MSP430F6779: Memory alignment and pointer variable using

TinK

Part Number: MSP430F6779

I test with code (IAR 5.60.1) below.

static unsigned long *test_value_p;

static unsigned long test_value_1;
static unsigned long test_value_2;
static unsigned char array[8] = {0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07};

test_value_p = (uint32_t *)(&array[1]);
test_value_2 = *test_value_p;

Expected output: test_value_2 , 0x04030201
Actual output: test_value_2, 0x03020100

Did it cause from compiler force the memory address to even number (test_value_2 = *test_value_p;)?

over 2 years ago

+1 David Schultz over 2 years ago

Guru 25505 points

Your expectations were wrong.

Do a word access from an odd address ("mov.w &0x2001,r15") and the hardware will ignore the lsb. Making it effectively zero. The compiler has no say in that.

0 TinK over 2 years ago in reply to David Schultz

Intellectual 815 points

Hi David Schultz

Thank you very much.

Is there any method (e.g IDE configuration) to warn word access from an odd address(in C language using pointer variable deference from odd address)?

0 Xiaodong LI over 2 years ago in reply to TinK

TI__Mastermind 23705 points

Hi Tink

You can check if varable type char or unsigned char can work.

Please refer to https://www.ti.com/lit/pdf/slau132 MSP430 Optimizing C/C++ Compiler UG

Thanks!

0 TinK over 2 years ago in reply to Xiaodong LI

Intellectual 815 points

Hi Xiaodong LI.

Thank for information.

That mean we have to manual check data type right?, does it mean complier (IAR or CCS) cannot warning about this issue?

Could i have automatic method to check access from an odd address?

https://www.iar.com/knowledge/support/technical-notes/compiler/accessing-unaligned-data/

0 David Schultz over 2 years ago in reply to TinK

Guru 25505 points

In general, the compiler cannot warn you about addresses that are determined at run time.

But this reminds me that I have had problems with the GNU C compiler where it propagates information in unexpected ways. Such that assigning an address that started out pointing to a char to an integer pointer (with a cast of course and aligned) caused the compiler to do byte at a time access. Very annoying.

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MSP430F6779: Memory alignment and pointer variable using