Re: Kernel size over 64K: what is needed, how to implement?

David Given <dg@xxxxxxxxxxx> · Sun, 19 Feb 2012 21:36:01 +0000

On 17/02/12 22:51, David Given wrote:
[...]
> Regarding C compilers --- has anyone ever tried Open Watcom? It claims
> to support all these weird 8086 segmentation modes, and there is a Linux
> version. It's also supposed to produce pretty decent code, which may
> well be more compact than bcc's and so save crucial kernel space.  Never
> touched it myself, though.

I just had a play. Apparently Open Watcom produces OMF object files,
which none of myself, binutils-multiarch or file have ever heard of.
Rumour has it that the Open Watcom linker is capable of being scripted
--- I don't know whether this is to a useful extent.

OTOH the code is definitely good. Here's a source file:

int global;

fnord(a, b)
	int a;
	int b;
{
	int i = a + b;
	int j = global - a;
	global += i;
	return j;
}

(K&R C because I wanted to use the same file for Open Watcom and bcc;
Open Watcom supports ANSI. It even supports some C99.)

In fastcall mode, Open Watcom produces:

0000                          @fnord:
0000    01 C2                     add         dx,ax
0002    8B 1E 00 00               mov         bx,word ptr _global
0006    29 C3                     sub         bx,ax
0008    89 D8                     mov         ax,bx
000A    01 16 00 00               add         word ptr _global,dx
000E    C3                        ret

In cdecl mode, it produces:

0000                          _fnord:
0000    55                        push        bp
0001    89 E5                     mov         bp,sp
0003    8B 46 04                  mov         ax,word ptr 0x4[bp]
0006    8B 56 06                  mov         dx,word ptr 0x6[bp]
0009    01 C2                     add         dx,ax
000B    8B 1E 00 00               mov         bx,word ptr _global
000F    29 C3                     sub         bx,ax
0011    89 D8                     mov         ax,bx
0013    01 16 00 00               add         word ptr _global,dx
0017    5D                        pop         bp
0018    C2 04 00                  ret         0x0004

bcc produces (and you may want to brace yourselves):

_fnord:
push	bp
mov	bp,sp
push	di
push	si
dec	sp
dec	sp
mov	ax,4[bp]
add	ax,6[bp]
mov	-6[bp],ax
dec	sp
dec	sp
mov	ax,[_global]
sub	ax,4[bp]
mov	-8[bp],ax
mov	ax,[_global]
add	ax,-6[bp]
mov	[_global],ax
mov	ax,-8[bp]
add	sp,*4
pop	si
pop	di
pop	bp
ret

Note that that last is without optimisation; when I try -O I get an
error message about not being able to find /usr/lib/bcc/rules.start. Is
optimisation supported for the 8086? Also I notice that the bcc code
seems to be returning the value in ax, so I may not have got the calling
conventions to match.

If anyone actually wanted to use this, I suspect the easiest thing to do
would be to compile with Open Watcom, disassemble the object files and
use a script of some kind to convert them to gas syntax. Which would
suck, but is almost certainly easier than adding OMF support to dev86.

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