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> DOS > how to check if EMM386 is ON and switch it off?Goto page 1, 2 Next |
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revolution 25 Feb 2010, 15:17
Code: smsw ax shr ax,1 jc WeAreInProtectedMode |
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25 Feb 2010, 15:17 |
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f0dder 25 Feb 2010, 16:46
revolution wrote:
 _________________  - carpe noctem |
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25 Feb 2010, 16:46 |
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zhak 25 Feb 2010, 23:25
i did some investigations... emm uses int 0x67, so to check if we're in V86 mode i use the following routine (seems to be working):
Code:
push es
xor ax, ax
mov es, ax
;
;check if int 0x67 vector is set up
;
mov si, 0x67*4
mov ax, [es:si]
mov cx, [es:si+2]
or ax, cx
jz short no_ems
;
;check if VCPI is present
;
mov ax, 0xde00
int 0x67
or ah, ah
jnz short no_ems
;
;check if cpu is in V86 mode
;
pushf
pop ax
bt ax, 17 ;check VM flag in EFLAGS
jnc not_v86
;
;switch from V86 mode to protected
;
;<<< Fn 0xDE0C Int 0x67 call >>>
;
for now, i'm able to detect V86 mode. Fn 0xDE0C Int 0x67 is used to switch from V86 mode into protected. But i can't make it work, yet. Dunno how initialize this function call correctly. Always get triple fault. Keep investigating... |
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25 Feb 2010, 23:25 |
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revolution 26 Feb 2010, 02:09
zhak wrote:
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26 Feb 2010, 02:09 |
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zhak 26 Feb 2010, 09:07
hehe really, it doesn't ) midnight coding can give you pretty funny errors )
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26 Feb 2010, 09:07 |
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zhak 26 Feb 2010, 09:11
but why didn't fasm give me compile-time error? is it a fasm bug?
changed to pushfd pop eax bt eax, 17 now this test returns that VM is cleared. but direct access to cr0 is not allowed, which means that cpu is in PM, ring3. I'll try to allocate a mem page later today. I've read that this will enter v86 mode. if this helps, then will think what to do next. |
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26 Feb 2010, 09:11 |
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revolution 26 Feb 2010, 09:57
Did you try the code I posted above? It works no matter what mode because reading the MSW is not protected.
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26 Feb 2010, 09:57 |
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revolution 26 Feb 2010, 10:00
zhak wrote: but why didn't fasm give me compile-time error? is it a fasm bug? |
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26 Feb 2010, 10:00 |
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zhak 26 Feb 2010, 10:13
revolution wrote: Did you try the code I posted above? It works no matter what mode because reading the MSW is not protected. smsw will work to check if in protected, but it's not enough. the main purpose is to disable protected or v86 mode - to go back to old good real mode |
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26 Feb 2010, 10:13 |
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zhak 26 Feb 2010, 10:24
i thought of an idea that if i'm sure that i'm in v86 mode, then i use Fn 0xDE0C Int 0x67 to switch to protected mode and then disable it, because i only need unreal mode for one segment register.
another option is to learn how to use vcpi services correctly and use them for loading files above 1 mb. but this will make code more complex - routines for vcpi and routines for not vcpi, relocating, and all other stuff. so i'd prefer just to switch to plain real mode and then initialize flat unreal mode. |
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26 Feb 2010, 10:24 |
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revolution 26 Feb 2010, 10:28
Why not just use protected mode? Why go to all the trouble to disable things when instead you can use them for what they were designed for?
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26 Feb 2010, 10:28 |
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zhak 26 Feb 2010, 10:43
i cannot use int 13h functions in protected. my proggie is intended to run either from dos, or not (loaded by boot sector). and i'm trying to keep it smaller and easier. using vcpi services will split the code into two parallel routines - one part is executed under dos with emm386, another part - if dos is not used (or dos w/o emm386). it makes code size at least twice bigger. then i suppose i'll need to add dpmi support which will split my code into 3 streams. and i have only 64kbytes for everything
i thought it could be easier to disable all third-party features (dos) and run full initialization in one stream. another option is to exit with notification "please, turn off EMM services" but it's not very user-friendly. (another reason i don't want to exit - because its tooooo simple ) |
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26 Feb 2010, 10:43 |
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DOS386 17 Mar 2010, 07:10
Code:
pushf
pop ax
bt ax, 17 ;check VM flag in EFLAGS
jnc not_v86
This is a "popular" way to do but invalid, even with EAX instead of AX: PUSHF is NOT realiable in V86, see Intel manuals. Code:
smsw ax
shr ax,1
jc WeAreInProtectedMode
This is sufficient and the only correct way to detect V86 (will hang on CPU's older than 80286). INT $67 is only useful if you want to use EMS or VCPI. Quote: i've read that cpu operates in virtual-8086 mode when emm386 is active. Right. NOT true for hardware (addon memory cards) or XMS based (EMSMAGIC) EMS services. Quote: now i'm stuck with turning this mode off No official way. Use VCPI to get into Ring0 (will not work inside NTVDM  ) and then switch to RM. Untested, may leave misstargeted IVT entries or other mess and hang 
Quote: I'll try to allocate a mem page later today. I've read that this will enter v86 mode. Popular "info" but seems wrong 
Quote: if this helps, then NO it won't. Test for V86 before INT $67. If in RM, be happy. If V86, try VCPI, no need for an EMS page. |
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17 Mar 2010, 07:10 |
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vid 17 Mar 2010, 12:41
Emm386 definitively can be turned off. Win9x loader does it.
Even if it is not official, as f0dder said, it ought to be de-facto standard, and no one is going to change anything in DOS "standards" until after World War 3. |
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17 Mar 2010, 12:41 |
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DOS386 18 Mar 2010, 06:54
vid wrote: Emm386 definitively can be turned off. Win9x loader does it. Even if it is not official, as f0dder said, it ought to be de-facto standard, and no one is going to change anything in DOS "standards" until after World War 3. So post working FASM code please ! |
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18 Mar 2010, 06:54 |
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vid 18 Mar 2010, 11:52
Quote: So post working FASM code please ! DOS is dead, and I don't feel like wasting my time with it now. If you really need to know, I think this code was located somewhere in WIN.COM, but I am not sure. More likely, it should still be floating somewhere in internet. It was some undocumented call. Without it, Windows 95 wouldn't be able to take over machine from DOS with EMM386 running. |
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18 Mar 2010, 11:52 |
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Tomasz Grysztar 18 Mar 2010, 12:29
I think it may have something to do with this function:
http://lrs.uni-passau.de/support/doc/interrupt-57/RB-4390.HTM Check out the function pointed to by DS:SI.  |
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18 Mar 2010, 12:29 |
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zhak 18 Mar 2010, 13:05
thank you guys for useful info.
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18 Mar 2010, 13:05 |
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Japheth 18 Mar 2010, 18:34
vid wrote: Emm386 definitively can be turned off. No. It MIGHT be possible that Emm386 can be turned off, but this cannot be guaranteed and since the intention behind using Emm386 often is to get UMBs it is usually impossible. Quote:
Yes, it get the Emm386 info and handles int 67h on its own then. But it most likely cannot deactivate EMM - because there might exist a DOS device driver or TSR loaded before Windows which has allocated EMS pages. |
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18 Mar 2010, 18:34 |
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