virtual "8086" mode

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b1528932

Joined: 21 May 2010
Posts: 287

b1528932 12 Feb 2011, 15:57

Why when runing in virtual8086 mode pop cs is unavaiable, push sp pushes old vlaue of sp, shifts by imm8 are possible? 8086 doesnt have those features. Its not 8086 emulation, like name would suggest. Its a real mode emulation.

q2: why fasm cant use dword operand on sgdt/sidt?

q3: why my idtr (real hardware) has initially 0xFFFF limit?

12 Feb 2011, 15:57

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20343
Location: In your JS exploiting you and your system

revolution 12 Feb 2011, 16:06

b1528932 wrote:

Why when runing in virtual8086 mode pop cs is unavaiable, push sp pushes old vlaue of sp, shifts by imm8 are possible? 8086 doesnt have those features. Its not 8086 emulation, like name would suggest. Its a real mode emulation.

All of these things are explained in the Intel manuals in the compatibility section. Except pop cs which has never been documented. You can use SSE4 in V8086 mode also. If you really must have real 8086 emulation then you have to use a real 8086 CPU. Finding one may be tricky though.

12 Feb 2011, 16:06

b1528932

Joined: 21 May 2010
Posts: 287

b1528932 12 Feb 2011, 16:10

But then why call it virtual 8086, not virtual real or virtual 80386.

12 Feb 2011, 16:10

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid 12 Feb 2011, 16:12

Maybe because 286 already supported protected mode?

12 Feb 2011, 16:12

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20343
Location: In your JS exploiting you and your system

revolution 12 Feb 2011, 16:17

b1528932 wrote:

But then why call it virtual 8086, not virtual real or virtual 80386.

Because it gives real mode addressing with the awful 64k segments and 16-bit registers by default and all the other real mode stuff but within a PM environment trapping the naughty hardware control instructions. Don't get so hung up on the names of things, just look at the functionality.

12 Feb 2011, 16:17

DOS386

Joined: 08 Dec 2006
Posts: 1902

DOS386 13 Feb 2011, 04:25

b1528932 wrote:

Why when runing in virtual8086 mode pop cs is unavaiable, push sp pushes old vlaue of sp, shifts by imm8 are possible?

mov [$FFFF], AX works on 8086 but not in NTVDM/V86 Smile

_________________
Bug Nr.: 12345

Title: Hello World program compiles to 100 KB !!!

Status: Closed: NOT a Bug

13 Feb 2011, 04:25

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