What is an 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.
What is unaligned access arm?
Hence, unaligned access simply means that a memory address that is being accessed is not aligned to the proper value, some instructions like LDRH require a 2-byte alignment, whereas instructions like LDR and STR require a 4-byte alignment for optimal performance.
Does MIPS support unaligned memory access?
Unaligned memory access on the MIPS R4000 is performed with pairs of instructions. They store the upper and lower part of the word to the corresponding parts of memory. …
Is aligned memory faster?
It is widely reported that data alignment improves performances even on processors that support unaligned processing such as your x86 laptop. For example, an answer on Stack Overflow states that it is significantly slower to access unaligned memory (as in, several times slower).
What is misaligned address?
“Misaligned address” usually means that you have a CPU that requires certain alignment for certain data types (e.g. a 32-bit integer must be at a 32-bit aligned address like 0x1000 or 0x1004), and your code is attempting to violate that requirement (by attempting to read a 32-bit integer from address 0x1001).
What is 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.
Does arm allow unaligned accesses?
By default, ARM7 and ARM9 based microcontrollers do not allow un-aligned accesses to 16-bit and 32-bit data types. Depending on the device implementation, unaligned accesses may either result in a DATA ABORT or swapped memory bytes (the strange output you have seen).
What does 8 byte aligned mean?
An object that is “8 bytes aligned” is stored at a memory address that is a multiple of 8. Many CPUs will only load some data types from aligned locations; on other CPUs such access is just faster.
Why is data alignment necessary?
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.
Which of the following are unaligned addresses?
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. The meaning in AXI4 would be the same.
What is misaligned pointer?
A pointer misaligns if its address isn’t a multiple of its type’s alignment. Dereferencing a misaligned pointer has undefined behavior, and may result in a crash or degraded performance. Alignment violations occur frequently in code that serializes or deserializes data.
What is an unaligned address?
An unaligned address is then an address that isn’t a multiple of the transfer size. The meaning in AXI4 would be the same.
Which is an example of an 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.
What does unaligned access in LDR mean?
Hence, unaligned access simply means that a memory address that is being accessed is not aligned to the proper value, some instructions like LDRH require a 2-byte alignment, whereas instructions like LDR and STR require a 4-byte alignment for optimal performance. More on performance, later.
What happens when unaligned access is set on arm?
This version also added an A bit into the SCTLR (System Control Register) where you can enable alignment checking. Essentially, if this bit is set then every unaligned access will result in the ARM trapping your code into the trap specified by the trap-vector beginning at address zero.
What happens when unaligned access is not supported?
In case unaligned access is not supported by the microprocessor, an exception can be triggered (e.g bus error exception) when such access is attempted. Memory access from the point of view of the software is just instructions for reading or writing bytes of data to or from memory.