-= PMode Tutorials in C & ASM =-
by Alexei A. Frounze (c) 2000
-= Disclaimer =-
PMode Tutorials in C & ASM
Copyright (C) 2000 Alexei A. Frounze
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
-= Introduction =-
I've started out this series of the PMode tutorials just for making PMode
more clear and easier to learn. Complexity level of the tutorials
increases from first tutes to last ones. It makes easier to get into
PMode. A beginner is supposed to learn PMode step by step.
-= Why C and ASM ? =-
Since pure assembly implies a lot of source code, starting something
completely new (PMode, for example) is very difficult with ASM. So I decided
to make the tutorials in C with a little of ASM. Such tutorials are about
2 times shorter than tutorials made in pure ASM. I think it's very handy
for a beginner. (s)he won't get lost in tons of the sources.
-= Why Turbo C 2.01 and NASM 0.98 ? =-
Turbo C 2.01 and NASM 0.98 are available for free. Hence, anyone who
wants to learn from my tutorials and make his/her own stuff using them,
can have them absolutely for free. Actually, there is yet another free
16-bit C compiler available from Borland: Turbo C++ 1.01, which is much better
due to improvements in IDE and syntax extensions to language. Needless to say,
we don't use OOP features of TC++. :)
-= Turbo C 2.01 Limitations =-
Since Turbo C 2.01 is a pretty outdated completely 16-bit compiler,
you may notice some problems with 32-bit values... For example,
when your program works in PMode, you're unable to use arithmetical shifts
for 32-bit (long) integers. That's because these shifts are made as
run-time C library subroutines and each such shift implies a far call to
a subroutine. And such far calls are not allowed in protected mode because
real mode segment values don't work in PMode. Be carful with 32-bit shifts,
muls, divs. If you have Borland C/C++ 3.1, it's possible to avoid this
problem because this compiler is capable to generate 32-bit instructions for
32-bit arithmetics. The mentioned limitations hold for Turbo C++ 1.01 as well
as for Turbo C 2.01.
-= System Requirements =-
- 386 or a better computer
- DOS 5.0+ or Windows 9x in command line prompt mode
(don't mess with DOS box)
- Absence of such drivers as EMM386.EXE (and sometimes HIMEM.SYS)
- 1MB RAM (sometimes a bit more)
- (S)VGA card
-= Where do I get Turbo C 2.01 and NASM 0.98 ? =-
Turbo C 2.01 and Turbo C++ 1.01 are available at Inprise web site:
You must register before the downloading.
NASM (Netwide Assembler) 0.98 is available from:
-= Acknowledgements =-
Thanks to Prashant TR <firstname.lastname@example.org> for his useful suggestions and
present and further work on documentation for this package.
-= Contact Information =-
Author name: Alexei A. Frounze
E-mail : email@example.com Please put "he110" (one hundred & ten) into the subject line
Homepage : http://alexfru.chat.ru
Mirror : http://alexfru.narod.ru
Home of this page : http://welcome.to/pmode or http://members.tripod.com/protected_mode/
-= Tutorials List =-
TUT01 Switches to PMode and back to real mode using CR0. On startup CPU
mode is checked using SMSW instruction. Bit 0 equals 0, if real mode
and 1, if PMode/V86. This is done to avoid conflicts with EMM386 and
similar drivers and Windows.
TUT02 Sets up GDT, goes to PMode, prints "Hello from PMode!" and goes back.
TUT03 The same GDT as before, IDT is setup for software ISR (Int 20h) and
divide by zero exception (Int 0). Tut shows work of these handlers.
Note, these handlers are defined as 286 ones (see ACS_INT_GATE in the
TUT04 The same GDT as before, IDT is setup for all exceptions,
you may now see type and address of an exception. Try different
exceptions and see what's going on.
Note, all handlers are defined as 386 ones now.
Note #2: since all exception handler wrappers call one common
exception handler and there is no additional stack provided, stack
faults are not handled properly. Basically such a design is good
for GPFs, TSS exception and similar things.
TUT05 The same GDT as before, almost the same IDT as in TUT04. Two IDT
entries and IRQ handlers are added -- IRQ0 (timer) and
IRQ1 (keyboard). The tut reprograms PIC in order to handle IRQs
TUT06 Almost the same as TUT05. Task switching is added. main() switches
to task() using a jump to TSS. Then task() waits for the ESC key.
After ESC, task() switches back to main(). Both tasks are PL#0.
Note, for such PL#0 tasks I/O map is not needed in TSS. Since there
are only PL#0 tasks, there is no need to setup stacks for interlevel
TUT07 Multitasking again. 3 PL#0 tasks (main(), task1(), task2()) work now.
Task switches are scheduled by a scheduler called from timer IRQ ISR.
I.e. preemptive multitasking.
TUT08 Demo of page translation. The demo shows 16 color bars in 3 cases:
1. page translation disabled (linear addresses equal physical ones)
Bars are drawn in the black, blue, ... yellow, white order
2. page translation enabled (linear addresses equal physical ones)
Bars are drawn in the black, blue, ... yellow, white order
3. page translation enabled (linear addresses don't equal physicals)
Bars are drawn in the opposite order: white,yellow,...,blue,black
BUG FIXED (21 June, 2000): Page Translation Cache is flushed after
page table has been modifyied.
2nd change (1 July, 2000): page translation cache is flushed
during swiching from PMode to real mode using "MOV CR3, zero value".
TUT09 Back to preemptive multitasking... :-) This tutorial is a bit
different to TUT07. It doesn't use TSS for task switchings now.
It uses stack-based switching instead. People say it's faster than
TSS-based method. Dunno, perhaps they're correct. MS Windows
uses this stuff a lot. Btw, this method also applicable to almost any
CPU. Even PC/XT made on 8086/8088 CPUs could have such task
TUT10 Well, let's have a rest. :-) This tutorial shows Big/Unreal Mode,
i.e. accessing RAM in real mode with use of just a 32-bit offset.
Yeah, it's really possible. In fact, you may access up to 4GB
of RAM from Real mode this way.
TUT11 It's almost the same as TUT07. The only difference is that task1()
and task2() tasks are 32-bit (32-bit code segments with 32-bit
instructions) and these tasks are in privilege level 3, IOPL=3 too.
New GDT entires added for PL#3 code and data segments respectively.
Screen segment is also redefined as PL#3.
TUT12 The same as TUT11. 2 LDTs are added for each PL#3 task.
TUT13 Multitasking. First time we run a Virtual 8086 machine here.
TUT14 Advanced V86 stuff. A software Int nn is now supported for V86 tasks.
TUT15 A working V86 monitor. It redirects IRQs to the V86 task now. So we
have our DOS stuff in V86 mode just like a DOS-box in Windows. An
asterisk flashing at the top-right corner of the screen tells you're
in V86 mode. You may try to type "dir" or run any real-mode program.
TUT16 The same V86 monitor. Quite a few changes (Int 1 -- single-step trap
is now passed to V86 task). It's possible to debug a real-mode
program using DEBUG.EXE (MSDOS debugger), TD.EXE (Borland Turbo
Debugger) or built-in debuggers such as in Turbo Pascal or Turbo C.
TUT17 is based on TUT07 and has TSS-based multitasking. There are two
tasks: main() and task(). main() is PL#0, task() is PL#3. The
tutorial shows the use of a call gate. The PL#3 task calls to a PL#0
procedure. This is an important thing for OS deelopment because
it provides access from user application programs to OS service
-= Changes =-
1 July, 2000
- NMIs are disabled before switching to PMode and enabled after
switching back to real mode (in all tutorials)
- BIOS timer (dword at 40h:6ch) is now updated, if a program takes some
time (in all tutorials)
- The package is now GPL'ed
3 July, 2000
- TUT17 added
A ZIP of everything can be download in pmtuts.zip.