Author Topic: Acessing the Master Boot Record! (Read 41889 times)

laskar01 · « **Reply #15 on:** July 12, 2012, 02:44:37 AM »

Thanks Steve I'll look it up.
Got the Win98 computer up and running, unfortunately no 1.4MB disqette yet. have to raid the cellar...

Switched to the code uploaded by Vortex, edited the output with adresses and read Wikipedia on MBR.

Structure of a master boot record
Address    Description    Size in bytes
Hex    Oct    Dec
0000    0000    0    code area    440
(max. 446)
01B8    0670    440    disk signature (optional)    4
01BC    0674    444    Usually nulls; 0x0000    2
01BE    0676    446    Table of primary partitions
(Four 16-byte entries, IBM partition table scheme)    64
01FE    0776    510    55h    MBR signature    2
01FF    0777    511    AAh
MBR, total size: 446 + 64 + 2 =    512

Here is the dump can somebody decode the first two or three bytes?

; File C:\Documents and Settings\laskar01\Skrivbord\ReadMBR11\master_boot_record_file.ext opened at 512 bytes

00 db 033h,0C0h,08Eh,0D0h,0BCh,000h,07Ch,08Eh,0C0h,08Eh,0D8h,0BEh,000h,07Ch,0BFh,000h
10 db 006h,0B9h,000h,002h,0FCh,0F3h,0A4h,050h,068h,01Ch,006h,0CBh,0FBh,0B9h,004h,000h
20 db 0BDh,0BEh,007h,080h,07Eh,000h,000h,07Ch,00Bh,00Fh,085h,00Eh,001h,083h,0C5h,010h
30 db 0E2h,0F1h,0CDh,018h,088h,056h,000h,055h,0C6h,046h,011h,005h,0C6h,046h,010h,000h
40 db 0B4h,041h,0BBh,0AAh,055h,0CDh,013h,05Dh,072h,00Fh,081h,0FBh,055h,0AAh,075h,009h
50 db 0F7h,0C1h,001h,000h,074h,003h,0FEh,046h,010h,066h,060h,080h,07Eh,010h,000h,074h
60 db 026h,066h,068h,000h,000h,000h,000h,066h,0FFh,076h,008h,068h,000h,000h,068h,000h
70 db 07Ch,068h,001h,000h,068h,010h,000h,0B4h,042h,08Ah,056h,000h,08Bh,0F4h,0CDh,013h
80 db 09Fh,083h,0C4h,010h,09Eh,0EBh,014h,0B8h,001h,002h,0BBh,000h,07Ch,08Ah,056h,000h
90 db 08Ah,076h,001h,08Ah,04Eh,002h,08Ah,06Eh,003h,0CDh,013h,066h,061h,073h,01Ch,0FEh
A0 db 04Eh,011h,075h,00Ch,080h,07Eh,000h,080h,00Fh,084h,08Ah,000h,0B2h,080h,0EBh,084h
B0 db 055h,032h,0E4h,08Ah,056h,000h,0CDh,013h,05Dh,0EBh,09Eh,081h,03Eh,0FEh,07Dh,055h
C0 db 0AAh,075h,06Eh,0FFh,076h,000h,0E8h,08Dh,000h,075h,017h,0FAh,0B0h,0D1h,0E6h,064h
D0 db 0E8h,083h,000h,0B0h,0DFh,0E6h,060h,0E8h,07Ch,000h,0B0h,0FFh,0E6h,064h,0E8h,075h
E0 db 000h,0FBh,0B8h,000h,0BBh,0CDh,01Ah,066h,023h,0C0h,075h,03Bh,066h,081h,0FBh,054h
F0 db 043h,050h,041h,075h,032h,081h,0F9h,002h,001h,072h,02Ch,066h,068h,007h,0BBh,000h
100db 000h,066h,068h,000h,002h,000h,000h,066h,068h,008h,000h,000h,000h,066h,053h,066h
110db 053h,066h,055h,066h,068h,000h,000h,000h,000h,066h,068h,000h,07Ch,000h,000h,066h
120db 061h,068h,000h,000h,007h,0CDh,01Ah,05Ah,032h,0F6h,0EAh,000h,07Ch,000h,000h,0CDh
130db 018h,0A0h,0B7h,007h,0EBh,008h,0A0h,0B6h,007h,0EBh,003h,0A0h,0B5h,007h,032h,0E4h
140db 005h,000h,007h,08Bh,0F0h,0ACh,03Ch,000h,074h,009h,0BBh,007h,000h,0B4h,00Eh,0CDh
150db 010h,0EBh,0F2h,0F4h,0EBh,0FDh,02Bh,0C9h,0E4h,064h,0EBh,000h,024h,002h,0E0h,0F8h
160db 024h,002h,0C3h,049h,06Eh,076h,061h,06Ch,069h,064h,020h,070h,061h,072h,074h,069h
170db 074h,069h,06Fh,06Eh,020h,074h,061h,062h,06Ch,065h,000h,045h,072h,072h,06Fh,072h
180db 020h,06Ch,06Fh,061h,064h,069h,06Eh,067h,020h,06Fh,070h,065h,072h,061h,074h,069h
190db 06Eh,067h,020h,073h,079h,073h,074h,065h,06Dh,000h,04Dh,069h,073h,073h,069h,06Eh
1A0db 067h,020h,06Fh,070h,065h,072h,061h,074h,069h,06Eh,067h,020h,073h,079h,073h,074h
1B0db 065h,06Dh,000h,000h,000h,063h,07Bh,09Ah,036h,067h,036h,067h,000h,000h,080h,020h
1C0db 021h,000h,007h,0FEh,0FFh,0FFh,000h,008h,000h,000h,000h,068h,0FCh,006h,000h,000h
1D0db 000h,000h,000h,000h,000h,000h,000h,000h,000h,000h,000h,000h,000h,000h,000h,000h
1E0db 000h,000h,000h,000h,000h,000h,000h,000h,000h,000h,000h,000h,000h,000h,000h,000h
1F0db 000h,000h,000h,000h,000h,000h,000h,000h,000h,000h,000h,000h,000h,000h,055h,0AAh
Kindest regards,

Lasse

Vortex · « **Reply #16 on:** July 12, 2012, 02:52:18 AM »

Hi laskar01,

Starman's site is a very good source to study MBR :

http://thestarman.pcministry.com

A nice study :

An Examination of the Windows 2000 ( NT5.0 ) and Windows XP ( NT5.1 ) MBR ( Master Boot Record )

http://thestarman.pcministry.com/asm/mbr/Win2kmbr.htm

Quote

The first 300 bytes (000h through 12Bh) of this 512-byte sector are executable code and the next 80 bytes (12Ch through 17Bh) contain error messages. The last 66 bytes of the sector contain the 64-byte Partition Table (1BEh through 1FDh); data in the Table area will depend upon the size, structure and file systems on each hard disk. The sector ends with the Word-sized signature ID of AA55h (often called the sector's Magic number); on PCs using an Intel (or x86 compatible) CPU, hex Words are stored with the Low-byte first and the High-byte last.

laskar01 · « **Reply #17 on:** July 12, 2012, 04:05:33 AM »

Hi all
Very nice tutorial Vortex.

Ran XVI32 and saw that
from 163h to 179h I have "Invalid partition table"
from 17Bh to 198fh "Error loading operating system"
from 19Ah to 1B1h "Missing operating system"

A slight difference from the author's thats start at 12Ch to17Bh.

It seems that I have code from 0h to 162h. That is 353 bytes of code, a some work to disassemble nothing else...
I'll start right off!

/Lasse

dedndave · « **Reply #18 on:** July 12, 2012, 05:04:49 AM »

Code: [Select]

0000:7C00 33C0           XOR    AX,AX
0000:7C02 8ED0           MOV    SS,AX
0000:7C04 BC007C         MOV    SP,7C00
0000:7C07 8EC0           MOV    ES,AX
0000:7C09 8ED8           MOV    DS,AX
0000:7C0B BE007C         MOV    SI,7C00
0000:7C0E BF0006         MOV    DI,0600
0000:7C11 B90002         MOV    CX,0200
0000:7C14 FC             CLD
0000:7C15 F3A4           REPZ   MOVSB

laskar01 · « **Reply #19 on:** July 12, 2012, 06:29:47 AM »

Holy! dedndave!

Which tool did you use for that!?!

dedndave · « **Reply #20 on:** July 12, 2012, 06:47:09 AM »

lol
open a console window and type in "debug"

once you get the '-' prompt, type '?' for a list of commands

i am not too fond of that particular boot sector
it doesn't seem to follow ms format very well

dedndave · « **Reply #21 on:** July 12, 2012, 07:39:25 AM »

i might add...

the easiest way to multi-boot is to modify the boot.ini files on each partition you want to boot from :t

http://support.microsoft.com/kb/289022

Ryan · « **Reply #22 on:** July 12, 2012, 08:51:15 AM »

There is no boot.ini in Windows 7.

Gunther · « **Reply #23 on:** July 12, 2012, 09:04:56 AM »

Ryan is right. Dedndave, Your link points to an article with the headline: How to edit the Boot.ini file in Windows XP.

Gunther

Ryan · « **Reply #24 on:** July 12, 2012, 09:15:54 AM »

Vista and 7 use bcdedit.

http://www.sevenforums.com/tutorials/2676-bcdedit-how-use.html

laskar01 · « **Reply #25 on:** July 12, 2012, 09:18:49 AM »

Hi again, Debug seemed to difficult, I found IDA that did a disassemble and also supplied comments :icon14:
(Mine starts with '--')

Need to understand this below now!

seg000:0000 ; Format : Binary file
seg000:0000 ; Base Address: 0000h Range: 0000h - 0200h Loaded length: 0200h
seg000:0000
seg000:0000 .686p
seg000:0000 .mmx
seg000:0000 .model flat
seg000:0000
seg000:0000 ; -----------------------
seg000:0000
seg000:0000 ; Segment type: Pure code
seg000:0000 seg000 segment byte public 'CODE' use16
seg000:0000 assume cs:seg000
seg000:0000 assume es:nothing, ss:nothing, ds:nothing, fs:nothing, gs:nothing
seg000:0000 xor ax, ax -- Clear ax
seg000:0002 mov ss, ax -- Clear ss
seg000:0004 mov sp, 7C00h -- Set stackpointer to 7C00h
seg000:0007 mov es, ax -- Clear es
seg000:0009 mov ds, ax -- Clear ds
seg000:000B mov si, 7C00h -- Load general purpose register si with immediate 7C00h =SOURCE
seg000:000E mov di, 600h -- Load general purpose register di with immediate 600h = TARGET
seg000:0011 mov cx, 200h -- Load counter cx with 200h= 512
seg000:0014 cld -- Clear direction flag will string instructions will increment (E)SI and (E)DI
seg000:0015 rep movsb -- Repeat while cx>0

--Until here 512 bytes are copied from adress 7C00h to 600h.

seg000:0017 push ax
seg000:0018 push 61Ch
seg000:001B retf --pops the stack first into (E)IP, and then into CS : IP=61Ch, CS=Ax=0

--Holy! IP=Instruction pointer. Execution starts at 61Ch now!
--This is the code of the active partition according to
-- http://thestarman.pcministry.com/asm/mbr/Win2kmbr.htm#CHS

seg000:001C ;

--My guess is that we are here because we didn't load the OS, and must do some sort of trouble shooting.

seg000:001C sti --Set interrupt enables maskable interrupt
seg000:001D mov cx, 4 --Sets counter register to 4, becasue partitiontable is 4*16=64 Bytes
seg000:0020 mov bp, 7BEh --Sets basepointer to 7BEh. This is at the beginning of the partition table of the copied MBR
--that is 600h+1BEh
seg000:0023
seg000:0023 loc_23:
seg000:0023 cmp byte ptr [bp+0], 0 --Compare the byte at adress bp+0 with zero. Valid partition table should start with 80h
seg000:0027 jl short loc_34 --Jump to shortloc_34 if the byte was negative
seg000:0029 jnz loc_13B --Jump to loc_13B if the byte was positive. Print out "Invalid partiton table", without further do.
seg000:002D add bp, 10h --The byte is zero, add 10h to the basepointer, the next entry in the partition-
--table.

seg000:0030 loop loc_23

seg000:0032 int 18h ; TRANSFER TO ROM BASIC
seg000:0032 ; causes transfer to ROM-based BASIC (IBM-PC)
seg000:0032 ; often reboots a compatible; often has no effect at all
seg000:0034
seg000:0034 loc_34:

--here because of the byte at bp+0 is less than zero from seg000:0027
-- Or becasue we had an error and jumped from seg000:00AE jmp short loc_34.
-- We read sectors from drive with error at seg000:007E int 13h , and for some reason
--we didn't want to jump to reset disks at seg000:00A2 or jump to loc_136 from seg000:00AC
seg000:0034

seg000:0034 mov [bp+0], dl --What is the value of the low byte of DX???
--Must have been filled in INT 18h, but if we came here from
--seg000:0027. If it is a repair it must be 80h.

seg000:0037 push bp
seg000:0038 mov byte ptr [bp+11h], 5 --5 is put at adress bp+11h. heads = 5
-- This is position +1 from the base of the second entry of the partitiontable
seg000:003C mov byte ptr [bp+10h], 0 -- 0 is put at adress bp+10h, base adress of the seond entry of the partitiontable
-- 0 in the 0:th place means, non-bootable partition.
seg000:0040
seg000:0040 loc_40:

--here because of the byte at bp+0 is less than zero and continuing from just above

seg000:0040 mov ah, 41h ; 'A'
seg000:0042 mov bx, 55AAh -- AA55h is always the last word on a master boot record. A signature.The so called "magic
-- number".
seg000:0045 int 13h -- http://en.wikipedia.org/wiki/INT_13H#INT_13h_AH.3D41h:_Check_Extensions_Present
seg000:0047 pop bp
seg000:0048 jb short loc_59 -- Jump if carry is set i.e. "not present",
seg000:004A cmp bx, 0AA55h -- We didn't jump result was "present" Compare Bx
--to the standard result of the above interrupt?
seg000:004A
seg000:004E jnz short loc_59 --The standard test didn't show OK, the interrupt didn't go well.
--Is the interpretation correct?
seg000:0050 test cx, 1 --implied and of cx with 1 (1=Device Access using the packet structure)
seg000:0054 jz short loc_59 --jump to short loc_59 if result of implied AND is 0.

seg000:0056 inc byte ptr [bp+10h] -- if ( cx==1) -> inc the byte in bp+10h
--(maybe bp+10h=7BEh+10=7CF, was 0, now =1, would make the partition entry
-- invalid)

seg000:0059
seg000:0059 loc_59:

--Here because the "standard test" after INT 13h, at seg000:004A cmp bx, 0AA55h above didn't return OK

seg000:0059
seg000:0059 pushad --Push all 32-bit registers
seg000:005B cmp byte ptr [bp+10h], 0 --0 is a valid number at the second entry of the partition table.

seg000:005F jz short loc_87
seg000:005F
seg000:0061 push large 0 --what is large?
seg000:0067 push large dword ptr [bp+8] -- Offset 8 in a partitiontable entry contains the
seg000:0067 --LBA of first absolut sector in the partition
seg000:006B push 0
seg000:006E push 7C00h
seg000:0071 push 1
seg000:0074 push 10h
seg000:0077 mov ah, 42h ; 'B'
seg000:0079 mov dl, [bp+0] --See
seg000:007C mov si, sp -- http://en.wikipedia.org/wiki/INT_13H#INT_13h_AH.3D42h:_Extended_Read_Sectors_From_Drive
seg000:007E int 13h ; DISK
seg000:0080 lahf --Load AH from flags
seg000:0081 add sp, 10h --Make place for local variables
seg000:0084 sahf --Restore flags from AH
seg000:0085 jmp short loc_9B
seg000:0087 ; --------
seg000:0087
seg000:0087 loc_87:

--Here because of instr. at seg000:005B cmp byte ptr [bp+10h], 0

seg000:0087 mov ax, 201h --We are doing the work of BIOS after POST, BIos read the content of (C,H,S)
seg000:008A mov bx, 7C00h -- = (0,0,1) to adress 0000:7C00h
seg000:008D mov dl, [bp+0]
seg000:0090 mov dh, [bp+1]
seg000:0093 mov cl, [bp+2]
seg000:0096 mov ch, [bp+3] -- http://en.wikipedia.org/wiki/INT_13H#INT_13h_AH.3D02h:_Read_Sectors_From_Drive
seg000:0099 int 13h ; DISK - READ SECTORS INTO MEMORY
seg000:0099 ; AL = number of sectors to read, CH = track, CL = sector
seg000:0099 ; DH = head, DL = drive, ES:BX -> buffer to fill
seg000:0099 ; Return: CF set on error, AH = status, AL = number of sectors read
seg000:009B
seg000:009B loc_9B:
seg000:009B popad --EDI=10h;ESI=?;EBP=?;ESP=?;EBX=?;EDX=?;ECX=?;EAX=?;
seg000:009D jnb short loc_BB --Same as JAE Jump if CF=0, that is no error
seg000:009F dec byte ptr [bp+11h] --No jump, we have an error, decrement byte at location [bp+11h]
seg000:00A2 jnz short loc_B0 --Jump below to reset discs
seg000:00A4 cmp byte ptr [bp+0], 80h ; 'Ç'
seg000:00A8 jz loc_136 -- Jump to loc_136 if byte at bp+0 was 80h. Valid partition table entry.
seg000:00AC mov dl, 80h ; 'Ç'
seg000:00AE jmp short loc_34
seg000:00B0 ; ---------------------------
seg000:00B0
seg000:00B0 loc_B0:
seg000:00B0 push bp
seg000:00B1 xor ah, ah
seg000:00B3 mov dl, [bp+0] -- http://en.wikipedia.org/wiki/INT_13H#INT_13h_AH.3D00h:_Reset_Disk_Drive
seg000:00B6 int 13h ; DISK - RESET DISK SYSTEM
seg000:00B6 ; DL = drive (if bit 7 is set both hard disks and floppy disks reset)
seg000:00B8 pop bp
seg000:00B9 jmp short loc_59
seg000:00BB ; --------------------
seg000:00BB
seg000:00BB loc_BB:
seg000:00BB cmp word ptr ds:7DFEh, 0AA55h --Check if the copied MBR contains the "magic number" at the end
seg000:00C1 jnz short loc_131 --Missing go to print out "Missing operating system"
seg000:00C3 push word ptr [bp+0]
seg000:00C6 call sub_156
seg000:00C9 jnz short loc_E2
seg000:00CB cli
seg000:00CC mov al, 0D1h ; 'Ð'
seg000:00CE out 64h, al ; AT Keyboard controller 8042. --Flashing of the Keyboard Leds?
seg000:00D0 call sub_156
seg000:00D3 mov al, 0DFh ; '¯'
seg000:00D5 out 60h, al ; AT Keyboard controller 8042. --Flashing of the Keyboard Leds?
seg000:00D7 call sub_156
seg000:00DA mov al, 0FFh
seg000:00DC out 64h, al ; AT Keyboard controller 8042.
seg000:00DC ; Reset the keyboard and start internal diagnostics
seg000:00DE call sub_156
seg000:00E1 sti
seg000:00E2
seg000:00E2 loc_E2:
seg000:00E2 mov ax, 0BB00h
seg000:00E5 int 1Ah -- http://maven.smith.edu/~thiebaut/ArtOfAssembly/CH13/CH13-3.html#HEADING3-193
seg000:00E7 and eax, eax
seg000:00EA jnz short loc_127
seg000:00EC cmp ebx, 41504354h
seg000:00F3 jnz short loc_127
seg000:00F5 cmp cx, 102h
seg000:00F9 jb short loc_127
seg000:00FB push large 0BB07h
seg000:0101 push large 200h
seg000:0107 push large 8
seg000:010D push ebx
seg000:010F push ebx
seg000:0111 push ebp
seg000:0113 push large 0
seg000:0119 push large 7C00h
seg000:011F popad
seg000:0121 push 0
seg000:0124 pop es
seg000:0125 int 1Ah
seg000:0127
seg000:0127 loc_127:
seg000:0127
seg000:0127 pop dx
seg000:0128 xor dh, dh
seg000:012A jmp far ptr 0:7C00h
seg000:012F ; -----------------------
seg000:012F int 18h ; TRANSFER TO ROM BASIC
seg000:012F ; causes transfer to ROM-based BASIC (IBM-PC)
seg000:012F ; often reboots a compatible; often has no effect at all
seg000:0131
seg000:0131 loc_131:

seg000:0131 mov al, ds:7B7h --600+1B7, 1B7 is the pointer to the third message
seg000:0134 jmp short loc_13E --"Missing operating system"
seg000:0136 ; --------------------
seg000:0136
seg000:0136 loc_136:

--We returned with an error when reading sectors at seg000:0099
--We didn't reset discs going from seg000:00A2 jnz short loc_B0 Jump below to reset dics
--And then because of evaluation at seg000:00A4 cmp byte ptr [bp+0], 80h ;

seg000:0136 mov al, ds:7B6h --600+1B6, 1B6 is the pointer to the second message
seg000:0139 jmp short loc_13E --"Error loading operating system"
seg000:013B ; ----------------------
seg000:013B
seg000:013B loc_13B:
--Here because of the byte at bp+0 was positive. Should always be 80h.

seg000:013B mov al, ds:7B5h --600+1B5, 1B5 is pointer to first message
seg000:013E --"Invalid partition table"

seg000:013E loc_13E:

--Here inderectly because of {seg000:00A4 cmp byte ptr [bp+0], 80h} and then from {seg000:0139 jmp short loc_13E}

seg000:013E
seg000:013E xor ah, ah
seg000:0140 add ax, 700h
seg000:0143 mov si, ax
seg000:0145
seg000:0145 loc_145:
seg000:0145 lodsb
seg000:0146 cmp al, 0 --The errormessagestring is terminated with zero
seg000:0148 jz short loc_153
seg000:014A mov bx, 7
seg000:014D mov ah, 0Eh
seg000:014F int 10h ; - VIDEO - WRITE CHARACTER AND ADVANCE CURSOR (TTY WRITE)
seg000:014F ; AL = character, BH = display page (alpha modes)
seg000:014F ; BL = foreground color (graphics modes)
--Prints out a character at the time
seg000:0151 jmp short loc_145
seg000:0153 ; ----------------------
seg000:0153
seg000:0153 loc_153:
seg000:0153 ; seg000:0154j
seg000:0153 hlt
seg000:0154 jmp short loc_153 --Do nothing after the errormessage is printed.
seg000:0156
seg000:0156 ; ¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦ S U B R O U T I N E ¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦
seg000:0156
seg000:0156
seg000:0156 sub_156 proc near ; CODE XREF: seg000:00C6p
seg000:0156 ; seg000:00D0p ...
seg000:0156 sub cx, cx
seg000:0158
seg000:0158 loc_158: ; CODE XREF: sub_156+8j
seg000:0158 in al, 64h ; AT Keyboard controller 8042.
seg000:015A jmp short $+2
seg000:015C and al, 2
seg000:015E loopne loc_158
seg000:0160 and al, 2
seg000:0162 retn
seg000:0162 sub_156 endp
seg000:0162
seg000:0162 ; ----------------------------
seg000:0163 aInvalidPartiti db 'Invalid partition table',0
seg000:017B aErrorLoadingOp db 'Error loading operating system',0
seg000:019A aMissingOperati db 'Missing operating system',0
seg000:01B3 db 2 dup(0), 63h, 7Bh, 9Ah, 36h, 67h, 36h, 67h, 2 dup(0)
seg000:01B3 db 80h, 20h, 21h, 0, 7, 0FEh, 2 dup(0FFh), 0, 8, 3 dup(0)
seg000:01B3 db 68h, 0FCh, 6, 30h dup(0), 55h, 0AAh
seg000:01B3 seg000 ends
seg000:01B3
seg000:01B3
seg000:01B3 end

Ryan · « **Reply #26 on:** July 12, 2012, 09:21:23 AM »

You can also change the default operating system and time delay once the options are set by going through System (via Control Panel or Computer->Properties), then Advanced system settings->Advanced tab->Startup and Recovery group->Settings button.

dedndave · « **Reply #27 on:** July 12, 2012, 11:13:04 AM »

Quote

Vista and 7 use bcdedit.

well - i got you close :P

laskar01 · « **Reply #28 on:** July 13, 2012, 12:45:09 AM »

Hi again all !

:( Forgive that I am taking up so much space, but my believe was that somebody would find the decoding interesting :icon_rolleyes: , and that I guessed that I would need help.
Feeling sorry though for occupying so many bits on the server :icon_redface:
I've decoded a couple of instructions but now I am stuck on INT 18h and the content of the low byte of DX.

I downloaded Ralf Brown's interrupt list, but I dont know how to read it. It doesn't seem to be what I need... :icon_confused:

I need to read the code at that interrupt vector. How? :shock:

Could you people help?

dedndave · « **Reply #29 on:** July 13, 2012, 02:05:52 AM »

INT 18h is really really old stuff - lol

on old computers (mainly the IBM XT), it would boot up BASIC-in-ROM
these machines had BASIC resident in EPROMS :P
if you had no boot disk (or a faulty one), it would come up with BASIC

on really old computers (mainly the IBM PC), it would boot from a cassette :shock:
it used an audio port with a connector similar to the keyboard connector
cassettes were terribly slow - you could boot up today - and come back tomorrow

INT 18h had no parameters for those machines - so the contents of BX didn't matter
some early third-party manufacturers used the interrupt for things like keyboards or sound cards

it may be used for something entirely different on modern machines - like accessing data in the RTC memory - i don't know - there are other functions for that

INT 19h reboots the system and also has no parameters

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Author Topic: Acessing the Master Boot Record! (Read 41889 times)

laskar01

Re: Acessing the Master Boot Record!

Vortex

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laskar01

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dedndave

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laskar01

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dedndave

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dedndave

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Ryan

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Gunther

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Ryan

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laskar01

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Ryan

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dedndave

Re: Acessing the Master Boot Record!

laskar01

Re: Acessing the Master Boot Record!

dedndave

Re: Acessing the Master Boot Record!