Align Data and RAM Gaps

I have a combined C and AS application.  I have to use AS because of the 
time restraints.

I have a 256 byte buffer that is accessed with two pointers, head and tail.

In the AS code, I want to only save the low order byte of the pointer. 
This is do-able, if the buffer is aligned at 0x0100.

In the C file,
char buf[256] __attribute__ ((aligned (256)));
In the AS file,
.global buf

Looking at the map file, I see

   0x0000000000800100                tx_buf_head_ptr
   0x0000000000800200                buf
   0x0000000000800300                rx_buf_tail_ptr
   0x0000000000800302                tx_buf_tail_ptr

There is a gap of 254 bytes.

How do I avoid this gap?  What determines the linker allocation order?

Tom Dean