What is the reason that memory alignment is necessary at times in assembly language?

What is the reason that memory alignment is necessary at times in assembly language?

The CPU can operate on an aligned word of memory atomically, meaning that no other instruction can interrupt that operation. This is critical to the correct operation of many lock-free data structures and other concurrency paradigms.

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.

How does memory alignment work?

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. Example: A 32bit memory that is byte addressable. Each row denotes a location with a fixed size of eight bits (1byte) labeled zero through seven.

Is aligned memory faster?

Alignment helps the CPU fetch data from memory in an efficient manner: less cache miss/flush, less bus transactions etc. Some memory types (e.g. RDRAM, DRAM etc.) need to be accessed in a structured manner (aligned “words” and in “burst transactions” i.e. many words at one time) in order to yield efficient results.

What is Posix_memalign?

DESCRIPTION top. The function posix_memalign() allocates size bytes and places the address of the allocated memory in *memptr. The address of the allocated memory will be a multiple of alignment, which must be a power of two and a multiple of sizeof(void *). This address can later be successfully passed to free(3).

Do all architectures use data alignment?

Yes, they do have different memory alignment requirements. In real life a specific type is usually supposed/required to be aligned at the boundary that is the same as the size of the type, although theoretically the concepts of size and alignment have no connection to each other.

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 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.

How do you align a pointer?

Hands down the fastest way to align a pointer is to use 2’s complement math. You need to invert the bits, add one, and mask off the 2 (for 32-bit) or 3 (for 64-bit) least significant bits. The result is an offset that you then add to the pointer value to align it. Works great for 32 and 64-bit numbers.

Why does the stack need to be aligned?

The compiler is maintaining a 16-byte alignment of the stack pointer when a function is called, adding padding to the stack as necessary. The compiler knows that the stack will always be aligned correctly, so it can emit instructions with alignment requirements without risk of triggering their fault conditions.

Why memory addresses are often aligned on two four or eight byte boundaries?

Aligned access is faster because the external bus to memory is not a single byte wide – it is typically 4 or 8 bytes wide (or even wider). This means that the CPU doesn’t fetch a single byte at a time – it fetches 4 or 8 bytes starting at the requested address.

What happens when data is not aligned in memory?

If your data isn’t aligned, when you write something that is not a full row, you will need to do read-modify-write (read the old content of the row, modify the relevant part and write the new content). Data from memory is typically delivered to the processor on a set of wires that matches the bus width.

Why do memory instructions take 4 cycles in arm assembly?

Memory latency adds another layer of complication to this idea. ARM employs a multi-level cache system which aims to have the most frequently used data available in the fewest cycles. Even a read from the fastest (L0) cache involves several cycles of latency.

Do you think memory alignment is still important?

Yes, memory alignment still matters. Some processors actually can’t perform reads on non-aligned addresses.

How does an unaligned 32-bit read work?

An unaligned 32-bit read would require some memory systems to return data from one address, and some to return data from the next higher address.

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