Atmel SAM4S Series, Preliminary Data Sheet

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11100B–ATARM–31-Jul-12
SAM4S Series [Preliminary]
11.12 Glossary
This glossary describes some of the terms used in technical documents from ARM.
Abort
A mechanism that indicates to a processor that the value associated with a memory access is invalid.
An abort can be caused by the external or internal memory system as a result of attempting to access
invalid instruction or data memory.
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.
Banked register
A register that has multiple physical copies, where the state of the processor determines which copy is
used. The Stack Pointer, SP (R13) is a banked register.
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 (LE)” .
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.