Date: Wed, 29 Sep 1999 16:18:02 -0400 (EDT) From: John Baldwin <jobaldwi@vt.edu> To: "Ronald G. Minnich" <rminnich@lanl.gov> Cc: hackers@freebsd.org Subject: RE: return to real mode Message-ID: <199909292018.QAA79832@server.baldwin.cx> In-Reply-To: <Pine.SGI.4.10.9909291116550.218081-100000@acl.lanl.gov>
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On 29-Sep-99 Ronald G. Minnich wrote:
> anybody got some reliable, tested, known-good code for getting back
> to
> real mode? I'm to the point where I have a working GDT, and paging is
> turned off, but the last step -- turning off protection enable -- is
> not
> working for me.
Well, initialize all of your selectors to descriptors that have 64k
limits (0xffff). Jump into a 16bit code segment with a 64k limit on
the CS selector, turn off bit 0 in cr0 to actually enter real mode,
then jump to the next instruction so that the cache is flushed. I
believe it needs to be a far jmp, and then you should be fine. For
example: (this is TASM, so it's Intel syntax and not AT&T)
GROUP CodeGroup _TEXT32, _TEXT16
ASSUME CS:CodeGroup, DS:_PMDATA
....
SEGMENT _TEXT32 Byte Public Use32 'CODE'
....
db 0EAh
dd OFFSET ExitPM
dw Sel_CS16
ENDP
ENDS
SEGMENT _TEXT16 Word Public Use16 'CODE'
ExitPM: mov ax,Sel_ESeg
mov es,ax
mov fs,ax
mov gs,ax
; mov ss,ax
mov eax,cr0
xor eax,eax ;clear bit 0, (i.e. leave PM)
mov cr0,eax ;leave protected mode
; jmp FAR CleanUp
db 0EAh ;jmp far CleanUp
dw OFFSET CleanUp
dw CodeGroup
.....
CleanUp: mov ax,_PMDATA
mov ds,ax ;restore DS
lss sp,[DWORD PTR OFFSET SaveSP] ;restore SS:SP
..... Now you are in Real Mode
> This is on a PII.
This code has been tested (and works) on 386, 386, and Pentium.
Presumably it should work on later chips as well.
> Thanks
>
> ron
---
John Baldwin <jobaldwi@vt.edu> -- http://www.cslab.vt.edu/~jobaldwi/
PGP Key: http://www.cslab.vt.edu/~jobaldwi/pgpkey.asc
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