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80386 Protected Mode Initialization

   By Neal Margulis, October 01, 1988

   In addition to discussing how to get from real- to protected-mode and
   back, Neal shows how to define greater tan 64K segments, and provides
   some general comments on the ins and outs of developing 32-bit
   software.

   Neal Margulis is an applications engineer for Intel Corp. and can be
   reached at 2625 Walsh Ave., SC4-40, Santa Clara, CA 95051.
     __________________________________________________________________

   The 32-bit mode of the Intel 80386 and the 80386SX provides significant
   architectural advantages over the 80286. ln addition, software that
   takes advantage of these advanced features has significant performance
   improvements. An application program running in the 80386's native
   32-bit mode typically executes from two to six times faster than the
   equivalent application written for the 80286. Furthermore, programs
   that manipulate large data structures are easier to write when you use
   the 32-bit mode of the 80386. Among the features that the 80386
   provides over the 80286 are support for large segment sizes, 32-bit
   data operations, and paged memory management.

   The program presented here shows how to initialize the 80386 into
   protected mode, how to define segments greater than 64K in size, and
   how to return to real mode. You can use this program as a template for
   coding applications that use the 80386 features. Although the 80386SX
   has a reduced physical addressing space of 16 Mbytes (the maximum
   address space of AT architecture), its programming model is the same as
   that of the 80386. Thus the template can be used with it as well.

   This article explains how the code works and briefly describes how to
   adapt the template to suit your individual needs. You may also find it
   helpful to refer to one of many 80386 programming articles, such as
   "Programming on the 80386" (DDJ, October 1986). Additional information
   can be found in the Intel 80386 Programmers Reference Guide and the
   80386 Data Sheet, as well as the book Programming the 80386 by Crawford
   and Gelsinger (Sybex Books).

32-Bit Data Operations

The ability to operate on 32 bits of data adds power to arithmetic and
logical instructions. While the 80286 generates only 16-bit data, the 80386
contains eight general-purpose 32-bit registers. Segments for 80386 protected
mode are set to either use16 or use32, which indicates the default sizes for
data and addressing. in real mode, the 80386 is limited to only use16
segments. An override prefix must be designated in order to perform 32-bit
operations within a protected mode use16 segment. This results in greater
program length and a possible decrease in performance. The 80386 in protected
mode allows for both use16 and use32 segments. No override prefixes are
necessary for 32-bit data operations or 32-bit addressing in a use32 segment.
In the program shown in [52]Listing One, page 84, CSEG and C3 are use16 code
segments because they must be executable from real mode, and PMODE segment is
a use32 code segment.

Large Segments

The 64K limit on the segment size of the 80286 and the real mode 80386 hinder
the addressing of large data arrays and of long sequences of code. Reloading
segments is time consuming, disrupts the task at hand, and causes an
unnatural breakup of procedures and data. The 80386 protected mode allows for
segments up to 4 gigabytes in size. The base, limit, and granularity fields
of segment descriptors specify the segment size and location in memory. The
base represents a linear address and the segment size is determined by the
limit and the granularity (G) bit. When the G bit is a zero, the actual limit
is the 20-bit limit field of the descriptor (Maximum size 2^20 = 1 Mbyte). If
the G bit is a one, then the limit field page granularity is multiplied by
4K. This gives a maximum limit of 4 gigabytes (2^20 * 2^12). The base's
linear address is 32-bits long, thus allowing it to be specified anywhere
within the 4-gigabyte address space.

Segmentation is the basis for protection. Data and code segments can reside
in separate, nonoverlapping areas of memory. In addition, privilege levels
assigned to different segments provide a mechanism for limiting access to
certain data or privileged instruction sequences (for both). The 80386
provides four privilege levels. In the program presented in [53]Listing One,
all segments are of the highest privilege level (0). You can change this by
modifying the segment descriptors and the selectors.

4-Gigabyte Addressing

To use the increased segment sizes, the 80386 has expanded the instruction
pointer to 32-bits and added new addressing modes. As a result, segment loads
and stores within a procedure can be eliminated and the entire physical
address space can be accessed as one segment. The template program sets up a
data segment that starts at the base of video memory 0B8000H. The entire
address space is accessible through a segment that begins at zero.

The effective address of a memory operand can be obtained through an absolute
address or through one of the register-base methods of the following form:

[base register] + [(index register *           scale) + displacement]

The 80386 also has page translation by which linear addresses can be resolved
to physical addresses. Page translation occurs when the PG bit in CR0 is set.
Two levels of tables are used to address each page of memory. The
higher-level table is the page directory, which addresses up to 1K
second-level page tables. These second-level page tables address up to 1K
pages, each being 4K.

Because all pages are of equal size, page translation can reduce the memory
fragmentation that occurs when using segmentation for on-demand memory
allocation. The template program does not use paging, so all linear addresses
are treated as the physical address.

Setting Up the Descriptor Tables

Before entering protected mode, you must set up descriptor tables and load
the 80386 with pointers to these tables. While in real mode, the program in
[54]Listing One sets up a global descriptor table (GDT) and does not require
a local descriptor table.

Starting at the memory location designated by the label GDT__table,
successive 8-byte descriptor entries make up the GDT. MASM's STRUC feature
makes coding of the entries much easier. Because DOS determines the memory
location of the program at run time, the absolute addresses must also be
calculated at run time.

The template program determines the bases for each of the segments and the
pointer to the descriptor table. This table pointer consists of a 32-bit
linear address and a 16-bit limit.

Such 48-bit (6-byte) objects are sometimes referred to as a PWORD or FWORD
data type. Using a QWORD (8 bytes), as in this program, helps to maintain
portability between assemblers.

The Type field of the descriptor determines whether segments that use this
descriptor contain code or data. Descriptors to specify gates, task state
segments, and local descriptor tables are also available, but are not used in
this example.

Entering Protected Mode

Having set up the descriptor table, it is simple to enter protected mode. The
PE bit of Control Register Zero (CR0) is set to one, and then a jump is
executed. The jump flushes the prefetch queue, which contains instructions
that were decoded for execution in real mode. Either a near or a far jump
will flush the prefetch queue. By using a far jump, the 80386 reloads the
code segment register (CS) and the internal segment descriptor cache. The far
jump instruction uses selector 08H, which is GDT entry 1. This is the PMODE
segment entry. Execution in this segment allows native 32-bit operations.
(Bits 15 through 3 of the selector determine the GDT table entry. Bit 2 is
Table indicator, and bits 1 and 0 are the privilege level.)

The Jump instructions are handcoded by using the define byte (DB) assembler
directive. Opcode 0EAH specifies an intersegment jump, with the next two
fields of the instruction being the offset and the segment selector operands.
In this example, all of the offsets are zero because the jump targets are at
the start of their respective segments. The offset size is either a 16-bit or
32-bit field. This is determined by the code segment type size in which the
jump instruction occurs. The segment selector field of the jump instruction
determines which descriptor table entry is the target segment for the jump.

Returning to Real Mode

Returning to real mode on the 80286 requires that you reset the processor. On
an PC AT, this means saving the required processor contents, placing a reset
code in the CMOS RAM, storing a return address in memory, and using the
keyboard controller to reset the processor. Although 386 based ATs support
this reset scheme, a much simpler and far faster way to return the 386 to
real mode is available. In the C3 segment of the example program, the 80386
is returned to real mode by clearing the PE bit then executing a jump
instruction to flush the instruction queue. To assure proper operation after
returning to real mode, the segment registers must be loaded with real mode
type selectors while still in protected mode. Entry 4 in the GDT represents
what should be in the segment descriptor caches during real mode. They have a
64K limit with the base at zero and the top 2 bytes set to zero.

Other Considerations

In the early days of PCs, some programs took advantage of addresses wrapping
around to zero after the limit of the 8088 was exceeded. When the 80286-based
AT was introduced, it was necessary to emulate this address wrapping. An
enable gate was added to address line 20. To prevent unwanted wrapping, you
must enable this gate. The code for doing this is located in the IBM
Technical References for the AT and PS/2. The procedures are different
because the AT uses the keyboard controller to enable the address line. The
programming example indicates where to insert the procedures for enabling and
disabling of the address line.

[LISTING ONE]
<a name="01ca_000c">
_80386 PROTECTED MODE INITIALIZATION_
by
Neal Margulis

comment #*****************************
Program by  Neal Margulis -- Use MASM 5.0
#*************************************

descriptor STRUC
    limit_0_15  dw  0      ;  lowest 16 bits of segment limit
    base_0_15   dw  0      ;  lowest 16 bits of base
    base_16_23  db  0      ;  base bits 16-23
    access      db  0      ;  Present bit, priv. level, type
    gran        db  0      ;  G bit, D/B bit , limit bits 16-19
    base_24_31  db  0      ;  base bits 24-31
descriptor ENDS

code_seg_access equ 09AH   ; Present, DPL=0, non-conforming,read/exec
data_seg_access equ 092H   ; Present, DPL=0, Expand-Up,writeable

; have screenbase equal B8000H for EGA or B0000H for monochrome
screenbase   EQU  0B8000H
screenseg    EQU  0B800H

CSEG segment word  use16 'code'
assume  cs:CSEG,ds:CSEG

       mov ax,CSEG
       mov ds, ax

; Make entries in GDT for PMODE segment as code or data
       mov ax, seg PMODE
       and eax, 0FFFFh
       shl eax, 4H
       mov ebx, eax
       shr eax, 16
       mov gdt_PM_1.base_0_15, bx
       mov gdt_PM_2.base_0_15, bx
       mov gdt_PM_1.base_16_23,al
       mov gdt_PM_2.base_16_23,al

; Make entry in GDT for C3 segment as code
       mov ax,seg C3
       and eax, 0FFFFH
       shl eax, 4H
       mov ebx, eax
       shr eax, 16
       mov gdt_c3_5.base_0_15, bx
       mov gdt_c3_5.base_16_23,al

; Set up gdtr for lgdt instruction
       mov ax, cs
       and eax, 0FFFFH
       shl eax, 4H
       add eax, offset gdttbl
       mov dword ptr gdtaddr+2,eax
       lgdt  gdtaddr               ; set GDT address
A20_ON:

       cld                         ; Clear direction flag
       cli                         ; Disable interrupts

; Enter Protected Mode
        mov     eax,cr0
        or      eax,1
        mov     cr0,eax            ; Enable protected mode

              ;flush prefetch queue
        DB 0EAH,0H,0H,08H,0H       ; jmp to PMODE and execute

gdtaddr label   qword
        dw      48
        dd      ?
        dw      0

; global descriptor table

gdttbl   label   dword
gdt_null  descriptor <,,,,,>  ;   GDT entry 0 (null descriptor)
gdt_PM_1  descriptor <0FFFFH,,,code_seg_access,0C0H,0>  ;  D bit
ON
gdt_PM_2  descriptor <0FFFFH,,,data_seg_access,08FH,>  ;
gdt_3     descriptor <0FFFFH,0,0,data_seg_access,08FH,0>
gdt_rm_4  descriptor <0FFFFH,0,0,data_seg_access,08fH,0>
gdt_c3_5  descriptor <0FFFFH,,,code_seg_access,080H,0>  ;  D bit
OFF

CSEG ends

PMODE segment para public use32 'code'
    assume cs:PMODE
        mov ax, 18h         ;selector 18H is 4 Gigabyte data
segment with
                                                          ;
base at 0
        mov es, ax
        mov fs, ax
        mov ax, 10h         ;  Data segment with base at 'c2seg'
        mov ds, ax
        mov cx, 025h
        mov     edi,screenbase  ;  Addressing screen memory from
protected mode
display:mov byte ptr es:[edi],'P'
        add     edi,2
        mov     byte ptr es:[edi],'M'
        add     edi,2
        mov     byte ptr es:[edi],' '
        add     edi,2
        loopne display

        db      0eah, 0h, 0h, 0h, 0h,28h, 0h ; jmp to c3 and
execute

align 16

                pdat  db 0ach
lastpm  label dword
PMODE ends

c3 segment para public use16 'code'
    assume cs:c3

        mov ax, 20h         ; Change segments back to have valid
        mov es, ax          ;  real mode attributes.
        mov ds, ax
        mov fs, ax
        mov eax,cr0
        and eax, 07ffffffeh
        mov cr0,eax               ; enter real mode
        jmp far ptr flushrl       ; flush queue
 flushrl:
         mov ax, screenseg        ; Address screen memory from real mode
         mov ds, ax
         sub edi, 0b8000h
         mov si,di
        mov byte ptr ds:[si],'C'    ;  Write to screen
        add si,2
        mov byte ptr ds:[si],'3'

A20_off:

        mov ah, 04ch                    ;  DOS termination
        mov al, 01h
        int 21h
c3 ends

end









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