Short/Char Memory Alignment

[Andy]

If I have a large array of shorts or chars in C, such as:

Code: Select all

const unsigned short huffTable[] = {
	0xf003, 0x3112, 0x3101, 0x2011....

How are these being packed into memory? I was wondering if they are word aligned by default and if so, can I pack 2 shorts or 4 chars into one 32 bit word to save space?

[richard]

There is no padding inside arrays. An array of n chars will take up n bytes. The ABI defines the layout of data types in memory, see the document XMOS XS1 32-Bit Application Binary Interface.

[Andy]

OK thanks - what kind of data does

All global objects are word aligned. Unsigned scalar types smaller than 32 bits are stored as zero extended 32-bit values

refer to?

[richard]

That refers to scalar globals such as "char c". If you have an array of chars as a global the base of the array will be word aligned but each element inside the array will not be word aligned.