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Glossary

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

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.

Base register

In instruction descriptions, 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.

See Also Index register.

Big-endian (BE)

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

See Also Byte-invariant, Endianness, Little-endian.

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 are removed after the program is successfully tested.

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. An ARM byte-invariant implementation also supports unaligned halfword and word memory accesses. It expects multi-word accesses to be word-aligned.

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, data, or instructions and data. This is done to greatly increase the average speed of memory accesses and so improve processor performance.

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.

Context

The environment that each process operates in for a multitasking operating system. In ARM processors, this is limited to mean the physical address range that it can access in memory and the associated memory access permissions.

Debugger

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

Direct Memory Access (DMA)

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

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 event that interrupts program execution. When an exception occurs, the processor suspends the normal program flow and starts execution at the address indicated by the corresponding exception vector. The indicated address contains the first instruction of the handler for the exception.

An exception can be an interrupt request, a fault, or a software-generated system exception. Faults include attempting an invalid memory access, attempting to execute an instruction in an invalid processor state, and attempting to execute an undefined instruction.

Exception vector

See Interrupt vector.

Halfword

A 16-bit data item.

Implementation-defined

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

Implementation-specific

The behavior is not architecturally defined, and does not have to be documented by individual implementations. Used when there are a number of implementation options available and the option chosen does not affect software compatibility.

Index register

In some load and store instruction descriptions, the value of this register is used as an offset to be added to or subtracted from the base register value to form the address that is sent to memory. Some addressing modes optionally enable the index register value to be shifted prior to the addition or subtraction.

See Also Base register.

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.

Little-endian (LE)

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

See Also Big-endian, Byte-invariant, 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.

Read

Reads are defined as memory operations that have the semantics of a load. Reads include the Thumb instructions LDM, LDR, LDRSH, LDRH, LDRSB, LDRB, and POP.

Region

A partition of 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.

Thumb instruction

One or two halfwords that specify an operation for a processor to perform. Thumb instructions must be halfword-aligned.

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.

Undefined

Indicates an instruction that generates an Undefined instruction exception.

Unpredictable

You cannot rely on the behavior. Unpredictable behavior must not represent security holes. Unpredictable behavior must not halt or hang the processor, or any parts 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.

WA

See Write-allocate.

WB

See Write-back.

Word

A 32-bit data item.

Write

Writes are defined as operations that have the semantics of a store. Writes include the Thumb instructions STM, STR, STRH, STRB, and PUSH.

Write-allocate (WA)

In a write-allocate cache, a cache miss on storing data causes a cache line to be allocated into the cache.

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. This is 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.

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.