Question

From jmorecfg.h:

#define PACK_TWO_PIXELS(l,r)   ((r<<16) | l)
#define PACK_NEED_ALIGNMENT(ptr) (((int)(ptr))&3)
#define WRITE_TWO_PIXELS(addr, pixels) do {     \
         ((INT16*)(addr))[0] = (pixels);        \
         ((INT16*)(addr))[1] = (pixels)>>16;    \
    } while(0)
#define WRITE_TWO_ALIGNED_PIXELS(addr, pixels)  ((*(INT32*)(addr)) = pixels

Can someone explain the difference between WRITE_TWO_PIXELS and WRITE_TWO_ALIGNED_PIXELS? If pixels is a stack allocated uint32_t and addr & 3 == 0, shouldn’t they be equivalent?

Thanks.

Answer 1

WRITE_TWO_PIXELS and WRITE_TWO_ALIGNED_PIXELS are equivalent for little endian machines but not for big endian architecture.

[Example edited: thanks to Steve Jessop]

Let, pixels = 0x0A0B0C0D

For big endian machines, WRITE_TWO_PIXELS work as follows:

---------------------
| 0B | 0A | 0D | 0C |
---------------------
  3    2    1    0          <--- addr

where WRITE_TWO_ALIGNED_PIXELS will write as follows:

---------------------
| 0D | 0C | 0B | 0A |
---------------------
  3    2    1    0          <--- addr