Glossary

This glossary describes some of the terms used in this manual. 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 a Prefetch Abort, a Data Abort, or an External Abort.

See Also Data Abort, External Abort, and Prefetch Abort.

Abort model

An abort model is the defined behavior of an ARM processor in response to a Data Abort exception. Different abort models behave differently with regard to load and store instructions that specify base register write-back.

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.

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.

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.

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 state

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

See Also Thumb state.

Base register

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

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.

Block address

An address that comprises a tag, an index, and a word field. The tag bits identify the way that contains the matching cache entry for a cache hit. The index bits identify the set being addressed. The word field contains the word address that can be used to identify specific words, halfwords, or bytes within the cache entry.

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.

Cache

A block of on-chip or off-chip fast access memory locations, situated between the processor and main memory, used for storing and retrieving copies of often used instructions and/or data. This is done to increase the average speed of memory accesses and therefore to increase processor performance.

Cache hit

A memory access that can be processed at high speed because the instruction or data that it addresses is already held in the cache.

Cache line

The basic unit of storage in a cache. It is always a power of two words in size (usually 4 or 8 words), and is required to be aligned to a suitable memory boundary.

See Also Cache terminology.

Cache lockdown

To fix a line in cache memory so that it cannot be overwritten. Cache lockdown enables critical instructions and/or data to be loaded into the cache so that the cache lines containing them are not subsequently reallocated. This ensures that all subsequent accesses to the instructions or data concerned are cache hits, and therefore complete as quickly as possible.

Cache miss

A memory access that cannot be processed at high speed because the instruction or data it addresses is not in the cache and a main memory access is required.

Cache set

A cache set is a group of cache lines (or blocks). A set contains all the ways that can be addressed with the same index. The number of cache sets is always a power of two.

Cast out

See Victim.

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.

Clean

A cache line that has not been modified while it is in the cache is said to be clean. To clean a cache is to write dirty cache entries into main memory. If a cache line is clean, it is not written on a cache miss because the next level of memory contains the same data as the cache.

See Also Dirty.

Coprocessor

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

CPU

See Central Processing Unit.

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.

Data Cache (DCache)

A block of on-chip fast access memory locations, situated between the processor and main memory, used for storing and retrieving copies of often used data. This is done to greatly increase the average speed of memory accesses and therefore to increase processor performance.

DCache

See Data Cache.

Debug 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 Communications Channel.

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.

Dirty

A cache line in a Write-Back cache that has been modified while it is in the cache is said to be dirty. A cache line is marked as dirty by setting the dirty bit. If a cache line is dirty, it must be written to memory on a cache miss because the next level of memory contains data that has not been updated. The process of writing dirty data to main memory is called cache cleaning.

See Also Clean.

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.

Exception 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 service routine.

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.

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.

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.

Host

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

ICache

See Instruction Cache.

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 Cache (ICache)

A block of on-chip fast access memory locations, situated between the processor and main memory, used for storing and retrieving copies of often used instructions. This is done to increase the average speed of memory accesses and therefore to increase processor performance.

Invalidate

To mark a cache line as being not valid by clearing the valid bit. This must be done whenever the line does not contain a valid cache entry. For example, after a cache flush all lines are invalid.

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.

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 an ARM processor, an Embedded Trace Macrocell, and a memory block) plus application-specific logic.

Monitor mode

One of two mutually exclusive debug modes. In monitor mode the ARM1136JF-S 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.

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 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.

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.

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.

Tag

The upper portion of a block address used to identify a cache line within a cache. The block address from the CPU is compared with each tag in a set in parallel to determine if the corresponding line is in the cache. If it is, it is said to be a cache hit and the line can be fetched from cache. If the block address does not correspond to any of the tags it is said to be a cache miss and the line must be fetched from the next level of memory.

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

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 Architecture 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.

Vector operation

An operation involving more than one destination register, perhaps involving different source registers in the generation of the result for each destination.

Victim

A cache line, selected to be discarded to make room for a replacement cache line that is required as a result of a cache miss. The way in which the victim is selected for eviction is processor-specific. A victim is also known as a cast out.

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.

WB

See Write-back.

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-back(WB)

In a write-back cache, data is only written to main memory when it is forced out of the cache on line replacement following a cache miss. Otherwise, writes by the processor only update the cache. (Also known as copyback).

Write buffer

A block of high-speed memory, arranged as a FIFO buffer, between the Data Cache and main memory, whose purpose is to optimize stores to main memory. Each entry in the write buffer can contain the address of a data item to be stored to main memory, the data for that item, and a sequential bit that indicates if the next store is sequential or not.

Write-through (WT)

In a write-through cache, data is written to main memory at the same time as the cache is updated.

WT

See Write-through.

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