Bona Fide OS Developer
View unanswered posts | View active topics It is currently Wed Sep 19, 2018 7:36 am



Post new topic Reply to topic  [ 3 posts ] 
 Meganet OS-4 GPT Enabled MBR (CRC32 Problem) 
Author Message
Site Admin

Joined: Sat Jul 25, 2009 7:44 am
Posts: 274
Location: United Kingdom
Post Meganet OS-4 GPT Enabled MBR (CRC32 Problem)
I am currently developing an MBr that can handle the GPT (GUID Partition Table) and still handle the Legacy MBR.

The idea is to be able to make the most use out of modern hard drives so the 64Bit GPT is ideal.

The main problem is that I need an ASM based CRC32 routine.

I have seen many C/C++ based CRC32 routines which dont use pre-compiled CRC tables and I wish to
have the same in ASM for my MBR.

If anyone could help, I would really appreciate it.

Also, here is the code I have so far:

Code:

; This is the Meganet OS-4 MBR
; It uses bot the MBR and the GPT, giving preference to
; entries in th GPT.

struc GPTHeader
   Signature     resq 1  ; Should be "EFI PART"
   Revision     resw 1  ; in format 00 00 MA MI (MA = Major, MI = Minor) so 00 00 01 00 is 1.0
   Headersize    resw 1  ; The size of this structure (Usually 5C 00 00 00) 92 Bytes
   HeaderCRC    resw 1  ; CRC32 of the header
   MBZ          resw 1  ; Should be zero
   CurrentLBA     resq 1  ; Location of this header (1)
   BackupLBA    resq 1  ; The location of the backup copy of this header LBA-1
   PartsFirstLBA resq 1  ; The first usable LBA for partitions
   PartsLastLBA    resq 1  ; The last usable LBA for partitions
   DiskGUID     resb 16 ; The disk GUID
   Partitions    resq 1  ; Should be 2
   PartCount    resw 1  ; The number of partiotn entries
   PartEntSize     resw 1  ; The size (In Bytes) or a partiotn entry
   PartCRC       resw 1  ; The CRC32 of the partiotn entry array
endstruc

struc GPTEntry
   PartitionTypeGUID    resb 16 ; The GUID representing the partition type
   PartitionGUID      resb 16 ; The partitions unique GUID
   FirstLBA            resq 1  ; The partition starting LBA (little-endian)
   LastLBA            resq 1  ; The partitions last LBA (Inclusive, usually odd
   Flags            resq 1  ; The partitions Attribute Flags
   PartitionName      resb 72 ; The name of the partition (36 UTF-16LE code units)
endstruc

struc MBRPartitionEntry   
   BootFlag        resb 1 ; 0x80 = Active 0x00 = Non-Active
   BeginHead      resb 1 ; Starting Sector CHS Head
   BeginSector    resb 1 ; Starting Sector CHS Head
   BeginCyl       resb 1 ; Starting Sector CHS Head
   SystemID       resb 1 ; The SystemID A.K.A. Partition Type
   EndHead        resb 1 ; End Sector CHS Head
   EndSector      resb 1 ; End Sector CHS Head
   EndCyl         resb 1 ; End Sector CHS Head
   RelSectorLow   resw 1 ;
   RelSectorHigh  resw 1
   NumSectorsLow  resw 1
   NumSectorsHigh resw 1
endstruc

[SECTION .text]
[BITS 16]
[ORG 0x0600]

; Disable interrupts
cli

; Initialise the segment registers
xor ax,ax
mov ds,ax
mov es,ax

; Initialise the stack
mov ss, ax
mov sp, 0x7C00

; enable interrupts
sti


mov si, _relocating_msg
call print_string_16

; Copy the MBR to 0x0600
cld
mov si, sp
mov di, 0x0600
mov cx, 512/2
repne movsw
jmp 0x0000:next

next:

; Save the Drive Number passed by the BIOS in DL
mov [BiosDriveNumber], DL

mov si, _done_msg
call print_string_16

; This is where we continue
; after being moved from 0x7C00 to 0x0600

; Check if drive supports LBA
mov ah,0x41
mov bx,0x55AA
mov dl,[BiosDriveNumber]
int 0x13
cmp bx,0xAA55
jne _no_gpt

;Read LBA1 (GPT Header) to 0x8000
push ds
.reset:
mov ax, 0
mov dl, [BiosDriveNumber]
int 0x13
jc .reset
pop ds
mov ah, 0x42
mov dl, [BiosDriveNumber]
lea si, [lbaadr]
int 0x13
cmp ah, 0x00
jne _no_gpt

mov si, _reading_gpt_msg
call print_string_16

; Check the GPT Signature
;db 0x45, 0x46, 0x49, 0x20, 0x50, 0x41, 0x52, 0x54
cmp dword [0x8000], 0x20494645
jne _no_gpt
cmp dword [0x8004], 0x54524150
jne _no_gpt



; Jump to _gpt_ok if the signature check was ok
je _gpt_ok

; We are here if the above signature check failed
_no_gpt:
mov si, _fail_msg
call print_string_16
jmp _mbr_fallback

; Protect from running through into data
jmp $ ; this is temporary

; We reach this point if the GPT signature check was ok
_gpt_ok:

; ####TEMP####
mov si, _GPT_sig_ok
call print_string_16
; ####TEMP####

jmp $

; We reach this point if we need to use legacy MBR
_mbr_fallback:

mov si, _reading_MBR_msg
call print_string_16

; check if first partition is active
mov ax, 1
cmp byte [0x7BBE], 0x80
je _found_mbr_active_part
inc ax
cmp byte [0x7BCF], 0x80
je _found_mbr_active_part
inc ax
cmp byte [0x7BDF], 0x80
je _found_mbr_active_part
inc ax
cmp byte [0x7BEF], 0x80
je _found_mbr_active_part

; At this point both GPT and MBR boot have failed
mov si, _fail_msg
call print_string_16

jmp $

; We reach this point if we have found an active MBR partition to boot from
_found_mbr_active_part:

; ####TEMP####
mov si, _MBR_part_act
call print_string_16
; ####TEMP####

jmp $

lbaadr:
   db 0x10      ; packet size (16 bytes)
   db 0x00      ; reserved, must be 0
   db 0x20      ; number of sectors to transfer
   db 0x00      ; reserved, must be 0
   dw 0x0000    ; Buffer's segment
   dw 0x0800    ; Buffer's offset
   dq 0x0000000000000001 ; 64-bit starting sector number

; 16-bit Function to print a sting to the screen
; input: SI - Address of start of string
print_string_16:         ; Output string in SI to screen
pusha
mov ah, 0x0E         ; int 0x10 teletype function
.repeat:
lodsb            ; Get char from string
cmp al, 0
je .done         ; If char is zero, end of string
int 0x10         ; Otherwise, print it
jmp short .repeat
.done:
popa
ret

; Data area
BiosDriveNumber db 0
SectorBufferOffset dw 0x8000
GPTHeaderCRC32 dw 0
SecondaryGPTHeader dq 0

; Permanent strings
_relocating_msg db 'Relocating MBR... ', 0
_reading_gpt_msg db 'Reading GPT... ', 0
_reading_MBR_msg db 'Reading MBR... ', 0
_fail_msg db 'FAILED!', 13, 10, 0
_done_msg db 'DONE.', 13, 10, 0

; Temporary string
_GPT_sig_ok db 'GPT SIG OK ', 0
_MBR_part_act db 'MBR PART ACTIVE FOUND ', 0


; Pad this stub MBR (With NOP's)
times 446-$+$$ db 0x90

; Space for the MBR Partition Table
PartitionTable:
db 0x80, '###############################################################'
;PartitionTable resb 4 * MBRPartitionEntry_size

; MBR's boot sig
sign dw 0xAA55

; Hardcoded GPT signature
db 0x45, 0x46, 0x49, 0x20, 0x50, 0x41, 0x52, 0x54
dd 0x00000100
dd 0x0000005C
dd 0xD69D5156 ; CRC32
dd 0x00000000
dq 0x0000000000000001
dq 0x0000000000000001
dq 0x0000000000000022
dq 0x0000000033D53F62

dq 0x2458777194255772
dq 0x2458777194255772

dq 0x0000000000000002
dd 0x00000001
dd 0x00000080
dd 0x2512CA3B

; 1st Partition
dq 0x2386776239599677
dq 0x4139766364236889
dq 0x2458777194255772
dq 0x2458777194255772
dq 0x0000000000000022
dq 0x0000000033D53F62
dq 0x00000000000000000000
times 72 db 0x20


_________________
Thank you for reading,

Kieran C G Foot


Tue Jan 18, 2011 8:39 pm
Profile WWW

Joined: Fri Aug 20, 2010 10:04 pm
Posts: 41
Post Re: Meganet OS-4 GPT Enabled MBR (CRC32 Problem)
TL;DR; (Too long, didn't read)

OSDev likes to give holier-than-thou, bad advice that really sucks. For example... if this were OSDev have I might ask, "Have you considered 2048 byte block drives that are coming-out in the future?" (I hear the hardware is backward compatible, so I wouldn't worry about it. OSDev might suggest making a 2048 byte MBR. I hate ideas that fork code.)


<rant>It's insanity having forks in code where some people have features and some people dont. If not everybody can have it, nobody can.</rant>


While it is true that MBR code can do anything partitioning they like, I'm pretty sure some BIOS's frown upon nonstandard MBR's.

You are aware of UEFI?
http://www.uefi.org/home/
I hear it's backward compatible, or at least I'm hoping so.


Thu Jan 20, 2011 7:43 am
Profile
Site Admin

Joined: Sat Jul 25, 2009 7:44 am
Posts: 274
Location: United Kingdom
Post Re: Meganet OS-4 GPT Enabled MBR (CRC32 Problem)
Yeah, I have looked into UEFI, however Im having problems targeting EFI with GCC under Cygwin.

So instead, Im going to use a standard MBR with a single 50/100MB FAT32 Partition with support code that will load my OS and handle the GPT.

_________________
Thank you for reading,

Kieran C G Foot


Fri Jan 21, 2011 3:53 am
Profile WWW
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 3 posts ] 


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
cron
Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group.
Designed by Vjacheslav Trushkin and tweaked by the BF Team.