Defining protected-mode segment-descriptors

1
Defining protected-mode segment-descriptors

• An example of a protected-mode bootsector
  application that draws a message to the video
  display

2
What will we do once there?

• Lets explore writing a bootsector program that
  will do something perceptible while in
  protected-mode, namely show a message
• We wont be able to call BIOS functions (theyre
  designed to work in real-mode)
• We must write directly to video memory

3
Recall PC Memory Layout
RAM
ROM-BIOS
0xF0000
0xC0000
VIDEO-BIOS
VRAM
0xA0000
1-MB
0x00000
4
Three VRAM zones
COLOR TEXT
32-KB
0xB8000
MONOCHROME TEXT
32-KB
0xB0000
GRAPHICS
64-KB
0xA0000
5
Array of picture-elements

• Text-mode VRAM is organized as an array
• Each array-element occupies one word
• Words LSB holds ascii character-code
• Words MSB holds a color-number pair

0
12
11
8
7
15
bgcolor
fgcolor
ASCII character-code
nybble
byte
nybble
6
Color-Attribute Byte
Blink
R
G
B
Intense
G
B
R
background color attribute
foreground color attribute
7
Screen-element locations
80 columns
characters 0..79
characters 80..159
25 rows
characters 1920..1999
Video screen
8
x86 Little-Endian storage

• Intels x86 CPUs use little-endian storage
• The little end of any multibyte value is stored
  at the smaller operand-address
• Example EAX 0x12345678
• mov 0x9000, EAX

0x12
0x34
0x56
0x78
Memory-addresses occupied by operand
0x9000
0x9001
0x9002
0x9003
9
Drawing a character-string

• Setup DSSI with strings starting address
• Setup ESDI with initial address on screen
• Clear DF-bit (Direction Flag) in FLAGS register
• Setup desired color attribute-byte in AH register
• again lodsb next
  character to AL
• or al, al is final null-byte?
• jz finis yes, exit from loop
• stosw write char colors
• jmp again go back for another
• finis

10
Planning our memory usage

• To draw a screen-message in protected- mode, our
  program will need to address these
  memory-segments
• its code (executable, at 0x07C00)
• its data (readable and writable, at 0x07C00)
• its stack (readable, writable, expand-down)
• the video ram (32KB, writable, at 0xB8000)
• For its return to real-mode, our program will
  need 64KB code and data segments

11
VRAM segment-descriptor
31
16
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
0
15
VRAM Base-Address 0x000B8000
VRAM Segment-Limit 0x07FFF (32-KB)
Segment-attributes P1, A0, S1, X0, D0,
W1 DPL0, G0, D0 (RSV0, AVL0)
.WORD 0x7FFF, 0x8000, 0x920B, 0x0000
12
CODE segment-descriptor
31
16
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
0
15
CODE Base-Address 0x00007C00
CODE Segment-Limit 0x0FFFF (64-KB)
Segment-attributes P1, A0, S1, X1, C0,
R1 DPL0, G0, D0 (RSV0, AVL0)
.WORD 0xFFFF, 0x7C00, 0x9A00, 0x0000
13
DATA segment-descriptor
31
16
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
0
15
DATA Base-Address 0x00007C00
DATA Segment-Limit 0x0FFFF (64-KB)
Segment-attributes P1, A0, S1, X0, D0,
W1 DPL0, G0, D0 (RSV0, AVL0)
.WORD 0xFFFF, 0x7C00, 0x9200, 0x0000
14
STACK segment-descriptor
31
16
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
0
15
STACK Base-Address 0x00007C00
STACK Segment-Limit 0x001FF (512-Bytes)
Segment-attributes P1, A0, S1, X0, D1,
W1 DPL0, G0, D0 (RSV0, AVL0)
.WORD 0x01FF, 0x7C00, 0x9600, 0x0000
15
Setting up the GDT

• Base-Address must be quadword-aligned .ALIGN 8
• NULL-Descriptor occupies first quadward
• theGDT .WORD 0, 0, 0, 0
• GDT base-address and segment-limit
• base 0x00007C00 theGDT
• limit 8 (number of descriptors) - 1

16
Loading register LDTR
BASE_ADDRESS
LIMIT

• We can load LDTR from our stack
• mov eax, 0x00007C00 boot location
• add eax, theGDT add GDT offset
• mov dx, 0x27 five descriptors
• push eax push bits 47..16
• push dx push bits 15..0
• lgdt esp load 48-bit LDTR
• add esp, 6 discard 3 words

GDTR
48-bits
17
Entering protected-mode

• No interrupts from any peripheral devices (since
  BIOSs real-mode ISRs wont work)
• Set the PE-bit to 1 (in register CR0)
• Do a far-jump (to load the CS attributes)
• Load SSSP with stacktop and attributes
• Setup DS and ES for data and vram
• Write character-string to video memory

18
Leaving protected-mode

• Be sure segment-registers are loaded with
  selectors for descriptors that have suitable
  segment-limits and segment-attributes for correct
  execution when back in real-mode
• Reset PE-bit to 0 (in register CR0)
• Do a far-jump (to load CS with paragraph)
• Load SSSP with real-mode stack-address
• Wait for users keypress before rebooting

19
Demo-program

• We have a bootsector program on website
  (pmhello.s) which illustrates the principles
  just discussed
• Try assembling and installing it
• as86 pmhello.s b pmhello.b
• dd ifpmhello.b of/dev/fd0
• Restart machine, use the GRUB memu to select this
  bootsector as execution-option

20
In-class exercises

• What happens if you changed the code
  descriptors access-rights byte from 0x9A to 0x9C
  (i.e., conforming code-segment)?
• Where exactly in does the expand-down
  stack-segment reside?
• BASE_ADDRESS 0x00007C00
• SEGMENT_LIMIT 0x001FF