What is unaligned memory access?

Unaligned memory accesses occur when you try to read N bytes of data starting from an address that is not evenly divisible by N (i.e. addr % N != 0). For example, reading 4 bytes of data from address 0x10004 is fine, but reading 4 bytes of data from address 0x10005 would be an unaligned memory access.

Does x86 support unaligned access?

There’s HW support for detecting unaligned accesses on x86, in the form of the AC bit in EFLAGS. But that’s normally unusable because compilers (and hand-written asm memcpy etc. in libc) sometimes use unaligned loads, e.g. to initialize or copy adjacent narrow members of a struct.

Does ARM support unaligned access?

Beginning with ARMv7, however, unaligned access began to be supported. It now does the expected, i.e. breaking up the access into multiple smaller reads and builds up the value as a “traditional” x86 CPU would do it.

Why is alignment important in memory?

Alignment refers to the arrangement of data in memory, and specifically deals with the issue of accessing data as proper units of information from main memory. First we must conceptualize main memory as a contiguous block of consecutive memory locations. Each location contains a fixed number of bits.

What do you mean by aligned and unaligned address?

The alignment of the access refers to the address being a multiple of the transfer size. For example, an aligned 32 bit access will have the bottom 4 bits of the address as 0x0, 0x4, 0x8 and 0xC assuming the memory is byte addressed. An unaligned address is then an address that isn’t a multiple of the transfer size.

What is alignment exception?

Note that the alignment check exception (#AC) is generated only for data types that must be aligned on word, doubleword, and quadword boundaries. A general-protection exception (#GP) is generated 128-bit data types that. are not aligned on a 16-byte boundary.

What does 4 byte aligned mean?

For instance, in a 32-bit architecture, the data may be aligned if the data is stored in four consecutive bytes and the first byte lies on a 4-byte boundary. Data alignment is the aligning of elements according to their natural alignment.

What is Pragma pack in C++?

#pragma pack instructs the compiler to pack structure members with particular alignment. Most compilers, when you declare a struct, will insert padding between members to ensure that they are aligned to appropriate addresses in memory (usually a multiple of the type’s size).

What is alignment restriction?

Alignment restrictions are constraints in the type of distributions and the indexing into the matrix that the user may utilize when calling a particular routine. For example, some routines will not accept submatrices whose starting index is not a multiple of the physical blocking factor.

What is aligned access?

An aligned memory access means that the pointer (as an integer) is a multiple of a type-specific value called the alignment. The alignment is the natural address multiple where the type must be, or should be stored (e.g. for performance reasons) on a CPU.

What is the purpose of alignment of data in cells?

With MS Excel, cell alignment is how your text or numbers are positioned in the cell. You can align vertically, meaning towards the top, the middle or the bottom. And you can also align horizontally, meaning to the left, the center or to the right. Excel actually has its own defaults for alignment.

What is an unaligned memory access?

This is called an unaligned memory access. On other systems, such an operation will result in a bus fault, which generally triggers some form of interrupt which may or may not be able to do something useful about it.

Why do we need to align memory accesses in a CPU?

CPUs used to perform better when memory accesses are aligned, that is when the pointer value is a multiple of the alignment value. This differentiation still exists in current CPUs, and still some have only instructions that perform aligned accesses.

Why is my arm memory pointer not word-aligned?

The ARM documentation of this instruction [3] states that the memory pointer must be word-aligned (a word is 2 bytes in our case). The problem arises because our main function uses a buffer passed by the libc loader to argv, which has no alignment guarantees. Why does this happen?