#Iar arm compiler v7 profiles how to#
#Iar arm compiler v7 profiles serial#
#Iar arm compiler v7 profiles manual#
#Iar arm compiler v7 profiles portable#
#Iar arm compiler v7 profiles code#

So, how can the compiler help us to avoid unaligned accesses? The answer is: We need to inform the compiler that the data is unaligned.

#Iar arm compiler v7 profiles portable#

Silently perform a different memory accessДЄs we can see, to create a portable application that can be used on many architectures, you should avoid unaligned accesses, and avoid relying on undefined behavior. Some architectures are not capable of unaligned memory access, but will Some architectures raise processor exceptions when unaligned accesses Transparently, but there is usually a significant performance cost. Some architectures are able to perform unaligned memory accesses Unaligned memory accesses are also described in the Linux Kernel documentation: The effects of performing an unaligned memory access vary In the Configuration and Control Register. To trap accidental generation of unaligned accesses, use the UNALIGN_TRP bit Therefore, ARM recommends that programmers ensure that accesses are aligned. In addition, some memory regions might not support unaligned accesses. Unaligned accesses are usually slower than aligned accesses. Instructions: LDR, LDRT, LDRH, LDRHT, LDRSH, LDRSHT, STR, STRT, STRH, STRHT This is from the Cortex-M3 Devices Generic User Guide: The Cortex-M3 processor supports unaligned access only for the following

#Iar arm compiler v7 profiles code#

With a Cortex-M3, the code might generate a UsageFault, depending on whether CCR.UNALIGN_TRP is set to 1 or not.ДЄs we can see, the behavior of the unaligned access is undefined and depends on the processor architecture. So, with a Cortex-M0, the code will always generate a UsageFault. Generate alignment faults when the CCR.UNALIGN_TRP bit is set to 1: The following data accesses support unaligned addressing, and only

#Iar arm compiler v7 profiles manual#

The following data accesses always generate an alignment fault:ДЇor Cortex-M3, M4, and M7, the ARM v7-M Architecture Reference Manual informs us: A3.2.1 Alignment behavior

On a Cortex-M3, the STR instruction accepts unaligned addresses.ДЇor Cortex-M0, M0+, and M1, the Armv6-M Architecture Reference Manual informs us: A3.2.1 Alignment behavior On a Cortex-M0, the STR instruction used by set_data requires an aligned address. With a Cortex-M3, the same function call works fine. With a Cortex-M0, the result of set_data is a UsageFault exception. With the cast, we have tried to 'fool' the compiler by saying that the uint8_t byte pointer &network_data really is an aligned uint32_t pointer, which is not true. Now, using the resulting data_p pointer gives undefined behavior.ДЇor example, calling our set_data function with data_p results in undefined behavior: set_data(data_p, 1200) If we refer to an 'odd' byte in our byte array and convert it to uint32_t with a cast, the behavior is undefined, because the resulting pointer is not correctly aligned for the pointed-to type: data_p = (uint32_t*) &network_data Now, assume that we have a byte array, like this: uint8_t network_data = Fooling the compiler (If not, the behavior would be undefined.) With the following function definition, we tell the compiler that out_p is a pointer to an aligned uint32_t variable: void set_data(uint32_t *out_p, uint32_t val) This is a pointer to uint32_t: uint32_t *data_p Д«ecause the 32-bit data type uint32_t has an alignment requirement, we know that the uint32_t pointer is correctly aligned. If the resulting pointer is not correctly aligned for the pointed-to type, the behavior is undefinedвЂќ. вЂњA pointer to an object or incomplete type may be converted to a pointer to a different object or incomplete type.

DiscussionДЄccording to the C language standard ISO/IEC 9899:

#Iar arm compiler v7 profiles how to#

This Technical Note shows how to inform the compiler about the unaligned data, and thereby avoid trouble.

Accessing the unaligned data in a safe and portable way can be tricky-the result can depend on the CPU architecture, the compiler optimization level, or even which memory region you are working with.

#Iar arm compiler v7 profiles serial#

Perhaps the data is in a buffer received from a network or serial link. Sometimes you want to access unaligned data.