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Glossary

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

Abort

A mechanism that indicates to a core that the attempted memory access is invalid or not allowed or that the data returned by the memory access is invalid. An abort can be caused by the external or internal memory system as a result of attempting to access invalid or protected instruction or data memory.

See Also Data Abort, External Abort and Prefetch Abort.

Addressing modes

Various mechanisms, shared by many different instructions, for generating values used by the instructions.

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.

Advanced Peripheral Bus (APB)

A simpler bus protocol than AXI and AHB. It is designed for use with ancillary or general-purpose peripherals such as timers, interrupt controllers, UARTs, and I/O ports. Connection to the main system bus is through a system-to-peripheral bus bridge that helps to reduce system power consumption.

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.

APB

See Advanced Peripheral Bus.

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.

ARM instruction

An instruction of the ARM Instruction Set Architecture (ISA). These cannot be executed by the processor.

ARM state

The processor state in which the processor executes the instructions of the ARM ISA. The processor only operates in Thumb state, never in ARM state.

Base register

A register specified by a load or store instruction that is used to hold the base value for the instruction’s address calculation. Depending on the instruction and its addressing mode, an offset can be added to or subtracted from the base register value to form the address that is sent to memory.

Base register write-back

Updating the contents of the base register used in an instruction target address calculation so that the modified address is changed to the next higher or lower sequential address in memory. This means that it is not necessary to fetch the target address for successive instruction transfers and enables faster burst accesses to sequential memory.

Beat

Alternative word for an individual data transfer within a burst. For example, an INCR4 burst comprises four beats.

BE-8

Big-endian view of memory in a byte-invariant system.

See Also LE, Byte-invariant and Word-invariant.

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.

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 AMBA are controlled using signals to indicate the length of the burst and how the addresses are incremented.

See Also Beat.

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.

See Also Word-invariant.

Cold reset

Also known as power-on reset.

See Also Warm reset.

Context

The environment that each process operates in for a multitasking operating system.

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.

Core reset

See Warm reset.

Data Abort

An indication from a memory system to the core of an attempt to access an illegal data memory location. An exception must be taken if the processor attempts to use the data that caused the abort.

See Also Abort.

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.

Exception handler

See Interrupt service routine.

Exception vector

See Interrupt vector.

Halfword

A 16-bit data item.

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 debug-mode.

Host

A computer that provides data and other services to another computer. Especially, a computer providing debugging services to a target being debugged.

Implementation-defined

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

Internal PPB

See Private Peripheral Bus.

Interrupt service routine

A program that control of the processor is passed to when an interrupt occurs.

Interrupt vector

One of a number of fixed addresses in low memory that contains the first instruction of the corresponding interrupt service routine.

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.

Macrocell

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

Monitor debug-mode

One of two mutually exclusive debug modes. In Monitor debug-mode the processor enables a software abort handler provided by the debug monitor or operating system debug task. When a breakpoint or watchpoint is encountered, this enables vital system interrupts to continue to be serviced while normal program execution is suspended.

See Also Halt mode.

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.

Power-on reset

See Cold reset.

PPB

See Private Peripheral Bus.

Prefetching

In pipelined processors, the process of fetching instructions from memory to fill up the pipeline before the preceding instructions have finished executing. Prefetching an instruction does not mean that the instruction has to be executed.

Prefetch Abort

An indication from a memory system to the core that an instruction has been fetched from an illegal memory location. An exception must be taken if the processor attempts to execute the instruction. A Prefetch Abort can be caused by the external or internal memory system as a result of attempting to access invalid instruction memory.

See Also Data Abort, Abort.

Private Peripheral Bus

Memory space at 0xE0000000 to 0xE00FFFFF.

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.

RealView ICE

A system for debugging embedded processor cores using a JTAG interface.

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.

SBO

See Should Be One.

SBZ

See Should Be Zero.

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.

Serial-Wire JTAG Debug Port

A standard debug port that combines JTAG-DP and SW-DP.

SWJ-DP

See Serial-Wire JTAG Debug Port.

System memory map

Address space at 0x00000000 to 0xFFFFFFFF.

TAP

See Test access port.

Test Access Port (TAP)

The collection of four mandatory and one optional terminals 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 nTRST. This signal is mandatory in ARM cores because it is used to reset the debug logic.

Thread Control Block (TCB)

A data structure used by an operating system kernel to maintain information specific to a single thread of execution.

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.

Warm reset

Also known as a core reset. Initializes the majority of the processor excluding the debug controller and debug logic. This type of reset is useful if you are using the debugging features of a processor.

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.

Word-invariant

In a word-invariant system, the address of each byte of memory changes when switching between little-endian and big-endian operation, in such a way that the byte with address A in one endianness has address A EOR 3 in the other endianness. As a result, each aligned word of memory always consists of the same four bytes of memory in the same order, regardless of endianness. The change of endianness occurs because of the change to the byte addresses, not because the bytes are rearranged.The ARM architecture supports word-invariant systems in ARMv3 and later versions. When word-invariant support is selected, the behavior of load or store instructions that are given unaligned addresses is instruction-specific, and is in general not the expected behavior for an unaligned access. It is recommended that word-invariant systems use the endianness that produces the desired byte addresses at all times, apart possibly from very early in their reset handlers before they have set up the endianness, and that this early part of the reset handler must use only aligned word memory accesses.

See Also Byte-invariant.

Write buffer

A pipeline stage for buffering write data to prevent bus stalls from stalling the processor.

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