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Message-ID: <4408940f$0$488$cc7c7865@news.luth.se>
From: Martin Str|mberg <ams AT speedy DOT ludd DOT ltu DOT se>
Subject: Re: djgpp for kernel dev.
Newsgroups: comp.os.msdos.djgpp
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Date: 03 Mar 2006 19:07:59 GMT
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To: djgpp AT delorie DOT com
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Rod Pemberton <do_not_have AT sorry DOT bitbucket DOT cmm> wrote:
> No.  But, there is a way around it.  I'm not going to post the code for my
> OS, but I'll explain to you the method for DJGPP to bypass this.

> When you call exit() in your program, int 0x21, ah=4c "DPMI Unload" is
> called in PM.  Then the DPMI host cleans up memory and returns to RM.  When
> in RM, int 0x21, ah=4c "DOS Exit" is called by DOS to clean up after your
> program exits.

Why, oh why, should he even enter DJGPP PM?

[Lot of unecessary stuff deleted.]

> Then, the RM int 0x21 routine does the following:
> 1) disable interrupts
> 2) creates and sets up a minimal GDT with three selectors
>    a) selector for PM application code using the saved base address
>    b) selector for PM application data using the saved base address
>    c) linear selector for interrupt routine code
> 3) set the CR0 PE bit, and switch to PM, do other Intel/AMD recommended
> stuff
> 4) restore ESP,EBP
> 5) setup DS,ES,FS,GS
> 6) far jump to reentry address (sets CS)

> Now, we've restarted our PM application:
[...]
> 3) Yeah!  Our program is running again.
> 4) create better GDT in C
> 5) create IDT in C
> 6) create ISR in C with _asm() wrappers
> 7) setup keyboard & mouse, enable interrupts, etc..

> Now we're running in PM with no DPMI host.  At this point everything can be
> done in C except:

> 1) special instructions lgdt,lidt,
> 2) assembly wrappers for interrupt service routines

Yes, so why bother to let DJGPP startup and shutdown stuff run?


Right,

						MartinS