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- Unaligned accesses in C C++: what, why and solutions to do it properly
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
- Memory Alignment Errors - Micro Focus
What is a Memory Alignment Error? A "Memory alignment error" occurs in response to a Bus Error signal This signal is generated on some Unix machines (but not all) when a multi-byte data item is accessed on an inappropriate memory boundary
- How to Manage Memory Alignment in Modern C++
For atomic instructions to perform correctly, the addresses you pass them must be at least four-byte aligned (to avoid memory access across pages) Otherwise, it would cause failure, or worse, silent corruption Some instructions (like some AVX-512 instructions) are designed to have memory alignment requirements, for speed concerns
- The Intel 80386, part 10: Atomic operations and memory alignment
Memory access on the 80386 to misaligned locations are supported, although they will operate more slowly than their aligned counterparts If a memory access straddles a page boundary, an access violation will be raised if either page does not support the desired operation (not readable or not writable), and the instruction will not appear to
- Unaligned Memory Accesses — The Linux Kernel documentation
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
- Did any x86 CPU optionally trap unaligned access?
The problem you actually want to solve is to find alignment undefined behaviour in your C programs, because that's what will compile to broken asm on machines where misaligned loads fault Alignment checking in x86 asm (with the AC flag) could have false positives, both in your own code when memcpy inlines or many other ways, and in libc
- Guide to Memory Alignment in C++ – lt;PRACTICAL CPP gt;
Misaligned data can cause the CPU to perform extra work, such as reading multiple memory locations Hardware Requirements: Some architectures strictly enforce alignment and will throw errors if data is not properly aligned Memory Padding: To maintain proper alignment, compilers add extra unused bytes (padding) between members of a struct
- Why Do We Need Memory Alignment and What’s the Problem with It?
Alignment requirements stem from how memory works Your first point might be slightly off since the CPU can select bits from any part of the data bus, not just the rightmost bits So, to clarify your third point: physical memory is organized in 32-bit (4-byte) slots
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