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Glossary

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

Abort

A mechanism that indicates to a core that it must halt execution of an attempted illegal memory access. An abort can be caused by the external or internal memory system as a result of attempting to access invalid instruction or data memory. An abort is classified as either a Prefetch or Data Abort, and an internal or External Abort.

See Also Data Abort, External Abort and Prefetch Abort.

Addressing modes

A mechanism, shared by many different instructions, for generating values used by the instructions. For four of the ARM addressing modes, the values generated are memory addresses (which is the traditional role of an addressing mode). A fifth addressing mode generates values to be used as operands by data-processing instructions.

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.

Advanced Peripheral Bus (APB)

The AMBA Advanced Peripheral Bus is a simpler bus protocol than 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.

See Also Advanced High-performance Bus.

AHB

See Advanced High-performance Bus.

AHB-AP

See AHB Access Port.

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

Aligned data items are stored so that their address is divisible by the highest power of two that divides their size. 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. Other related terms are defined similarly.

AMBA

See Advanced Microcontroller Bus Architecture.

APB

See Advanced Peripheral Bus.

Application Specific Integrated Circuit (ASIC)

An integrated circuit that has been designed to perform a specific application function. It can be custom-built or mass-produced.

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 architecture.

ARM instruction

A word that specifies an operation for an ARM processor to perform. ARM instructions must be word-aligned.

ARM state

A processor that is executing ARM (32-bit) word-aligned instructions is operating in ARM state.

ASIC

See Application Specific Integrated Circuit.

ATPG

See Automatic Test Pattern Generation.

Automatic Test Pattern Generation (ATPG)

The process of automatically generating manufacturing test vectors for an ASIC design, using a specialized software tool.

Banked registers

Those physical registers whose use is defined by the current processor mode. The banked registers are r8 to r14.

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 transfer within a burst. For example, an INCR4 burst comprises four beats.

See Also Burst.

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.

Boundary scan chain

A boundary 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.

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 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.

See Also Beat.

Byte

An 8-bit data item.

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.

Cold reset

Also known as power-on reset. Starting the processor by turning power on. Turning power off and then back on again clears main memory and many internal settings. Some program failures can lock up the processor and require a cold reset to enable the system to be used again. In other cases, only a warm reset is required.

See Also Warm reset.

Communications channel

The hardware used for communicating between the software running on the processor, and an external host, using the debug interface. When this communication is for debug purposes, it is called the Debug Comms Channel. In an ARMv6 compliant core, the communications channel includes the Data Transfer Register, some bits of the Data Status and Control Register, and the external debug interface controller, such as the DBGTAP controller in the case of the JTAG interface.

Condition field

A four-bit field in an instruction that specifies a condition under which the instruction can execute.

Conditional execution

If the condition code flags indicate that the corresponding condition is true when the instruction starts executing, it executes normally. Otherwise, the instruction does nothing.

Control bits

The bottom eight bits of a Program Status Register (PSR). The control bits change when an exception arises and can be altered by software only when the processor is in a privileged mode.

Coprocessor

A processor that supplements the main processor. It carries out additional functions that the main processor cannot perform. Usually used for floating-point math calculations, signal processing, or memory management.

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.

CPSR

See Current Program Status Register.

Current Program Status Register (CPSR)

The register that holds the current operating processor status.

Data Abort

An indication from a memory system to a core that it must halt execution of an attempted illegal memory access. A Data Abort is attempting to access invalid data memory.

See Also Abort, External Abort, and Prefetch Abort.

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.

Debug Test Access Port (DBGTAP)

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 DBGTDI, DBGTDO, DBGTMS, and TCK. The optional terminal is TRST. This signal is mandatory in ARM cores because it is used to reset the debug logic.

Direct Memory Access (DMA)

An operation that accesses main memory directly, without the processor performing any accesses to the data concerned.

DMA

See Direct Memory Access.

DNM

See Do Not Modify.

Do Not Modify (DNM)

In Do Not Modify fields, the value must not be altered by software. DNM fields read as Unpredictable values, and must only be written with the same value read from the same field on the same processor.

DNM fields are sometimes followed by RAZ or RAO in parentheses to show which way the bits should read for future compatibility, but programmers must not rely on this behavior.

Doubleword

A 64-bit data item. The contents are taken as being an unsigned integer unless otherwise stated.

Doubleword-aligned

A data item having a memory address that is divisible by eight.

DSM

See Design Simulation Model.

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, when connected to a processor core, outputs instruction and data trace information on a trace port. The ETM provides processor driven trace through a trace port compliant to the ATB protocol.

Endianness

Byte ordering. The scheme that determines the order in which 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.

ETM

See Embedded Trace Macrocell.

Exception

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 handler to deal with the exception.

Exceptional state

When a potentially exceptional instruction is issued, the VFP11 coprocessor sets the EX bit, FPEXC[31], and loads a copy of the potentially exceptional instruction in the FPINST register. If the instruction is a short vector operation, the register fields in FPINST are altered to point to the potentially exceptional iteration. When in the exceptional state, the issue of a trigger instruction to the VFP11 coprocessor causes a bounce.

See Also Bounce, Potentially exceptional instruction, and Trigger instruction. .

Exception service routine

See Interrupt handler.

Exception vector

See Interrupt vector.

Exponent

The component of a floating-point number that normally signifies the integer power to which two is raised in determining the value of the represented number.

External Abort

An indication from an external memory system to a core that it must halt execution of an attempted illegal memory access. An External Abort is caused by the external memory system as a result of attempting to access invalid memory.

See Also Abort, Data Abort and Prefetch Abort.

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.

High vectors

Alternative locations for exception vectors. The high vector address range is near the top of the address space, rather than at the bottom.

IMB

See Instruction Memory Barrier.

Index register

A register specified in some load or store instructions. The value of this register is used as an offset to be added to or subtracted from the base register value to form the virtual address, which is sent to memory. Some addressing modes optionally enable the index register value to be shifted prior to the addition or subtraction.

Instruction cycle count

The number of cycles for which an instruction occupies the Execute stage of the pipeline.

Instruction Memory Barrier (IMB)

An operation to ensure that the prefetch buffer is flushed of all out-of-date instructions.

Internal scan chain

A series of registers connected together to form a path through a device, used during production testing to import test patterns into internal nodes of the device and export the resulting values.

Interrupt handler

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, or in high memory if high vectors are configured, that contains the first instruction of the corresponding interrupt handler.

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 Access Port (JTAG-AP)

An optional component of the DAP that provides JTAG access to on-chip components, operating as a JTAG master port to drive JTAG chains throughout a SoC.

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.

Load Store Unit (LSU)

The part of a processor that handles load and store transfers.

LSU

See Load Store Unit.

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.

Microprocessor

See Processor.

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-ICE

A JTAG-based tool for debugging embedded systems.

Penalty

The number of cycles in which no useful Execute stage pipeline activity can occur because an instruction flow is different from that assumed or predicted.

Power-on reset

See Cold reset.

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 a core that it must halt execution of an attempted illegal memory access. 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, External Abort and Abort.

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.

Read

Reads are defined as memory operations that have the semantics of a load. That is, the ARM instructions LDM, LDRD, LDC, LDR, LDRT, LDRSH, LDRH, LDRSB, LDRB, LDRBT, LDREX, RFE, STREX, SWP, and SWPB, and the Thumb instructions LDM, LDR, LDRSH, LDRH, LDRSB, LDRB, and POP. Java instructions that are accelerated by hardware can cause a number of reads to occur, according to the state of the Java stack and the implementation of the Java hardware acceleration.

Region

A partition of instruction or data memory space.

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.

Saved Program Status Register (SPSR)

The register that holds the CPSR of the task immediately before the exception occurred that caused the switch to the current mode.

SBO

See Should Be One.

SBZ

See Should Be Zero.

SBZP

See Should Be Zero or Preserved.

Scalar operation

A VFP coprocessor operation involving a single source register and a single destination register.

See Also Vector operation.

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.

SCREG

The currently selected scan chain number in an ARM TAP controller.

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.

Significand

The component of a binary floating-point number that consists of an explicit or implicit leading bit to the left of the implied binary point and a fraction field to the right.

SPSR

See Saved Program Status Register.

TAP

See Test access port.

TCM

See Tightly coupled memory.

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 TRST. This signal is mandatory in ARM cores because it is used to reset the debug logic.

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.

Tightly coupled memory (TCM)

An area of low latency memory that provides predictable instruction execution or data load timing in cases where deterministic performance is required. TCMs are suited to holding:

  • critical routines (such as for interrupt handling)

  • scratchpad data

  • data types whose locality is not suited to caching

  • critical data structures (such as interrupt stacks).

Trap

An exceptional condition in a VFP coprocessor that has the respective exception enable bit set in the FPSCR register. The user trap handler is executed.

Undefined

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

UNP

See Unpredictable.

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.

Unsupported values

Specific data values that are not processed by the VFP coprocessor hardware but bounced to the support code for completion. These data can include infinities, NaNs, subnormal values, and zeros. An implementation is free to select which of these values is supported in hardware fully or partially, or requires assistance from support code to complete the operation. Any exception resulting from processing unsupported data is trapped to user code if the corresponding exception enable bit for the exception is set.

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.

Write

Writes are defined as operations that have the semantics of a store. That is, the ARM instructions SRS, STM, STRD, STC, STRT, STRH, STRB, STRBT, STREX, SWP, and SWPB, and the Thumb instructions STM, STR, STRH, STRB, and PUSH. Java instructions that are accelerated by hardware can cause a number of writes to occur, according to the state of the Java stack and the implementation of the Java hardware acceleration.

Write completion

The memory system indicates to the processor that a write has been completed at a point in the transaction where the memory system is able to guarantee that the effect of the write is visible to all processors in the system. This is not the case if the write is associated with a memory synchronization primitive, or is to a Device or Strongly Ordered region. In these cases the memory system might only indicate completion of the write when the access has affected the state of the target, unless it is impossible to distinguish between having the effect of the write visible and having the state of target updated.

This stricter requirement for some types of memory ensures that any side-effects of the memory access can be guaranteed by the processor to have taken place. You can use this to prevent the starting of a subsequent operation in the program order until the side-effects are visible.

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