; 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