16-bit code and L4

[Valery V. Sedletski]

On Wed, 20 Jun 2007 13:42:57 +0200, Marcus Voelp wrote:

>Hi Valery,
>
>> 2) Must all the segment registers be CS == DS == ES == FS == GS == SS == FLAT in L4 usermode 
programs (in Intel 
>> architecture implementation)? Or, there exist a possibility to use segments other than FLAT?
>>   
>Fiasco preserves some of these segments (e.g., GS to support thread 
>local storage). However, unless OS/2 makes use of these segments in 
>32-bit mode there is no need to change these from a flat setting.

It seems, that OS/2 does not use GS (GS seems to be always == 0), at least, in application programs.

But, if the program contains 16-bit segments, there can be the need to execute 16-bit code in these 
segments. In OS/2, 16-bit protected mode segments can coexist with 32-bit flat segments. When 32-bit part  
of a program calls the 16-bit part, it comes through the thunk (assembler code, which converts 32-bits flat 
pointers into 16:16 pointers and passes control from 32-bit part to 16-bit one). So, during the passing through 
thunk, segment registers are loaded with 16-bit segment selector. So, there is a need in segments, other than 
flat (mostly, 16-bit ones).

>> 3) Is it possible to cause the L4 kernel to create 16-bit segment descriptor on Intel architecture?
>>   
>What you are looking for is microkernel support for the virtual x86 
>mode.

But, V86 mode can help emulate Real mode. It is mostly useful for creating DOS emulator, but how can it 
help with creating 16-bit protected mode support? So, 16-bits != real mode (or I don't understand)

>Unfortunately, only the old L4 assembler kernels supported this 
>processor mode and these are no longer supported. If you really want to 
>run the 16-bit code on bare metal (and not in something like qemu) you 
>probably need to add support for VM86 into the kernel (e.g., by adding 
>an architecture specific systemcall which executes some code in VM86 
>mode and which returns with any abnormalities to a 32-bit exception 
>handler).
>

So, the V86 support can be implemented through architecture specific system call.
And, probably, we need not only V86 support, but also a means to cause the L4 kernel
to allocate GDT selector for 16-bit protected mode code support?
So, the only way to execute 16-bit code (i.e., code in segmens with "16-bit" descriptor attribute)
in L4 is to fork microkernel source code, adding custom changes?

>Best regards
>
>    Marcus
>
>-- 
>Marcus V÷lp

Thanks for answering my questions,
WBR,
Valery