is intended.

Advanced High-performance Bus (AHB)

The AMBA Advanced High-performance Bus system connects embedded processors such as an ARM core to high-performance peripherals, DMA controllers, on-chip memory, and interfaces. It is a high-speed, high-bandwidth bus that supports multi-master bus management to maximize system performance.

See Also Advanced Microcontroller Bus Architecture and AHB-Lite.

Advanced Microcontroller Bus Architecture (AMBA)

AMBA is the ARM open standard for multi-master on-chip buses, capable of running with multiple masters and slaves. It is an on-chip bus specification that details a strategy for the interconnection and management of functional blocks that make up a System-on-Chip (SoC). It aids in the development of embedded processors with one or more CPUs or signal processors and multiple peripherals. AMBA complements a reusable design methodology by defining a common backbone for SoC modules. AHB conforms to this standard.

See Also Advanced High-performance Bus and AHB-Lite.

AHB

See Advanced High-performance Bus.

AHB-Lite

AHB-Lite is a subset of the full AHB specification. It is intended for use in designs where only a single AHB master is used. This can be a simple single AHB master system or a multi-layer AHB system where there is only one AHB master on a layer.

Aligned

Refers to data items stored so that their address is divisible by the highest power of two that divides their size. Aligned words and halfwords therefore have addresses that are divisible by four and two respectively. The terms word-aligned and halfword-aligned therefore refer to addresses that are divisible by four and two respectively. The terms byte-aligned and doubleword-aligned are defined similarly.

AMBA

See Advanced Microcontroller Bus Architecture.

Big-endian

Memory organization in which the least significant byte of a word is at a higher address than the most significant byte.

See Also Little-endian and Endianness.

Breakpoint

A breakpoint is a mechanism provided by debuggers to identify an instruction at which program execution is halted unconditionally. Breakpoints are inserted by programmers to allow inspection of register contents, memory locations, and/or variable values at fixed points in the program execution to test that the program is operating correctly. Breakpoints are removed after the program is successfully tested. See also Watchpoint.

Burst

A group of transfers to consecutive addresses. Because the addresses are consecutive, there is no requirement to supply an address for any of the transfers after the first one. This increases the speed at which the group of transfers can occur. Bursts over AHB buses are controlled using the HBURST signals to specify if transfers are single, four-beat, eight-beat, or 16-beat bursts, and to specify how the addresses are incremented.

Central Processing Unit (CPU)

The part of a processor that contains the ALU, the registers, and the instruction decode logic and control circuitry. Also commonly known as the processor core.

Clock gating

Gating a clock signal for a macrocell with a control signal, and using the modified clock that results to control the operating state of the macrocell.

CPU

See Central Processing Unit.

DBGTAP

See Debug Test Access Port.

Debugger

A debugging system that includes a program, used to detect, locate, and correct software faults, together with custom hardware that supports software debugging.

An application that monitors and controls the operation of a second application. Usually used to find errors in the application program flow.

Debug Test Access Port (DBGTAP)

The collection of four mandatory terminals and one optional terminal that form the input/output and control interface to a JTAG boundary-scan architecture. The mandatory terminals are DBGTDI (TDI), DBGTDO (TDO), DBGTMS (TMS), and TCK. The optional terminal is DBGnTRST (TRST).

EmbeddedICE logic

An on-chip logic block that provides TAP-based debug support for ARM processor cores. It is accessed through the TAP controller on the ARM core using the JTAG interface.

EmbeddedICE-RT

The JTAG-based hardware provided by debuggable ARM processors to aid debugging in real-time.

Embedded Trace Macrocell (ETM)

A hardware macrocell that outputs instruction and data trace information on a trace port.

Endianness

Byte ordering. The scheme that determines the order in which successive bytes of a data word are stored in memory.

See Also Little-endian and Big-endian.

ETM

See Embedded Trace Macrocell.

Exception

An event that occurs during program operation that makes continued normal operation inadvisable or impossible, and so makes it necessary to change the flow of control in a program. Exceptions can be caused by error conditions in hardware or software. The processor can respond to exceptions by running appropriate exception handler code that attempts to remedy the error condition, and either restarts normal execution or ends the program in a controlled way.

Halt mode

One of two mutually exclusive debug modes. In halt mode all processor execution halts when a breakpoint or watchpoint is encountered. All processor state, coprocessor state, memory and input/output locations can be examined and altered by the JTAG interface. See also Monitor mode.

Little-endian

Memory organization where the least significant byte of a word is at a lower address than the most significant byte.

See Also Big-endian and Endianness.

Macrocell

A complex logic block with a defined interface and behavior. A typical VLSI system comprises several macrocells (such as an ARM processor, an Embedded Trace Macrocell, and a memory block) plus application-specific logic.

Processor

A contraction of microprocessor. A processor includes the CPU or core, plus additional components such as memory, and interfaces. These are combined as a single macrocell, that can be fabricated on an integrated circuit.

Register

A temporary storage location used to hold binary data until it is ready to be used.

Reserved

A field in a control register or instruction format is reserved if the field is to be defined by the implementation, or produces Unpredictable results if the contents of the field are not zero. These fields are reserved for use in future extensions of the architecture or are implementation‑specific. All reserved bits not used by the implementation must be written as zero and are read as zero.

SBO

See Should Be One.

SBZ

See Should Be Zero.

SBZP

See Should Be Zero or Preserved.

Scan chain

A scan chain is made up of serially-connected devices that implement boundary scan technology using a standard JTAG TAP interface. Each device contains at least one TAP controller containing shift registers that form the chain connected between TDI and TDO, through which test data is shifted. Processors can contain several shift registers to enable you to access selected parts of the device.

Should Be One (SBO)

Should be written as 1 (or all 1s for bit fields) by software. Writing a 0 produces Unpredictable results.

Should Be Zero (SBZ)

Should be written as 0 (or all 0s for bit fields) by software. Writing a 1 produces Unpredictable results.

Should Be Zero or Preserved (SBZP)

Should be written as 0 (or all 0s for bit fields) by software, or preserved by writing the same value back that has been previously read from the same field on the same processor.

TAP

See Test Access Port.

Test Access Port (TAP)

The collection of four mandatory terminals and one optional terminal that form the input/output and control interface to a JTAG boundary-scan architecture. The mandatory terminals are TDI, TDO, TMS, and TCK. The optional terminal is TRST.

Unaligned

Memory accesses that are not appropriately word-aligned or halfword-aligned.

See Also Aligned.

Undefined

Indicates an instruction that generates an Undefined instruction trap. See the ARM Architectural Reference Manual for more information on ARM exceptions.

Unpredictable

For reads, the data returned when reading from this location is unpredictable. It can have any value. For writes, writing to this location causes unpredictable behavior, or an unpredictable change in device configuration. Unpredictable instructions must not halt or hang the processor, or any part of the system.

Watchpoint

A watchpoint is a mechanism provided by debuggers to halt program execution when the data contained by a particular memory address is changed. Watchpoints are inserted by the programmer to enable inspection of register contents, memory locations, and variable values when memory is written to test that the program is operating correctly. Watchpoints are removed after the program is successfully tested. See also Breakpoint.

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