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



Wow. Just wow. I am not keen on assembler, but I can quite well
understand what is going on here. OpenWatcom cdecl is nicer for
debugging because it generates a frame pointer on the stack. Fastcall
makes smaller (and likely faster) code. Both blow bcc away (which I'm
not even sure what all the code it's generating is doing, or much less
even needed)

OMF is the Object Module Format defined by Intel for the 16-bit PC
OSes. It's nearly universal as a format during the 16-bit DOS, OS/2
and Windows era. Watcom still uses it for two reasons: 1.) It has
support for segments, including in the OMF-386 variant. 2.) Near
universal compatibility with the vast majority of 16-bit development
systems for DOS, OS/2 1.x, and Win16 in existence.

See http://en.wikipedia.org/wiki/Relocatable_Object_Module_Format for
a basic brief. As for interpreting the format, see
http://www.azillionmonkeys.com/qed/Omfg.pdf (it's worth noting that
"omfg" is probably an accurate response to OMF as an object format.
Especially after reading this doc.)

Sent from my HTC Inspire™ 4G on AT&T

On Sun, Feb 19, 2012 at 16:36, David Given <dg@xxxxxxxxxxx> wrote:
> 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.
>
> --
> ┌─── dg@cowlark.com ───── http://www.cowlark.com ─────
> │
> │ "Never attribute to malice what can be adequately explained by
> │ stupidity." --- Nick Diamos (Hanlon's Razor)
>
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