Path: utzoo!utgpu!water!watmath!clyde!rutgers!husc6!bbn!uwmcsd1!ig!agate!ucbvax!hplabs!decwrl!labrea!rocky!rokicki
From: rokicki@rocky.STANFORD.EDU (Tomas Rokicki)
Newsgroups: comp.sys.amiga
Subject: Re: Structure alignment in Manx C
Message-ID: <1047@rocky.STANFORD.EDU>
Date: 14 Feb 88 20:23:47 GMT
References: <8802141808.AA27862@cory.Berkeley.EDU>
Reply-To: rokicki@rocky.UUCP (Tomas Rokicki)
Organization: Stanford University Computer Science Department
Lines: 32

> 	You might want a more compact structure.
> 
> 	C-A sure did, and...
> 
> 	If you were to make longword alignment the default, your C compiler
> would suddenly become totally incompatible with almost every Amiga structure
> in existance.
> 
> 	For example:  struct Menu, &menu.MenuName relative to &menu.NextMenu
> is on a word boundry.  struct MenuItem, Node, Message, and thus IORequest,
> IOStdReq, etc.....
> 
> 					-Matt

Oh my, I didn't realize the problem was quite so deep seated!  Even the
Amiga software wasn't really designed with the 68020 in mind, then!  I
always order my structures so longwords stay on longword boundaries.  You
should lose at most three bytes per structure.  And the structures are
ingrained enough so they will not change . . .  You are absolutely correct,
though, about Amiga structures not being organized around a 32-bit word,
even at the lowest level (struct Node{struct Node *ln_Succ,*ln_Pred;
UBYTE ln_type;BYTE ln_Pri;char *ln_Name;};).  Ouch, ouch, double ouch.
Putting in a SHORT ln_Pad would have added 2 bytes to this 14-byte
structure; I think that would have been a worthwhile tradeoff.  No use
crying over spilt milk, I guess.

But if some new, later Amiga does have such changes that most code needs
recompiling to run (for instance, MEMF_PUBLIC enforced due to an MMU or
some such) then it might be worthwhile to investigate the cost of making
this change.

-tom