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What is the difference between aligned and unaligned memory access?
I work on an TMS320C64x DSP, and I want to use the intrinsic functions (C functions for assembly instructions) and it has
ushort & _amem2(void *ptr); ushort & _mem2(void *ptr); where _amem2 does an aligned access of 2 bytes and _mem2 does unaligned access.
When should I use which?
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A memory access is said to be aligned when the data being accessed is n bytes long and the datum address is n-byte aligned. When a memory access is not aligned, it is said to be misaligned. Note that by definition byte memory accesses are always aligned.
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.
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.
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.
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. For example, a CPU might require that all two-byte loads or stores are done through addresses that are multiples of two. For small primitive types (under 4 bytes), the alignment is almost always the size of the type. For structs, the alignment is usually the maximum alignment of any member.
The C compiler always puts variables that you declare at addresses which satisfy the "correct" alignment. So if ptr points to e.g. a uint16_t variable, it will be aligned and you can use _amem2. You need to use _mem2 only if you are accessing e.g. a packed byte array received via I/O, or bytes in the middle of a string.
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