.186
.model small
.code

kernel segment at 1000h
org 0
start label far
kernel ends

org 7c00h

first:
jmp short Main
nop

OSName            db "FIRSTOS "
BytesPerSector    dw 512
SectorsPerCluster db 1
ReservedSectors   dw 1
NumberOfFATs      db 2
RootDirEntries    dw 224
NumberOfSectors   dw 2880
MediaType         db 0f0h
SectorsPerFAT     dw 9
SectorsPerTrack   dw 18
NumberOfHeads     dw 2
HiddenSectors     dd 0
DWSectors         dd 0
DriveNumber       db 0
Reserved          db 0
BootSignature     db 29h
SerialNumber      dd 0
VolumeLabel       db "FOSBOOTDISK"
FileSystem        db "FAT12   "

Main:
; set up segments & stack
cli
xor ax, ax
mov ds, ax
mov es, ax
jmp es:MainSetCS
MainSetCS:
mov ss, ax
mov sp, 0f000h
sti
; load root directory
mov ax, 0500h
mov es, ax
mov cx, 4
MainRetryRoot:
push cx
xor ax, ax
mov al, [NumberOfFATs]
mul [SectorsPerFAT]
add ax, [ReservedSectors]
mov [LBAFirstData], ax
call LBAToCHS
mov ax, 0020h
push dx
mul [RootDirEntries]
div [BytesPerSector]
pop dx
add [LBAFirstData], ax
mov ah, 02h
xor bx, bx
int 13h
pop cx
jnc MainSuccessRoot
loop MainRetryRoot
int 18h
MainSuccessRoot:
; read root directory
mov cx, [RootDirEntries]
xor di, di
MainNextEntry:
push cx
mov cx, 11
mov si, offset KernelFile
push di
repe cmpsb
pop di
pop cx
je MainEntryFound
add di, 0020h
loop MainNextEntry
int 18h
MainEntryFound:
add di, 26
mov ax, es:[di]
mov [Cluster], ax
; load FAT
mov cx, 4
MainRetryFAT:
push cx
mov ax, [ReservedSectors]
call LBAToCHS
mov ax, [SectorsPerFAT]
mov ah, 02h
int 13h
pop cx
jnc MainSuccessFAT
loop MainRetryFAT
int 18h
MainSuccessFAT:
; read FAT and load clusters
MainReadCluster:
mov ax, 1000h
mov es, ax
mov cx, 4
MainRetryCluster:
push cx
mov ax, [Cluster]
sub ax, 2
xor dx, dx
mov dl, [SectorsPerCluster]
mul dx
add ax, [LBAFirstData]
call LBAToCHS
mov al, [SectorsPerCluster]
mov ah, 02h
int 13h
pop cx
jnc MainNextCluster
loop MainRetryCluster
int 18h
MainNextCluster:
mov ax, 0500h
mov es, ax
mov ax, [Cluster]
mov dx, ax
shr ax, 1
add ax, dx
push bx
mov bx, ax
mov ax, es:[bx]
pop bx
test dx, 0001h
jnz MainOddCluster
and ax, 0fffh
jmp MainClusterNumber
MainOddCluster:
shr ax, 4
MainClusterNumber:
add bx, [BytesPerSector]
mov [Cluster], ax
cmp ax, 0ff0h
jb MainReadCluster
; clear screen and move cursor
mov ax, 0b800h
mov es, ax
mov ax, 0720h
xor bx, bx
MainNextChar:
mov es:[bx], ax
add bx, 2
cmp bx, 4000
jb MainNextChar
xor bx, bx
xor dx, dx
mov ah, 02h
int 10h
; set up registers and call kernel
mov ax, kernel
mov ds, ax
mov es, ax
xor ax, ax
xor bx, bx
xor cx, cx
xor dx, dx
xor di, di
jmp kernel:start

LBAToCHS:
xor dx, dx
div [SectorsPerTrack]
mov cl, dl
inc cl
xor dx, dx
div [NumberOfHeads]
mov ch, al
mov dh, dl
mov dl, [DriveNumber]
ret

KernelFile db "KERNEL  BIN"
LBAFirstData dw 0
Cluster dw 0
;db (510 - ($ - first)) dup (0)
;dw 0aa55h
end

.model tiny
.code
mov si, offset msg
Next:
lodsb
or al, al
jz Done
mov ah, 0eh
int 10h
jmp Next
Done:
jmp $

msg db "In the kernel! :)", 0
end

jmp es:MainSetCS

jmp MainSetCS
nop
nop