ac> Does anyone know of any other DOS or OS/2 compilers that align structures
 ac> by default?

Most do. Borland is the exception rather than the norm in its default
setting. Either way, it is an implementation option.

 ac> Also, does aligning structures at 2- or 4-byte boundaries
 ac> really provide a distinct speed increase in an 'average' program?

The issue isn't quite that simple, but certainly accessing words on word
boundaries in 16-bit mode is faster than odd-byte word fetches.

On some platforms (not intel), the option of accessing data at other than
alignment boundaries of a size determined by the machine is not available.
While you can fetch bytes at a time from a block of memory, the code
produced will usually load the data at the required alignment, then mask
and shift it into the CPU register for convenience to deal with it.  A
statement such as:

    char b = char_block[3];

might produce something like:

    load reg_a,char_block
    and  reg_a,0x00ff0000
    shr  reg_a,16

... which is a whole *lot* less efficient than your typical byte fetch.

Such compilers will often require alignment of longs and (almost always)
floats no matter what, since generating code that attempts to fetch with
adjustment for alignment is pretty hairy. On those machines, the usual
result of generating bad code which doesn't do it correctly is a trap or
crash.

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