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Protected Mode Tutorial v.0.02

Tutorial for Protected Mode programming in asm

(by till gerken)

This text contains a short and simple step-by-step tutorial for Protected Mode beginners. It shows you everything you need to program your own PM environment and is intended for those who don't have any experiences with it yet. All you need to understand this text is your brain and a bit assembler knowledge.
This article is online from 4924 days and has been seen 22103 times

Protected Mode Tutorial
V 0.02

Written by Till Gerken



This text contains a short and simple step-by-step tutorial for Protected Mode
beginners. It shows you everything you need to program your own PM environment
and is intended for those who don't have any experiences with it yet. All you
need to understand this text is your brain (you have one, do you? :) ) and a
bit assembler knowledge.

Do everything you want with the information contained in this document, BUT I

If you are doing something commercial with my code, please send me a postcard
from the place where you live (you can even send me a postcard if don't do
anything with my code but don't send e-mail as a substitute for a real

My address is:

Till Gerken
Wiefelsteder Str. 2a
26127 Oldenburg

Internet address:

Fido address:


Any comments, suggestions, criticism or whatever can be sent to one of these

The text will try to teach you the principles of the 80386's Protected Mode
and contains the complete source for a mode switcher. This little program was
made to show you the basic rules of Protected Mode code, so it is unoptimized
and kept simple, but very well documented. If you have questions/suggestions
for an extension, _please_ mail me.

The whole code is written in assembler, using Ideal mode TASM 2.01 syntax.
Newer versions of TASM will work, too, but you'll have to add a macro
to convert DWORDS to WORDS in order to compile the assembler file.

There are some figures in this text, they look better if you print them.
All tabulators have to be set to 8.

Any code can also be found as a complete program in the file PMTUT.ASM
(contained in this archive). Please do not rewrite the stuff contained below,
it is not always as complete as needed.

To port the code in PMTUT.ASM to other assemblers like MASM, you have to notice
that I used the feature of TASM to automatically group segments if an address
couldn't be resolved. Look at the GDT setup in START16. You'll see something
like that:
mov [ds:code16_descriptor.base0_15],ax
Using the DS prefix causes TASM to resolve this address (it is in CODE32 and
can't normally be accessed from CODE16).

To port the code, add a group containing CODE16 and CODE32, change the ASSUME
directive of CODE16 to link DS with the group and remove all DS prefixes. Only
remember to change the lines in CODE16 containing relative offsets in CODE32,
add eax,offset dummy_descriptor
add eax,offset code32:dummy_descriptor
else TASM would insert an offset relative to the group start address.

I didn't tried the things above, but it should work. Please mail me if you
port this code to other assemblers, I'd really like to know how it goes on.

I'm currently thinking about splitting the text into several files, each
containing information about a different topic. (like mode switching under
DPMI, VCPI, XMS and RAW; Exception handling; etc.) This would made it a bit
handier and easier to read. Someone has any opinion?



Well, "getting started" is what this document is all about... Here you'll
learn what you need to prepare for Protected Mode. There are several things
you have to take care of, at first you have to check on which processor your
program's currently running (trying to do PM on a 8086 may hurt... :) )

The best way to test for a 80386 is to test the processor's flag register.
Since the 80386, flag bits 12-14 are used for the I/O Privilege Level (IOPL)
and the Nested Task (NT) flag, so the only thing to do is to test if these bits
are modifyable (the 8086, 8088, 80186 don't use these flags and set them
implicitely to zero, the 80286 uses them, but they can only be modified in
Protected Mode. DOS can't run in Protected Mode, so if the program runs on a
80286, it is in Real Mode, and there the flags can't be modified).

; checks for a 386

no386e db 'Sorry, at least a 80386 is needed!',13,10,'$'

proc check_processor
pushf ; save flags
xor ah,ah ; clear high byte
push ax ; push AX onto the stack
popf ; pop this value into the flag register
pushf ; push flags onto the stack
pop ax ; ...and get flags into AX
and ah,0f0h ; try to set the high nibble
cmp ah,0f0h ; the high nibble is never 0f0h on a
je no386 ; 80386!
mov ah,70h ; now try to set NT and IOPL
push ax
pop ax
and ah,70h ; if they couldn't be modified, there
jz no386 ; is no 80386 installed
popf ; restore the flags
ret ; ...and return
no386: ; if there isn't a 80386, put a msg
mov dx,offset no386e ; and exit
jmp err16exit
endp check_processor

; exits with a msg
; In: DS:DX - pointer to msg

proc err16exit
mov ah,9 ; select DOS' print string function
int 21h ; do it
mov ax,4cffh ; exit with 0ffh as exit code
int 21h ; good bye...
endp err16exit

Now we have a function to determine if there is a 80386 installed and one
which provides a quick and dirty exit. The second thing to be done now is
to check in which mode we are running. Expanded Memory Managers like EMM386,
QEMM, etc. usually switch to V86 mode to provide their services. Our little
program only works in Real Mode, so another function has to be coded.

To distinguish between Real Mode and V86 mode, we have to look at the Control
Register 0: bit 0 is clear when we are running in Real Mode, otherwise it is

; checks if we are running in Real Mode

nrme db 'You are currently running in V86 mode!',13,10,'$'

proc check_mode
mov eax,cr0 ; get CR0 to EAX
and al,1 ; check if PM bit is set
jnz not_real_mode ; yes, it is, so exit
ret ; no, it isn't, return
mov dx,offset nrme
jmp err16exit
endp check_mode

Now we can be sure that nothing will disturb us while setting up for Protected
Mode. DPMI and VCPI (even BIOS) can be used for mode switching too, but this is
left as an exercise to you. (the interface however is described below)

Please notice that I used MOV EAX,CR0 in this example. Jerzy Tarasiuk pointed
out that this is not allowed in a Protected Mode environment, especially not
on a 286. If the program doesn't work on your computer, try SMSW AX. This
instruction is only supported on the 386/486 for compatibility reasons and
shouldn't be used any more, but it works in any environment.

When switching to Protected Mode, you also have to change MOV CR0,EAX to

Please note that you can use LMSW only to get _into_ Protected Mode. The
instruction cannot be used to get _out_ of Protected Mode. To handle this
weird behaviour, you have to force the processor to enter shutdown mode and
then reset it. This is, however, not described here, as it would be a too
complex thing for a beginner's tutorial. ;)



Before the actual mode switch, we have to set up a few tables and descriptors.
What I'm talking about is the GDT, the LDT and the IDT.

The GDT is the Global Descriptor Table and contains basic segment descriptors.
These segment descriptors are keeping information about different parts of
the memory. In Real Mode, one segment is 64kb big followed by the next segment
in a 16 byte distance. In Protected Mode however, you can decide yourself
where to put a segment. Every segment can be as big as 4Gb (in words: four
giga-bytes!). The LDT is optional and contains segment descriptors, too, but
these are usually used for applications. An Operating System, for example,
could set up the GDT with it's own system descriptors and for every task a LDT
which contains the application descriptors.

The LDT is a descriptor table like the GDT. It's usage is to provide different
tasks different memory-layouts. In our program, LDTs aren't needed.
The IDT contains the interrupt descriptors. These are used to tell the
processor where to find the interrupt handlers. It contains one entry per
interrupt, just like in Real Mode, but the format of these entries is totally

Here is the basic format of these descriptors:

Segment Descriptor

15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
[ Base address 31:24 ] [G] [D] [0] [AVL] [Sg. length 19:16]
[P] [DPL] [DT] [ Type ] [ Base address 23:16 ]
[ Base address 15:00 ]
[ Segment length 15:00 ]

As you can see in this figure, the basic segment descriptor has a size of
4*16=64 bits.
To give you a help understanding the structure (my ASCII graphics are not very
good at all), here is the same a bit more compact in assembler:

; contains a segment descriptor

struc segment_descriptor
seg_length0_15 dw ? ; low word of the segment length
base_addr0_15 dw ? ; low word of base address
base_addr16_23 db ? ; low byte of high word of base addr.
flags db ? ; segment type and misc. flags
access db ? ; highest nibble of segment length
; and access flags
base_addr24_31 db ? ; highest byte of base address
ends segment_descriptor

Using this structure makes handling the GDT and LDT much easier. The same
applies to the IDT, but here the format is different:

15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
[ Offset 31:16 ]
[P] [ DPL ] [0] [1] [1] [1] [0] [0] [0] [0] [0] [0] [0] [0] [0]
[ Selector 00:15 ]
[ Offset 00:15 ]

And, as above, the same in assembler:

; contains an interrupt descriptor

struc interrupt_descriptor
offset0_15 dw ? ; low word of handler offset
selector0_15 dw ? ; segment selector
zero_byte db 0 ; unused in this descriptor format
flags db ? ; flag-byte
offset16_31 dw ? ; high word of handler offset
ends interrupt_descriptor

Now you know the segment and interrupt descriptor format. But what to fill in?
Good question. Before explaining you this, I'll give you a short table where
all mentioned bit names are described:

Bit name Meaning
G Granularity.
G=0 => 1 Byte granularity
G=1 => 4k granularity
This bit specifies the granularity of the segment. If the bit
is clear, the length field in the descriptor reflects the
real length of the segment in bytes. If the bit is set, you
have to multiply the length field in the descriptor by 4096
to get the real length in bytes.
D Default Operand Size.
D=0 => 16 bit operands
D=1 => 32 bit operands
This bit specifies the default operand size which has to be
used by special opcodes (like REP xxx). If the bit is clear,
the default operand size is 16 bit and the processor behaves
similar to a 80286. If the bit is set, the default operand
size is 32 bit. D=0 does not mean you can't use 32 bit
instructions, it only affects the default operand sizes.
AVL Available for System.
This bit is not used in 80286/80386/80486 machines. If
somebody has information about how it is used on Pentium
machines, please mail me. For now, better keep it to zero to
keep compatibility. However, if your program only runs on
machines lower than Pentium, you can use it as a mark for your
own software or whatever.
P Presence.
P=0 => segment is not present (or invalid)
P=1 => segment is present and valid
With this bit you can easily implement a virtual memory
manager (VMM). If the application wants to allocate more memory
than available, save the least used segment (determined with
help of the A bit) to disk, then clear the P bit in its
descriptor. The next access to that segment will be followed by
a General Protection Fault. Catch the fault, reload the segment
into memory and set its P bit. Done. The processor checks only
the P bit before generating the General Protection Fault, so
if P is set to zero, the rest of the descriptor is available
to keep information for your VMM.
DPL Descriptor Privilege Level.
0 <= DPL <= 3
The DPL bits contain the Descriptor Privilege Level. The
Privilege Level has a range from 0 (highest privilege level)
to 3 (lowest privilege). If a program tries to access a
segment with a higher privilege level than its own, the
processor will generate a General Protection Fault.
REMARK: Every time I speak of Privilege Levels in this text,
I mean that HIGH Privilege Level is a LOW number in DPL,
LOW Privilege Level is a HIGH number in DPL.
Example: segment 1: DPL=1 \
-> segment 1 is more privileged
segment 2: DPL=3 /
DT Descriptor Type.
DT=0 => System Descriptor (System-Segment or Gate)
DT=1 => Application Descriptor (Data or Code)
If this bit is clear, the Descriptor describes a segment that
is (a) available for the System Software, or (b) a
Type Segment type.
These four bits select the segment type.

Bit 3 2 1 0 Type Description
Name T E W A
0 0 0 0 Data read-only
0 0 0 1 Data read-only, accessed
0 0 1 0 Data read/write
0 0 1 1 Data read/write, accessed
0 1 0 0 Data read-only, expand down
0 1 0 1 Data read-only, exp. down, acc.
0 1 1 0 Data read-write, expansion down
0 1 1 1 Data read-write, exp. down, acc.
1 0 0 0 Code exec-only
Name T C R A
1 0 0 1 Code exec-only, accessed
1 0 1 0 Code exec-read
1 0 1 1 Code exec-read, accessed
1 1 0 0 Code exec-only, conforming
1 1 0 1 Code exec-only, conf., acc.
1 1 1 0 Code exec-read, conforming
1 1 1 1 Code exec-read, conf., acc.

Read-only means that you are only allowed to read this segment.
Read-write means that you can read and write from/to the
Exec-only segments can only be executed, but no read-access is
Exec-read segments can be read and executed. Unlike as in Real
Mode, you aren't allowed to use self-modifying code. A way
around this can be found a few lines ahead in this text.
The Accessed bit is set everytime a program tried to access
this segment _and_ the bit isn't already set. If you want to
figure out which segment to swap (the famous VMM example),
increase a counter if the A bit is set and then clear this bit.
The segment with the lowest counter position can safely be
swapped out. BUT WATCH OUT: If the A bit is set, a program
might run in this segment! Swapping these segments may hurt!
Expansion Direction is a weird thing. If the bit is clear, the
Expansion Direction is upwards, that means the segment grows
upwards. To grow it, increase the length. You are allowed to
access every address that is
0 <= Address <= Limit
Limit means the actual length of the segment. If the segments
Granularity is set to 0, the limit is equal to the length.
But if Granularity is set to 1, you first have to multiply the
length by 4096 (4k) to get the length.
If the bit is set however, welcome to hell. Now the Expansion
Direction is _downwards_, that means to grow the segment,
you'll have to _decrease_ the Length. You are allowed to
access every address that is
G=0 : Limit-1 <= Address <= 0ffffh
G=1 : Limit-1 <= Address <= 0ffffffffh.
Because of the 4G Wrap-Around, these addresses are just the
ones that would cause a General Protection Fault if E would be
Conforming means that a segment with C=1 can call another
segment with a lower or equal Privilege Level. The Current
Privilege Level however isn't changed!
If you call directly from a segment with C=0 (and not through
a Task-Gate) to a segment that has another Privilege Level
than the Current Privilege Level, a General Protection Fault

That wasn't too hard, wasn't it? At first all this might look a bit confusing,
but when you look at the code, it will be that simple...
So the only thing left before starting to do the _real_ thing (no, not
drinking Coke, I mean coding! :) ) is to explain what a Selector is:
A Selector selects something, and this something are Segment Descriptors!
The format is simple enough to understand:

15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
[ Pointer into a Descriptor Table ] TI [ RPL ]

RPL is the Requested Privilege Level. If the Descriptor in the Descriptor
Table has a higher Privilege Level than RPL, a General Protection Fault will
be caused.
TI selects the Descriptor Table to get the Descriptor from : TI=0 means GDT,
TI=1 means LDT.
The pointer contains the offset into the Descriptor Table where the wanted
Descriptor is to find.



If you understood the above, the rest is easy to do: set up the appropriate
tables, then switch to PM. EASY!!!
At first, we declare the two segments where to put all data and code in:

segment code16 para public use16 ; <- the 16-bit code and data segment
assume cs:code16, ds:code16

ends code16

segment code32 para public use32 ; <- the 32-bit code and data segment
assume cs:code32, ds:code32

ends code32

There is only one little bug with these two segments: both are code segments.
If you listened carefully, you'd remember that in Protected Mode it is not
allowed to write to a code segment. Well, where's the problem? (I hear them
saying: let's only use static data!! :) ) Everytime your program would try
to write to data located in one of these segments, a General Protection Fault
will occur. What about an extra 32-bit data segment? Hmm, nice, but not the
best way. Here's the probably easiest way to have code _and_ data together in
one segment, even allowing self-modifying code: we have to create a so called
"alias"-segment. This segment isn't really a new segment, it's just another
descriptor in the GDT. The descriptor points to the same memory that is defined
in the code segment descriptor. The only thing that we have to take care of now
is that CS is loaded with the _code_ selector and DS, ES, FS and GS are loaded
with the _data_ selector.

; GDT data

gdt_reg dw gdt_size,0,0
dummy_dscr segment_descriptor <0,0,0,0,0>
code32_dscr segment_descriptor <0ffffh,0,0,9ah,0cfh,0>
data32_dscr segment_descriptor <0ffffh,0,0,92h,0cfh,0>
core32_dscr segment_descriptor <0ffffh,0,0,92h,0cfh,0>
code16_dscr segment_descriptor <0ffffh,0,0,9ah,0,0>
data16_dscr segment_descriptor <0ffffh,0,0,92h,0,0>
gdt_size=$-(offset dummy_dscr)

The first line contains three words (better: one word and one dword) that
contain information for the GDT register. To inform the CPU where our GDT
is located in memory, we have to use the LGDT instruction. This instruction
sets an internal CPU register with the data pointed to by the instruction
parameter. The format of this data is

- GDT size in bytes (word)
- GDT base address (dword)

so to set up the register, we have to use the following line:

lgdt [fword ds:gdt_reg]

And the same for the IDT:

lidt [fword ds:idt_reg]

In our GDT, we have defined six descriptors: 32-bit code (4G size, 32-bit
operands, code type), 32-bit data (4G size, 32-bit operands, data type),
32-bit core (4G size, 32-bit operands, data type), 16-bit code (64k size,
16-bit operands, code type) and finally 16-bit data (64k size, 16-bit operands,
data type).

Whoops, there's one descriptor missing: the dummy descriptor! Why do we have to
include something like this? Good question! This descriptor can't be used and
has to be set to zero. But that doesn't explain why it is included! The LDT
definitely does _not_ have something like this...

The reason is the concept of the _Protected_ Mode. The CPU provides several
protection mechanisms, and one of them is the "invalid" (zero) descriptor.
If a segment is loaded with a zero-descriptor, every try to access memory
through it will be followed by a General Protection Fault, so it can be used
as a "marker". This is handy for debuggers if they want to find out when a
segment register is used, but there are lots of other possibilities to take
advantage of this feature.

A second thing is that the CPU validates every selector before it is loaded
into a segment register. This means, that if you want to load a Real Mode
segment address (like 1234h) into a segment register, the CPU checks in the GDT
if there is a valid descriptor. At offset 1234h, there probably won't, so again
a General Protection Fault is generated. In V86 Mode however, the processor
works with segment addresses like that. To solve this problem, the CPU saves
every segment register before calling an interrupt handler and loads them with
the zero selector. The Protected Mode interrupt handler won't notice if it has
been called from Protected or V86 Mode, so one handler will work in both modes.

Now, disable interrupts (a Real Mode Interrupt in Protected Mode does you no
good, a Protected Mode Interrupt with no IDT may be even worse!),


and switch to Protected Mode:

mov eax,cr0
or al,1
mov cr0,eax

(you may use LMSW AX, too)

The next thing is dirty but there's no way around it:

db 0eah
dw offset start32, code32_idx

0EAh is the opcode for JMP FAR. If you use the instruction JMP FAR, TASM tries
to resolve the jump with the optimal opcode, but we need JMP FAR to flush the
Instruction Prefetch Queue and to load CS with the new Protected Mode
Descriptor. This has to be done because the CPU doesn't set its descriptor
caches and the Instruction Prefetch Queue might contain instructions decoded in
a way only valid for Real Mode.

After this, the CPU is setup for Protected Mode and starts execution at the
label START32. There we just load the other segment registers with their
Protected Mode selectors and call MAIN.

The rest is easy.



There are lots of features not contained in the sample source. I've done this
to keep the program as simple as possible and to demonstrate as much as
possible, so there is an interrupt-system missing. This thing should intercept
every interrupt and redirect them to Real Mode. Second, a routine to toggle
the A20 address line should be added. Mail me if you want something to be
included or if you already coded something for it. You can also look at Tran's
PMODE package. It contains a complete Protected Mode header with complete
environment management. You won't notice what it does, you can start coding
as if you were in Real Mode, it is very powerful. However, I encourage you to
write your own Protected Mode system sometime, it helps to understand the



This section just can't explain all of the advantages of Protected Mode, better
buy a good book, but some of them I'll show you here.

1. To use the JMP FAR back to a 16-bit Real Mode segment from a 32-bit segment,
you have to use

db 0eah
dw offset real_mode_proc, 0 , segment_selector

This little zero-word can cost some time of debugging.

2. You can use _every_ register as an index.

mov eax,[edx]

3. You can use displacements _and_ factors in pointers.

mov eax,[ecx*8+edx]

All factors have to be powers of 2.

4. Sometimes this feature can be used for quick multiplies:

lea eax,[eax*8+eax] -> EAX=EAX*9

5. IMUL accepts immediates:

imul ecx,5 -> ECX=ECX*5

6. IMUL accepts immediates _and_ a register:

imul eax,ecx,5 -> EAX=ECX*5

7. Extended form of SHR/SHL to shift across registers:

shrd eax,edx,5
shld eax,edx,5

In this example, the 5 bits that become free in EAX will be filled with
bits of EDX. EDX is not modified.

8. This feature can be used to get rid of one MOV instruction:

Assume ECX contains a linear address that you want to convert into a
segment:offset notation. Normally, you would use something like this:

mov eax,ecx
shr eax,4 ; EAX now contains segment
and ecx,0fh ; ECX now contains offset

With the SHRD instruction, you can modify it to

shld eax,ecx,28 ; EAX now contains segment
and ecx,0fh ; ECX now contains offset

REMEMBER: The CPU only uses the 5 lower bits of the shift factor, so
shld eax,ecx,32 will _not_ copy ECX to EAX!!!

9. You can push immediates:

push 12345



Below is a complete list of the Faults, Traps and Exceptions that may occur.
If somebody has information about Exception 17 (Arrangement Error), please
mail me.

No. Name Type Error Code Cause
0 Division by Fault no Someone tried to
Zero divide by zero. Same
as in Real Mode
1 Single Step Trap,Fault no This interrupt is
called after each
instruction if the
Trap Flag is set
2 Non Maskable Abort no Heavy hardware failure.
Interrupt (NMI) Same as in Real Mode.
3 Breakpoint Trap no Used for debugging
purposes. Called by
special INT3 opcode.
4 Overflow Trap no Called if INTO is
executed and the
Overflow Bit is set.
5 Bound Range Fault no BOUND failed
6 Invalid Opcode Fault no CPU found an invalid
opcode. Same as in
Real Mode.
7 Coprocessor Fault no Called if CPU tries to
Not Available execute ESC or WAIT and
EM bit is clear.
8 Double Fault Abort yes (always 0) An exception occured
while another exception
handler is active.
9 Coprocessor Abort no The middle operand
Segment Overrun of a FPU instruction
can't be accessed.
Dunno what this should
be, i486 doesn't has
this exception any
10 Invalid TSS Fault yes Tried to switch to a
task with an invalid
11 Segment not Fault yes Someone tried to access
Present a segment that had it's
Present bit clear.
12 Stack Exception Fault yes Called if stack exceeds
it's limits or if
selector for SS is
13 General Fault yes Someone tried to access
Protection Fault invalid, protected or
not-present data.
14 Page Fault Fault yes Called if paging is
enabled and and an
access to an invalid,
protected or
not-present page
16 Coprocessor Fault no The FPU saw that it
Error was doing something
totally wrong... :)
17 Arrangement ???? ?? This exception occurs
Error only if AC=1, AM=1 and
CPL=3. If memory isn't
accessed at integral
addresses, EXCP17 is
generated. (see table
0..255 Software Trap no If you call one of
Interrupts these interrupts from
your program (INT xx),
they are handled like

Faults are documented and recoverable errors. The return address for the IRET
instruction (CS:EIP) points to the opcode that caused the Fault. Some Faults
have an error code on the stack (see below). To solve the problem, the handler
only has to read in the failed opcode and react on it.

Traps are interrupts that are caused by your program (INT xx instruction) or
by a debugging mechanism (INT3 or Trap Mode). An error code is never generated.
CS:EIP points to the opcode following the one that caused the Trap.

Aborts are only caused if the system tables (GDT, IDT, LDT) are invalid or
if there was a hardware failure. They don't allow you to return to your
program, nor there's an error code.

The format of the error code:

15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
[ Reserved ]
[ Selector ] TI IDT EXT

Bit name Meaning
Selector Selector of segment where the error occured
TI Table Indicator (applies only if IDT=0)
TI=0 - Selector from GDT
TI=1 - Selector from LDT
IDT Interrupt Descriptor
IDT=0 - No Interrupt Gate
IDT=1 - Selector from IDT
EXT External
EXT=0 - Program caused Exception
EXT=1 - Exception caused by external event

Arrangement Check Errors

Data Type Address has to be dividable by
Word 2
Double Word 4
Floating Point, Single Precision 4
Floating Point, Double Precision 8
Floating Point, Extended Precision 8
Selector 2
48-Bit Farpointer 4
Contents of GDTR and IDTR (48 Bits) 4
32-Bit Farpointer 2
32-Bit Address 4
Bitstrings 4
FPU Environment Blocks or FPU State 4 or 2, depending on Operand Length

An example how to handle these exceptions is not yet included in the demo
source. If someone would like to see more about this -> mail me! (as you might
have noticed, I definitely WANT your mails! :) )



In this section I'll give you some useful interrupt calls that provide handy
information or services for Protected Mode.

Format of the entries:

Function name
Interrupt number (or CALL address) in hexadecimal numbers
Input registers
Return registers

Func: Get RAM Size
Call: INT 12h
Input: ---
Return: AX - Memory size in kb
Notes: Returns only size of conventional memory

Func: Move Block (AH=87h)
Call: INT 15h
Input: AH=87h
CX=Number of Words in Buffer
ES:SI=Address of Descriptor Table
Return: CY=0 - ok
CY=1 - error
Notes: Transfers a block of max. 64k (max. CX=8000h) via Protected Mode
to any memory location.
ES:SI -> Dummy <--,
Table begin --'
Source Descriptor
Destination Descriptor
BIOS Codesegment
Every entry is 8 bytes long. First entry has to be set to zero.
Entry structure: - Segment size (word)
- Low Word 24-bit Segment Address (word)
- High Byte 24-bit Segment Address (byte)
- Access Flag (byte, =93h)
- Reserved (word)
The last two entries have to be set to zero.
Error code in AH: 00 - Transfer completed without error
01 - RAM Parity Error
02 - Exception
03 - A20 failure

Func: Extended Memory Size (AH=88h)
Call: INT 15h
Input: AH=88h
Return: CY=0 - ok
AX=ext. memory size in kb
CY=1 - error
AH=error code (80h - invalid command, 86h - function not supported)
Notes: Function returns extended memory size stored at address 20h and 31h
in CMOS RAM of clock chip. Extended Memory can only be used if
base memory is at least 512k big. If HIMEM.SYS is loaded, the function
returns 0.

Func: Virtual Mode (AH=89h)
Call: INT 15h
Input: AH=89h
BH=first PIC start
BL=second PIC start
CX=ofsfet of code segment
ES:SI=pointer to Descriptor Table
Return: CY=0 - ok
CY=1 - error
Notes: Function destroys all registers. ES:SI points to Descriptor Table.
ES:SI -> Dummy <--,
Table begin --'
Interrupt Table
User Data Segment (DS)
User Extra Segment (ES)
User Stack Segment (SS)
User Code Segment (CS)
Internal Code Segment
Entries structured like in function Move Block (AH=87h). Dummy has to
be set to zero. Third entry points to IDT built by program (Real Mode
structure). Last Descriptor has to be set to zero. BH contains mappings
for first PIC (start of first 8 hardware interrupts, i.e. BH=8 if IRQ0
should be shifted to IRQ8). BL contains mappings for second PIC.
Carry flag has to be cleared before function call. CX contains code
segment where execution in V86 should start. After function call no
BIOS is available, return to Real Mode only by resetting CPU.



The VDS provides services to use DMA transfers in Protected Mode with enabled
paging mechanism.

I am not sure about the information contained here. Please mail me if there are
errors. I hoped it would be handy for you, so it is included.

Error codes used in all functions:
01h region not in contigous memory
02h region crossed a physical alignment boundary
03h unable to lock pages
04h no buffer available
05h region too large for buffer
06h buffer currently in use
07h invalid memory region
08h region wasn't locked
09h number of physical pages greater than table length
0ah invalid buffer ID
0bh copy out of buffer range
0ch invalid DMA channel number
0dh disable count overflow
0eh disable count underflow
0fh function not supported
10h reserved flag bits set in DX

Sometimes Descriptor Tables are needed (DMA Descriptor Structure - DDS).
Format as following:

Offset Bytes Meaning
00h 4 size of region
04h 4 region offset
08h 2 region segment
0ah 2 buffer ID
0ch 4 linear address

Some functions use an extended format (EDDS):

Offset Bytes Meaning
00h 4 size of region
04h 4 region offset
08h 2 region segment
0ah 2 reserved
0ch 2 number available
0eh 2 number used
10h 4 linear address (region 0)
14h 4 size in bytes (region 0)
18h 4 linear address (region 1)
1ch 4 size in bytes (region 1)

If there are page tables contained, the following structure applies:

Offset Bytes Meaning
00h 4 size of region
04h 4 region offset
08h 2 region segment
0ah 2 reserved
0ch 2 number available
0eh 2 number used
10h 4 Page Table Entry 0
14h 4 Page Table Entry 1

Bits 1-12 of a Page Table Entry should be cleared. Bit 0 has to be set if the
page is present and locked.

Func: VDS Get Version (AX=8102h)
Call: INT 4Bh
Input: AX=8102h
Return: CY=1 - error
AL=error code
CF=0 - ok
AH=major version number
AL=minor version number
BX=product number
CX=revision number
SI:DI=buffer size
Notes: Flag bits in DX:
Bit Meaning
0 PC/XT Bus System (1Mb addressable)
1 physical buffer/remap region in 1st Mb
2 automatic remap enabled
3 all memory physically contigous
4-15 reserved
SI:DI contains maximal size of DMA buffer.

Func: VDS Lock DMA Region (AX=8103h)
Call: INT 4Bh
Input: AX=8103h
ES:SI=DMA Descriptor
Return: CY=1 - error
AL=error code
CY=0 - ok
Notes: DX is used as flag register to control the operation.
Bit Meaning
0 reserved (cleared)
1 copy data to buffer (ignored if bit 2 set)
2 don't allocate buffer if region not contigous or
exceeds physical boundaries (bit 4,5)
3 don't try to automatically remap
4 region must not exceed 64kb
5 region must not exceed 128kb
6-15 reserved (cleared)
Region Size Field in DDS contains size of maximal contigous memory
area. If Carry Flag is clear, area is locked and may not be swapped.
Physical Address and Buffer ID are filled by the function. If Buffer ID
is 0, no buffer has been allocated.

Func: VDS Unlock DMA Region (AX=8104h)
Call: INT 4Bh
Input: AX=8104h
ES:DI=DMA Descriptor
Return: CY=1 - error
AL=error code
CY=0 - ok
Notes: Flag bits in DX:
Bit Meaning
0 reserved (cleared)
1 Copy data from buffer
2-15 reserved (cleared)
Region Size, Physical Address and Buffer ID in DDS have to be filled.

Func: VDS Scatter/Gather Lock Region (AX=8105h)
Call: INT 4Bh
Input: AX=8105h
BX=page offset (not sure about it)
ES:DI=DMA Descriptor
Return: CY=1 - error
AL=error code
CY=0 - ok
Notes: Function is used to lock parts of memory. Useful if parts of memory
are swapped out.
Flag bits in DX:
Bit Meaning
0-5 reserved (cleared)
6 return EDDS with page table entries
7 only lock existing pages, fill not existing pages
with 0
8-15 reserved (cleared)
Region Size, Linear Segment, Linear Offset and Number Available have to
be set. Region Size Field in EDDS will be filled with size of largest
contigous memory block. Number Used will be filled with the number of
used pages. If bit 6 in DX is set, lower 12 bits of BX should contain
offset of first page (not sure about that).

Func: VDS Scatter/Gather Unlock Region (AX=8106h)
Call: INT 4Bh
Input: AX=8106h
ES:DI=DMA Descriptor
Return: CY=1 - error
AL=error code
CY=0 - ok
Notes: Flag bits in DX:
Bit Meaning
0-5 reserved (cleared)
6 EDDS contains page table entries
7 EDDS may contain not present pages
8-15 reserved (cleared)
ES:DI contains EDDS initialised by function 8105h.

Func: VDS Request DMA Buffer (AX=8107h)
Call: INT 4Bh
Input: AX=8107h
ES:DI=DMA Descriptor
Return: CY=1 - error
AL=error code
CY=0 - ok
Notes: Flag bits in DX:
Bit Meaning
0 reserved (cleared)
1 Copy data to buffer
2-15 reserved (cleared)
ES:DI contains pointer to DDS. Region Size has to be filled. If bit 1
in DX is set, Region Offset and Region Segment have to be filled, too.
Function returns Physical Address, Buffer ID and Region Size.

Func: VDS Release DMA Buffer (AX=8108h)
Call: INT 4Bh
Input: AX=8108h
ES:DI=DMA Descriptor
Return: CY=1 - error
AL=error code
CY=0 - ok
Notes: Flag bits in DX:
Bit Meaning
0 reserved (cleared)
1 copy data from buffer
2-15 reserved (cleared)
Buffer ID in DDS has to filled. If bit 1 in DX is set, Region Size,
Region Offset and Region Segment have to be initialised, too.

Func: VDS Copy into DMA Buffer (AX=8109h)
Call: INT 4Bh
Input: AX=8109h
ES:DI=DMA Descriptor
Return: CY=1 - error
AL=error code
CY=0 - ok
Notes: BX:CX contains offset into DMA Buffer. ES:DI contains pointer to DDS.
Buffer ID, Region Offset, Region Segment and Region Size have to be

Func: VDS Copy out of DMA Buffer (AX=810ah)
Call: INT 4Bh
Input: AX=810ah
ES:DI=DMA Descriptor
Return: CY=1 - error
AL=error code
CY=0 - ok
Notes: BX:CX contains offset into DMA Buffer. ES:DI contains pointer to DDS.
Buffer ID, Region Offset, Region Segment and Region Size have to be

Func: VDS Disable DMA Translation (AX=810bh)
Call: INT 4Bh
Input: AX=810bh
BX=DMA channel
Return: CY=1 - error
AL=error code
CY=0 - ok
Notes: Function stops DMA transfer on channel BX.

Func: VDS Enable DMA Translation (AX=810ch)
Call: INT 4Bh
Input: AX=810ch
BX=DMA channel
Return: CY=1 - error
AL=error code
CY=0 - ok
Notes: Function starts DMA transfer on channel BX.



The Virtual Control Program Interface (VCPI) was the first standard to manage
memory in a Protected Mode or Virtual 86 Mode environment. In has been founded
in 1987 by many different companies. (PharLap, Quarterdeck, Qualitas, LOTUS,
Autodesk and others)

The communication between the interface and the application is divided into
a Server and a Client. The program that provides the interface services is
recognized as the Server. The application will be the Client.

To call the Server, there are two ways: in Real Mode, you have to use INT 67h,
in Protected Mode, you have to use a FAR CALL.

Everytime I speak of page addresses, I mean the common format to address a
page (bits 31-22=page directory, bits 21-12=directory entry, bits 11-0=offset).
The offset part of the page address is normally always set to 0.
[Page Directory

I am not sure about the information contained here. Please mail me if there are
errors. I hoped it would be handy for you, so it is included.

Func: VCPI Installation Check (INT 67h, AX=DE00h)
Call: INT 67h
Input: AX=DE00h
Return: AH=0 - VCPI is available
BH=major version number
BL=minor version number
AH!=0 - VCPI not available
Notes: Some docs say that AH=84h on return if VCPI isn't available, but EMM
is enabled.

Func: VCPI Get Protected Mode Interface (INT 67h, AX=DE01h)
Call: INT 67h
Input: AX=DE01h
DS:SI=pointer to descriptors
ES:DI=pointer to client pages
Return: AH=0 - ok
DI=pointer into page directory
EBX=offset of entry point
AH!=0 - error
Notes: To call the Server in Protected Mode, you have to use the returned
address. The memory at DS:SI has to have enough space for three GDT
Descriptors, the first descriptor will be filled with the VCPI
code segment. Use a FAR CALL into this segment, offset EBX, to reach
the Server dispatcher. The space has to be in the first page in the
applications code segment. ES:DI has to contain a pointer to a list of
pages used by the Client. In DI, a pointer to the first unused page is

Func: VCPI Get Maximum Physical Memory (INT 67h, AX=DE02h)
Call: INT 67h
Input: AX=DE02h
Return: AH=0 - ok
EDX=page address
AH!= - error
Notes: EDX contains the address of the highest 4kb page in memory. The lowest
12 bits are set to zero. Some Clients are using this call to
initialize their data structures.

Func: VCPI Get Number of Free 4K Pages (INT 67h / CALL FAR, AX=DE03h)
Call: INT 67h / CALL FAR
Input: AX=DE03h
Return: AH=0 - ok
EDX=number of free pages
AH!=0 - error
Notes: The call returns the number of free pages that are available for all
tasks. This function is available in Protected Mode, too. (CALL FAR...)

Func: VCPI Allocate a 4K Page (INT 67h / CALL FAR, AX=DE04h)
Call: INT 67h / CALL FAR
Input: AX=DE04h
Return: AH=0 - ok
EDX=page address
AH!=0 - error
Notes: The function allocates a 4K page for the Client. The lowest 12 bits of
EDX are set to 0. This function is available in Protected Mode, too.

Func: VCPI Free a 4K Page (INT 67h / CALL FAR, AX=DE05h)
Call: INT 67h / CALL FAR
Input: AX=DE05h
EDX=page address
Return: AH=0 - ok
AH!=0 - error
Notes: The page has to be allocated by function DE04h. The lowest 12 bits of
EDX are set to 0. This function is available in Protected Mode, too.

Func: VCPI Get Physical Address of Page in First MB (INT 67h, AX=DE06h)
Call: INT 67h
Input: AX=DE06h
CX=page number
Return: AH=0 - ok
EDX=page address
Notes: The function returns the address of a page in the first MB. The lowest
12 bits of EDX are set to zero. The page number in CX is the address of
the page SHL 12. (This is written in my VCPI docs, but quite illogical,
because then there are only the 4 highest bits available for the page

Func: VCPI Read CR0 (INT 67, AX=DE07h)
Call: INT 67h
Input: AX=DE07h
Return: AH=0 - ok
AH!=0 - error
Notes: The function returns CR0 in EBX because MOV xxx,CR0 isn't allowed in
V86 Mode. However, EMM386 and QEMM simulate this instruction and you
don't have to use an interrupt call.

Func: VCPI Read Debug Register (INT 67h, AX=DE08h)
Call: INT 67h
Input: AX=DE08h
ES:DI=pointer to buffer
Return: AH=0 - ok
AH!=0 - error
Notes: ES:DI has to provide enough space for 8 entries, every entry has a size
of 4 bytes. The function stores DR0, DR1, ..., DR7. DR4 and DR5 are

Func: VCPI Set Debug Register (INT 67h, AX=DE09h)
Call: INT 67h
Input: AX=DE09h
ES:DI=pointer to buffer
Return: AH=0 - ok
AH!=0 - error
Notes: ES:DI has to point to a table with 8 entries, every entry has a size of
4 bytes. The function loads DR0, DR1, ..., DR7. DR4 and DR5 are unused.

Func: VCPI Get 8259 Interrupt Vector Mappings (INT 67h, AX=DE0Ah)
Call: INT 67h
Input: AX=DE0Ah
Return: AH=0 - ok
BX=1st PIC Vector Map
CX=2nd PIC Vector Map
AH!=0 - error
Notes: The Server returns the mapping from the Master PIC in BX (start of
first 8 hardware IRQs) and the mapping from the Slave PIC in CX (start
of next 8 hardware IRQs). If there's no Slave PIC installed, CX is

Func: VCPI Set 8259 Interrupt Mappings (INT 67h, AX=DE0Bh)
Call: INT 67h
Input: AX=DE0Bh
BX=1st PIC Vector Map
CX=2nd PIC Vector Map
Return: AH=0 - ok
AH!=0 - error
Notes: Master PIC is programmed with BX, Slave PIC is programmed with CX.
Interrupts have to be disabled before calling this function.

Func: VCPI Switch to Protected Mode (INT 67h, AX=DE0Ch)
Call: INT 67h
Input: AX=DE0Ch
ESI=pointer to data structure
Return: AH=0 - ok
AH!=0 - error
Notes: The data structure has to be setup by the Client in the first MB. ESI
has to contain the linear address of it. Structure as follows:
Offset Bytes Meaning
00h 4 new value for CR3
04h 4 linear address in first MB of value for
GDT register (6 bytes)
08h 4 linear address in first MB of value for
IDT register (6 bytes)
0Ch 2 selector for LDT register
0Eh 2 selector for Task Register
10h 4 EIP of Protected Mode entry point
14h 2 CS of Protected Mode entry point
The function loads GDTR, IDTR, LDTR and TR. SS:ESP has to point to a
stack with at least 16 bytes available on entry. EAX, ESI, DS, ES, FS
and GS are destroyed.
The CPU continues execution in Protected Mode at address CS:EIP
specified in the table.
Interrupts are disabled on return.

Func: Switch from Protected Mode to V86 Mode (CALL FAR, AX=DE0Ch)
Input: AX=DE0Ch
DS=segment selector
Return: ---
Notes: The stack has to be shifted in the first MB on entry. DS has to contain
a selector for a segment that includes the address area returned by
function DE01h.
The function switches to V86 mode. Interrupt have to be disabled.
GDTR, IDTR, LDTR and TR are initialised by the Server. SS:ESP has to
contain the following structure:
Offset Meaning
-28h GS
-24h FS
-20h DS
-1Ch ES
-18h SS
-14h ESP
-10h reserved
-0Ch CS
-08h EIP
00h return address
EAX is destroyed.



After the successful VCPI standard, a new standard was founded: DPMI. With the
DPMI, some "bugs" of VCPI were removed, for example VCPI allowed to run a task
on CPL=0. DPMI has been published in 1990 by Microsoft and Intel with
version 0.9, in 1991 version 1.0 has been published.

All DPMI functions are reentrant. Many implementations use a VMM, so be sure to
lock your memory if you don't want it to be swapped to disk.

Every DPMI task uses four stacks: - Protected Mode Application Stack (CPL=0)
- Locked Protected Mode Stack
- Real Mode Stack
- DPMI Host Stack (CPL=0).

The Protected Mode Stack is used by the Client when switching from Real- to
Protected Mode. The Locked Protected Mode Stack is used by the DPMI Server to
simulate hardware IRQs. The Real Mode Stack is used for Real Mode IRQs and the
DPMI Host Stack is used by (who else could it have been... ;) ) the DPMI Host.

Unlike VCPI, all Protected Mode function can be reached via INT 31h.

I didn't include the interface here, because it is nearly 70 (140 on screen)
pages "small". I'd really like to know if someone knows a source, but if
there's enough demand, I'll include the whole interface here.



This document has been created to be part of the comp.lang.asm.x86 FAQ,
section 13 (Real Mode/Protected Mode).
Jerzy Tarasiuk (the author of the directly in the FAQ included text) and I
cooperate on writing and extending the FAQ contribution. (Again, please mail us
about something you want to have included, I LOVE RECEIVING MAILS! ;) )

His texts are on* . There is also a
listserver, to get the files mail to with the
body of the letter containing


to get all of his files (about 200k). Subjects are switching from Real Mode
to Protected Mode, V86 Mode, basic multitasking and using Protected Mode with
Turbo Pascal (NOT the Turbo Pascal DPMI stuff!!).

Jerzy's internet address is:

Another really good choice is Tran's Protected Mode package. I don't know which
ftp server has this file available, but it should be on
( However, if you have Fidonet access, you can request it at
2:2426/2030 (file PMC100.ZIP). This file contains a complete DOS Extender for
BC++ 4.0 (can be used with Watcom, too), including all sources.

For those German speaking people out there, I suggest reading the following

Author: Dr. Wolfgang Matthes
Title: Intel's i486
Architektur und Befehlsbeschreibung
der xxx86er-Familie
Published by: Elektor
ISBN: 3-928051-29-6



At first I like to thank Jerzy Tarasiuk who agreed to cooperate with me in
writing the comp.lang.asm.x86 FAQ Section 13. He gave me useful corrections.

Second, a big thanks goes out to Jouni Miettounen and the whole x2ftp crew for
approving this text at It is the first text for the net I wrote
in my whole life and I didn't expect that something like this could be done
without any problems at all.

And, of course, I would like to thank Jason Steinberg <> and Poom
Malakul Na Ayudhya <> (the first two guys who wrote to me
about this text!) and all the others that helped me getting rid of some bugs.

Then I want to say something that flies around in my head for quite a while:

When I got my internet access, one of the first things I saw was Raymond Moons
frequently posting of the comp.lang.asm.x86 FAQ. I looked through the subjects
and saw that the Real Mode/Protected Mode section was open. As for that time I
was writing quite a lot of stuff for Protected Mode, I mailed him asking if I
could make a little contribution.

The result was this text, that I hope will give you answers to many of your
questions. All information contained here has been collected from only one
processor description, thousands of little notes and millions of hardware
crashes caused by my never-ending stupid tries making it work. I give it to you
now without asking for anything (well, the postcard would be nice :) ).

So, please, if you discover something new, don't post on the net something
"I discovered a totally new mind-blasting thingy, but you won't
get the sources dudes, 'cuz they're mine..."
It is so stupid!

Share your experiences with others so they can learn from you! Remember how
hard it was for you learning to code really good programs?

I hear them saying: Money! I won't get money by sharing my ideas!

Wrong. Again: WRONG! You don't have to publish your rip-roaringly fast
3D-engine, your totally cool multitasking environment or your mind-blasting
DTP software you just wrote.

All I ask from you is to share the concepts. Give away your algorithms, sample
codes or whatever you've made. Let the dudes who really want to get on with it
do the work. You'll be already on other topics when others just finished
understanding what you meant. You'll be ahead of them. And the first gets the

Tell a friend
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