A protectedmode exploration

1
A protected-mode exploration

• A look at the steps needed to build
  segment-descriptors for displaying a message
  while in protected-mode

2
Segment-Descriptor Format
63
32
Base31..24
G
D
R S V
A V L
Limit 19..16
P
D P L
S
X
C / D
R / W
A
Base23..16
Base15..0
Limit15..0
31
0
Legend DPL Descriptor Privilege Level
(0..3) G Granularity (0 byte, 1
4KB-page) P Present (0 no, 1 yes) D
Default size (0 16-bit, 1 32-bit) S System
(0 yes, 1 no) X eXecutable (0 no, 1
yes) A Accessed (0 no, 1
yes) code-segments R Readable (0 no, 1
yes) C Conforming (0no, 1yes) data-segments
W Writable (0 no, 1 yes) D
expands-Down (0no, 1yes) RSV Reserved for
future use by Intel AVL Available for users
purposes
3
Example the vram segment

• The video display-memory for color text occupies
  a 32KB physical address-range from 0x000B8000
  to 0x000BFFFF
• Its segment-limit can be described with byte
  granularity as equal to 0x07FFF (or with
  page granularity as 0x00007 )
• It needs to be a writable data-segment
• Its privilege-level ought to be 0 (restricted)

4
Descriptor Implementations
00
92
0B
0
0
00
92
0B
0
8
8000
7FFF
8000
0007
Using byte granularity
Using page granularity
vram-segment descriptor using byte
granularity .word 0x7FFF, 0x8000, 0x920B,
0x0000 vram-segment descriptor using page
granularity .word 0x0007, 0x8000, 0x920B, 0x0080
5
Code and data segments

• Our programs code and data will reside at the
  base memory-address 0x00010000
• For simplicity when returning to real-mode, we
  can keep segment-limits as 0x0FFFF
• Both segments can retain privilege-level 0
• Code-segment readable executable
• Data-segment writable readable

6
Descriptors implemented
data-segment descriptor
code-segment descriptor
00
92
01
0
0
00
9A
01
0
0
0000
FFFF
0000
FFFF
Using byte granularity
Using byte granularity
data-segment descriptor using byte
granularity .word 0xFFFF, 0x0000, 0x9201,
0x0000 code-segment descriptor using byte
granularity .word 0xFFFF, 0x0000, 0x9A01, 0x0000
7
Global Descriptor Table

• We can put all of our segment-descriptors into
  the Global Descriptor Table
• Our program executes at privilege-level 0
• Every GDT must have a null descriptor
• Thus our GDT will need four descriptors

.align 8 the Pentium requires quadword
alignment theGDT .word 0x0000, 0x0000, 0x0000,
0x0000 null descriptor .word 0xFFFF, 0x0000,
0x9A01, 0x0000 code-descriptor .word 0xFFFF,
0x0000, 0x9201, 0x0000 data-descriptor .word 0x
7FFF, 0x8000, 0x920B, 0x0000 vram-descriptor
8
GDTR register-format
47
16
15
0
Segment Base-Address
Segment Limit
32 bits
16 bits
The register-image (48-bits) is prepared in a
memory-location
regGDT .word 0x001F, theGDT, 0x0001
register-image for GDTR
then the register gets loaded from memory via a
special instruction
lgdt regGDT initializes register GDTR
9
segment-selector format
15
3 2 1 0
INDEX
T I
RPL
16 bits
Legend RPL Requested Privilege Level
(0..3) TI Table Indicator (0 GDT, 1
LDT) INDEX 8 number of bytes in table that
precede the descriptor
10
segment-selectors defined

• Assembly language source-code is easier for
  humans to read if meaningful symbols are used as
  names for magic numbers

These equates provide symbolic names for our
segment-selectors .equ sel_cs0, 0x0008
code-segment selector .equ sel_ds0, 0x0010
data-segment selector ,equ sel_es0, 0x0018
vram-segment selector
11
Our pmhello.s demo

• Use these commands to assemble, link, and install
  our demo program (in class)
• as pmhello.s o pmhello.o
• ld pmhello.o -T ldscript -o pmhello.b
• dd ifpmhello.b of/dev/sda4 seek1
• It also needs a boot-sector program that can
  load it at the proper memory-address and then
  transfer control to its entry-point

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Our quikload.s loader

• We have provided a boot-sector program that you
  can use in our classroom or labs (its not
  designed to work at other places), or you can use
  your own loader program
• Heres how to assemble, link, and install our
  quikload.s example
• as quikload.s -o quikload.o
• ld quickload.o -T ldscript -o quikload.b
• dd ifquikloab.b of/dev/sda4

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In-class exercise-set 1

• Find out what will happen if you modify the
  segment-descriptor for video memory so it uses
  page granularity for its limit-field
• Find out what will happen if you do NOT set the
  ES-registers segment-limit to 64K before
  clearing the PE-bit in register CR0
• Find out what will happen if you change the DPL
  and/or RPL to values other than 0

14
In-class exercise-set 2

• Redesign the pmhello.s program so that it
  expects to be loaded at a higher address
• Say at address 0x00040000 (i.e., at 256KB)
• Say at address 0x01000000 (i.e., at 16MB)
• You will need to change the disk-address packet
  in our quikload.s program so that it will
  transfer your pmhello.b code from the disk to
  your higher memory address