A20 Address line - XMS issue

[S.Gopi]

Hi all,

First of all sorry if I am talking about something which is very silly and stupid topic.

I want to use HIMEM.SYS under DOSEMU with FreeDOS. I heard that this is 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 or some other method to access high memory access.

I tried loading HIMEM.sys and it bounced back with error message
'Unable to Control A20 Line', 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? If DOSEmu can allow these interrupts then programs like himem.sys can use of this and provide the memory manager suppor with VCPI support etc. Is there any technical difficulty or memory blocking issues with this or it is disabled for some anonymous reasons.

Is there any alternative way for this.

Thank you and sorry if this has been discussed already,
gopi


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