Forum Discussion

Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
15 years ago

gcc alignment

I'm surprised. I have a C++ code that fills a char array and there seems to be an alignment problem with g++. See below: char* response = new char; response = 0; unsigned int* dataSize = (uns...
Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
15 years ago

From the manual:

--- Quote Start ---

STW

Description

Computes the effective byte address specified by the sum of rA and the instruction's signed 16-bit immediate value. Stores rB to the memory location specified by the effective byte address. the effective byte address must be word aligned. If the byte address is not a multiple of 4, the operation is undefined.

--- Quote End ---

Seems that it is not gcc doing this. The compiled code actually uses the value pointer+1 to do a STW (store word) instruction. It is the NIOS instruction set that aligns the store word instruction (it appears to truncate the low 2 bits of the store address for STW, but the manual says "undefined", meaning that it might be different in a different version of the processor).

No way around it. To store at byte addresses, you are going to have to use bytes, or use a different processor, or write your own routine for storing words at byte addresses (another version of "using bytes");