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Glossary

This glossary describes some of the terms used in technical documents from ARM Limited.

Advanced High-performance Bus (AHB)

A bus protocol with a fixed pipeline between address/control and data phases. It only supports a subset of the functionality provided by the AMBA AXI protocol. The full AMBA AHB protocol specification includes a number of features that are not commonly required for master and slave IP developments and ARM Limited recommends only a subset of the protocol is usually used. This subset is defined as the AMBA AHB-Lite protocol.

See Also Advanced Microcontroller Bus Architecture and AHB-Lite.

Advanced Microcontroller Bus Architecture (AMBA)

A family of protocol specifications that describe a strategy for the interconnect. AMBA is the ARM open standard for on-chip buses. 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

See Advanced High-performance Bus.

AHB Access Port (AHB-AP)

An optional component of the DAP that provides an AHB interface to a SoC.

AHB-AP

See AHB Access Port.

AHB-Lite

A subset of the full AMBA AHB protocol specification. It provides all of the basic functions required by the majority of AMBA AHB slave and master designs, particularly when used with a multi-layer AMBA interconnect. In most cases, the extra facilities provided by a full AMBA AHB interface are implemented more efficiently by using an AMBA AXI protocol interface.

Aligned

A data item stored at an address that is divisible by the number of bytes that defines the data size is said to be aligned. Aligned words and halfwords have addresses that are divisible by four and two respectively. The terms word-aligned and halfword-aligned therefore stipulate addresses that are divisible by four and two respectively.

AMBA

See Advanced Microcontroller Bus Architecture.

Architecture

The organization of hardware and/or software that characterizes a processor and its attached components, and enables devices with similar characteristics to be grouped together when describing their behavior, for example, Harvard architecture, instruction set architecture, ARMv6-M architecture.

Big-endian

Byte ordering scheme in which bytes of decreasing significance in a data word are stored at increasing addresses in memory.

See Also Little-endian and Endianness.

Big-endian memory

Memory in which:

  • a byte or halfword at a word-aligned address is the most significant byte or halfword within the word at that address

  • a byte at a halfword-aligned address is the most significant byte within the halfword at that address.

See Also Little-endian memory.

Breakpoint

A breakpoint is a mechanism provided by debuggers to identify an instruction at which program execution is to be halted. Breakpoints are inserted by the programmer to enable inspection of register contents, memory locations, variable values at fixed points in the program execution to test that the program is operating correctly. Breakpoints can be removed after the program is successfully tested.

See Also Watchpoint.

Byte

An 8-bit data item.

Byte-invariant

In a byte-invariant system, the address of each byte of memory remains unchanged when switching between little-endian and big-endian operation. When a data item larger than a byte is loaded from or stored to memory, the bytes making up that data item are arranged into the correct order depending on the endianness of the memory access. The ARM architecture supports byte-invariant systems in ARMv6 and later versions.

Core

A core is that part of a processor that contains the ALU, the datapath, the general-purpose registers, the Program Counter, and the instruction decode and control circuitry.

Debug Access Port (DAP)

A TAP block that acts as an AMBA, AHB or AHB-Lite, master for access to a system bus. The DAP is the term used to encompass a set of modular blocks that support system wide debug. The DAP is a modular component, intended to be extendable to support optional access to multiple systems such as memory mapped AHB and APB through a single debug interface.

Debugger

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

Endianness

Byte ordering. The scheme that determines the order that successive bytes of a data word are stored in memory. An aspect of the system’s memory mapping.

See Also Little-endian and Big-endian.

Exception

An error or event which can cause the processor to suspend the currently executing instruction stream and execute a specific exception handler or interrupt service routine. The exception could be an external interrupt or NMI, or it could be a fault or error event that is considered serious enough to require that program execution is interrupted. Examples include attempting to perform an invalid memory access, external interrupts, and undefined instructions. When an exception occurs, normal program flow is interrupted and execution is resumed at the corresponding exception vector. This contains the first instruction of the interrupt service routine to deal with the exception.

Halfword

A 16-bit data item.

Implementation-defined

The behavior is not architecturally defined, but is defined and documented by individual implementations.

Joint Test Action Group (JTAG)

The name of the organization that developed standard IEEE 1149.1. This standard defines a boundary-scan architecture used for in-circuit testing of integrated circuit devices. It is commonly known by the initials JTAG.

JTAG

See Joint Test Action Group.

JTAG Debug Port (JTAG-DP)

An optional external interface for the DAP that provides a standard JTAG interface for debug access.

JTAG-DP

See JTAG Debug Port.

LE

Little endian view of memory in both byte-invariant and word-invariant systems. See also Byte-invariant, Word-invariant.

Little-endian

Byte ordering scheme in which bytes of increasing significance in a data word are stored at increasing addresses in memory.

See Also Big-endian and Endianness.

Little-endian memory

Memory in which:

  • a byte or halfword at a word-aligned address is the least significant byte or halfword within the word at that address

  • a byte at a halfword-aligned address is the least significant byte within the halfword at that address.

See Also Big-endian memory.

Load/store architecture

A processor architecture where data-processing operations only operate on register contents, not directly on memory contents.

Multi-layer

An interconnect scheme similar to a cross-bar switch. Each master on the interconnect has a direct link to each slave, The link is not shared with other masters. This enables each master to process transfers in parallel with other masters. Contention only occurs in a multi-layer interconnect at a payload destination, typically the slave.

Processor

A processor is the circuitry in a computer system required to process data using the computer instructions. It is an abbreviation of microprocessor. A clock source, power supplies, and main memory are also required to create a minimum complete working computer system.

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 0 and read as 0.

Serial Wire Debug Port (SW-DP)

The interface for Serial Wire Debug.

Serial Wire Debug (SWD)

A debug implementation that uses a serial connection between the SoC and a debugger. This connection normally requires a bi-directional data signal and a separate clock signal, rather than the four to six signals required for a JTAG connection.

SWD

See Serial Wire Debug.

SW-DP

See Serial Wire Debug Port.

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.

Thumb instruction

A halfword that specifies an operation for an ARM processor in Thumb state to perform. Thumb instructions must be halfword-aligned.

Thumb state

A processor that is executing Thumb (16-bit) halfword aligned instructions is operating in Thumb state.

Unaligned

A data item stored at an address that is not divisible by the number of bytes that defines the data size is said to be unaligned. For example, a word stored at an address that is not divisible by four.

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.

Word

A 32-bit data item.