Re: A20 Address line - XMS issue

[Stas Sergeev <stssppnn@yahoo.com>]

Hello.

S.Gopi wrote:
> not possible and I was advised to use XMS and EMS settings available 
> under DOSEmu. But I read from FAQs is that I cannot use them if my 
> program uses VCPI
Not "them", you can't use *dosemu* if
your program uses VCPI. Use bochs
instead. The reason is already described
in the nearby message.

> or some other method to access high memory access.
Not exactly. XMS and EMS are both
supported so you can use them for
accessing a high memory. DPMI is
also supported (16bit and 32bit)
so that you can run your programs
in protected mode.

> after some quick hack into dos code I 
> found that DOSEmu is explicitly disabling A20 line and denying any 
> further interrupt calls on that (i guess it is int15,0x87 and 0x88)
> May i know why is it so?
I think it is because there is no
use to control a20 under dosemu.
It is emulated however, so I think
dosemu may not be so picky about
that.

> If DOSEmu can allow these interrupts then 
> programs like himem.sys can use of this
himem uses so called "Big Real Mode".
To enable this mode it must be started
from real mode. himem cannot start in v86
by any means.

> and provide the memory manager suppor
Internally provided by dosemu.

> with VCPI support etc.
himem have nothing to do with VCPI.

> Is there any technical difficulty or 
> memory blocking issues with this or it is disabled for some anonymous 
> reasons.
A20 emulation was implemented much
later than an xms support, I think
this is a reason why that emulation
is not fully used.

> Is there any alternative way for this.
Internal XMS support of dosemu is a
good alternative to himem.
If you have the program that uses
int15/ah=0x87 for memory transferrs
and that fails under dosemu, then I
think this can be fixed, but I need
this program to expirement with.