🖱️ 👨🏼‍🎤 🧔🏾 Zurück zu den Wurzeln: Wir fahren einen Computer direkt vom MBR aus 🔷 👍🏻 👲🏻

Wieder einmal habe ich ein Linux-Image auf einem USB-Laufwerk bereitgestellt (aus irgendeinem Grund stellte sich heraus, dass es sich um Manjaro handelte, aber das ist eine ganz andere Geschichte), und seltsame Gedanken kamen mir in den Sinn: Das BIOS sah ein USB-Flash-Laufwerk, und was dann? Ja, es gibt einen MBR, höchstwahrscheinlich GRUB, und ... Und sobald der benutzerdefinierte Code eines anderen MBR-Benutzers angezeigt wurde, bedeutet dies, dass eine einfache Person aus Adygea etwas auf einem "großen" Computer programmieren kann, jedoch außerhalb des Betriebssystems.

Und da es zu mutig ist, solche Dinge in Hochsprachen zu tun, und wir keine Assembler kennen, werden wir die Opcodes für 8086 direkt schonen.

Einführung

Planen:

Abheben #
Abheben Hello, Habrauser!
Zeigen Sie Eingabezeichen an (Sie können bereits Kinder unterhalten).

Haftungsausschluss und Haftungsausschluss

Um das Dröhnen des Floppers nicht zu stören, werden wir weiter trainieren ~~Katzen~~ QEMU. , , dd x86- . .

MBR, - (?) (?) — , Enter. .

— , ( !) - . ( .) ! .

MBR

, Bootstrap Area .
, ( ) 0x7c00 ( Compaq).
, , ( , --). : 640KB, . ( , .)

, — , .

, , , , (sic!) . , ( - ), , .

, - «» 80- «-» 90-, : «» «». «», Lode Runner, «».

«» +

-, , , , , — errata.

, , , .

, MOV, INT, ADD, DIV — , , , . , , , .
, , , : imm8, r16, r/m32, rel8. , , (, ) , , DIV BL F6 F3 (DIV r/m8, , , — .) F6 — DIV, NEG, ( opcode extension — .)

, , dd, , , , , — , , -. -, … , POSIX , !

➜  $ echo "48 65 6c 6c 6f 2c 20 48 61 62 72 21" | xxd -r -p 
Hello, Habr!%

➜  $ echo -e "# Comment\n48 65 6c 6c 6f 2c 20  # First line\n48 61 62 72 21        # Last line" | sed 's/#.*$//g' | xxd -r -p
Hello, Habr!%

( sed-, xxd , hex-dump. ?)

, , .

, sh-

#!/bin/sh

IN="${1:-/dev/stdin}"
OUT="${2:-/dev/stdout}"

> $OUT

while read line
do
    echo "$line" | sed 's/#.*$//' | xxd -r -p >> $OUT
done < $IN

: LF (aka \n), . , , , , - , — .

— , !

➜  $ ./build loader.mbr loader.img && stat -f %z loader.img
512

512 — , . , .

, bin- 512B, . « dd /dev/zero, !» — . , c ( , ipython) !

# 0x0000:0x007F (0-127)
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00

00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00

# 0x0080:0x00FF (128-255)
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00

00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00

# 0x0100:0x017F (256-383)
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00

00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00

# 0x0180:0x0200 (384-512)
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00

00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00

, , , QEMU, Exception'.

, , ( ), MBR.

...

# 0x0180:0x0200 (384-445)
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00

# Partition 1     0x01BE:0x01CD (446-461) 
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00

# Partition 2     0x01CE:0x01DD (462-477)
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00

# Partition 3     0x01DE:0x01ED (478-493)
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00

# Partition 4     0x01EE:0x01FD (494-509)
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00

# MBR Signature   0x1FE:0x1FF (510-511)
00 00

, :

# MBR Signature   0x1FE:0x1FF (510-511)
55 AA

$ qemu-system-i386 -nic none loader.img

-nic none , PXE, — .

, BIOS , ! , !

`PRINT "#"`

, !

, . :

# 0x0000:0x007F (0-127)
B4 0E                      # Set a console output mode
B0 23                      # Set an octothorp sign
CD 10                      # Call a print function

00 00
00 00 00 00 00 00 00 00

( , 512).

, QEMU !

, , , , :

B4 0E — AH 0E ( mov ah, 0e), BIOS ( ), , .
B0 23 — : AL #. ? «»? — ASCII- man ascii!
CD 10 — : BIOS-, . , AL AH, , : .

, AL :

$ (B0 24)
% (B0 25)
á (B0 A0, )

`PRINT "Hello, Habrauser!"`

, , . .

- . — printf('Hell of word') .

, , :

# 0x0000:0x007F (0-127)
B4 0E    # Set a console output mode

B0 0A    # LF
CD 10

B0 48    # H
CD 10    # print 

B0 65    # e
CD 10

B0 6C    # l
CD 10

B0 6C
CD 10

B0 6F    # o
CD 10

B0 2C    # ,
CD 10

B0 20    # SPC
CD 10

B0 48    # H
CD 10

B0 61    # a
CD 10

B0 62    # b
CD 10

B0 72    # r
CD 10

B0 61    # a
CD 10

B0 75    # u
CD 10

B0 73    # s
CD 10

B0 65    # e
CD 10

B0 72    # r
CD 10

B0 21    # !
CD 10

00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00

, , :

,
, ,
, BIOS' — - , Booting from Hard Disk....

, :

# 0x0000:0x007F (0-127)
B8 00 06          # Clear screen
CD 10

B4 0E             # Set a console output mode
BE 80 7C          # Place 0x0080 + 0x7c00 = 0x7c80 into SI
AC                # Load a byte at address SI into AL, increment SI
3C 00             # AL == 00?
74 06             # If yes, go to +6 bytes (to zeroes)
CD 10             # Print a char in AL
EB F7             # Go to -7 bytes (to AC opcode)

00 00 00 00 00
00 00 00 00 00 00 00 00

...

# 0x0080:0x00FF (128-255)
48 65 6C 6C 6F 2C 20 48   # Hello, H
61 62 72 61 75 73 65 72   # abrauser
21                        # !

00 00 00 00 00 00 00
00 00 00 00 00 00 00 00

: , 0x7c00, . ? data- ( ), ( !), SI — .
SI AC ( LODS). , SI AL, SI! , !
. , . , , ?
AL, , — ( , 6 ), 00 .
AL - , BIOS-...
… 7 — LODS!

, . , .

`PRINT user_input$`

, - . : BIOS- . … . — ( , — ).

, . printer.mbr typewriter.mbr.

, :

# 0x0000:0x007F (0-127)
B4 07   # Clear screen
B0 00   #
CD 10   #

B4 00   # Set Get keystroke mode
CD 16   # Read a char -> AL

3C 0D   # AL == 0D? (CR, Return pressed)
75 06   # If no, go to +6 bytes

B4 0E   # Print CR
CD 10   #
B0 0A   # Then print NL

B4 0E   # Print a char
CD 10   # 

EB EC   # Go to -20 bytes

( QEMU) .
AH = 00h 16h, , , AL.
, : OD (aka CR aka ), Return/Enter, , ( -), .
CR CR, LF, , Enter.
AL OD, , .
: -- , ! 20 .

, :

, «» ,
, ,
«»: Backspace , .

. emacs- .

, .

, - :

, . , :

# 0x0000:0x007F (0-127)
B8 12 00          # Set VGA mode 640x480x16
CD 10

B4 0E             # Set a console output mode
B3 00             # Set FG color to black
FE C3             # Color++

BE 80 7C          # Place 0x0080 + 0x7c00 = 0x7c80 into SI
AC                # Load a byte at address SI into AL, increment SI
3C 00             # AL == 00?
74 F6             # If yes, go to -10 bytes (to FE C3)

CD 10             # Print a char in AL
EB F7             # Go to -9 bytes (to AC)

00
00 00 00 00 00 00 00 00

...

# 0x0080:0x00FF (128-255)
48 65 6C 6C 6F 2C 20 48   # Hello, H
61 62 72 61 68 61 62 72   # abrahabr
21 20                     # ! 

00 00 00 00 00 00
00 00 00 00 00 00 00 00

color-printer.mbr

(? ? BIOS?) , ,
BL (. , .)
FE BL
: , , , .

, .

x86 Opcode Cheat Sheet — , .
Intel 80386 Reference Programmer's Manual — . :
X86 Opcode and Instruction Reference — , .
x86 Instruction Set Reference — - .
Intel 80x86 Assembly Language OpCodes — , , Ctrl-F-.
Ralf Brown's Interrupt List — BIOS- , .
Values for standard video mode — ( Ralf Brown's Interrupt List ).
(, : , ):
- - ( )
- -
, .
man ascii , .

, :
— !
— .
, Sublime Text :
— --, ! !
.

- .

UPD: eisaev, Andrew_Pinkerton, MrSmith33, Anthony1025 , , .

Zurück zu den Wurzeln: Wir fahren einen Computer direkt vom MBR aus

Einführung

Planen:

Haftungsausschluss und Haftungsausschluss

MBR

PRINT "#"

PRINT "Hello, Habrauser!"

PRINT user_input$

More articles:

`PRINT "#"`

`PRINT "Hello, Habrauser!"`

`PRINT user_input$`