Question about how DOSEMU works

Question about how DOSEMU works

[complaw]

Hi:

I've downloaded the source code for DOSEMU, but was wondering if someone could
help me to look for the code for a specific feature.

I know that DOSEMU does some form of binary translation on instructions from the
DOS application.  However, for performance gains, it is usually faster to simply
allow direct (native) execution of the DOS application's instruction.  (See,
e.g., http://www.deatech.com/deatech/articles/linux_pc_vc.html ).  Does DOSEMU
allow (enable) direct execution of instructions?  I presume that all
instructions coming out of the DOS application are trapped.  Is that correct? 
Does that trap first look at the instruction and, depending on what it is,
simply allow the instruction to be executed (and, if not, do a binary
translation)?   If so, where in the source code is that decision logic?

Best wishes,

Ron

Re: Question about how DOSEMU works

[Bart Oldeman]

On Tue, 13 Aug 2002 complaw@hal-pc.org wrote:

> I know that DOSEMU does some form of binary translation on instructions from the
> DOS application.  However, for performance gains, it is usually faster to simply
> allow direct (native) execution of the DOS application's instruction.  (See,
> e.g., http://www.deatech.com/deatech/articles/linux_pc_vc.html ).  Does DOSEMU
> allow (enable) direct execution of instructions?  I presume that all
> instructions coming out of the DOS application are trapped.  Is that correct? 
> Does that trap first look at the instruction and, depending on what it is,
> simply allow the instruction to be executed (and, if not, do a binary
> translation)?   If so, where in the source code is that decision logic?

DOSEMU runs code natively by default. Either using vm86(2) or just by
jumping into it for DPMI. It traps privileged instructions using faults.

Search for cpuemu in the source if you want CPU emulation too.

CPU emulation is also used for planar video mode emulation under X
(for instance vga 640x480x16 colours)

Bart