start:
.code16
-# Disable interrupts.
-# String instructions go upward.
-
- cli
- cld
-
-# Set up data segments.
+# Set up segment registers.
+# Stack grows downward starting from us.
sub ax, ax
- mov es, ax
mov ds, ax
-
-# Set up stack segment.
-# Stack grows downward starting from us.
-# We don't ever use the stack so this is strictly speaking
-# unnecessary.
-
mov ss, ax
mov sp, 0x7c00
-#### Enable A20. Address line 20 is tied to low when the machine
-#### boots, which prevents addressing memory about 1 MB. This code
-#### fixes it.
-
-# Poll status register while busy.
-
-1: in al, 0x64
- test al, 0x2
- jnz 1b
+# Scan floppy disks.
-# Send command for writing output port.
-
- mov al, 0xd1
- outb 0x64, al
-
-# Poll status register while busy.
-
-1: in al, 0x64
- test al, 0x2
- jnz 1b
-
-# Enable A20 line.
-
- mov al, 0xdf
- out 0x60, al
-
-#### Get memory size, via interrupt 15h function 88h, which returns CF
-#### clear if successful, with AX = (kB of physical memory) - 1024.
-#### This only works for memory sizes <= 65 MB, which should be fine
-#### for our purposes. We cap memory at 64 MB because that's all we
-#### prepare page tables for, below.
-
- mov ah, 0x88
- int 0x15
- jc panic
- cli # BIOS might have enabled interrupts
- add eax, 1024 # Total kB memory
- cmp eax, 0x10000 # Cap at 64 MB
- jbe 1f
- mov eax, 0x10000
-1: shr eax, 2 # Total 4 kB pages
- mov ram_pages, eax
+ sub dl, dl
+ sub ebx, ebx
+ mov eax, 0x7e00
+ mov es, ax
+1: call scan_partitions
+ inc dl
+ jnc 1b
+
+# Scan hard disks.
+ mov dl, 0x80
+1: call scan_partitions
+ inc dl
+ jnc 1b
+
+ call panic
+ .asciz "No boot partition"
+
+scan_partitions:
+ # EBX = sector number of partition table
+ # DL = drive number
+ # ES:0000 -> buffer for partition table
+ # Returns CF set if drive exists, CF clear otherwise.
+ call read_sector
+ jnc no_such_drive
+2: cmp word ptr [es:510], 0xaa55
+ jnz no_boot_partition
-#### Create temporary page directory and page table and set page
-#### directory base register.
-
-# Create page directory at 64 kB and fill with zeroes.
- mov ax, 0x1000
+ mov si, 446
+1: mov al, [es:si+4]
+ cmp al, 0x20
+ jz found_boot_partition
+ cmp al, 0x05
+ jz found_extended_partition
+ cmp al, 0x0f
+ jz found_extended_partition
+ cmp al, 0x85
+ jz found_extended_partition
+ cmp al, 0xc5
+ jz found_extended_partition
+next_parttbl_entry:
+ add si, 16
+ cmp si, 510
+ jb 1b
+
+no_boot_partition:
+ clc
+ ret
+no_such_drive:
+ stc
+ ret
+
+found_extended_partition:
+ # DL = drive number.
+ # ES:SI -> partition table entry for extended partition.
+ # Recursively examine it.
+ pusha
+ mov ebx, es:[si+8]
+ mov ax, es
+ add ax, 0x20
mov es, ax
- sub eax, eax
- sub edi, edi
- mov ecx, 0x400
- rep stosd
-
-# Add PDEs to point to PTEs for the first 64 MB of RAM.
-# Also add identical PDEs starting at LOADER_PHYS_BASE.
-# See [IA32-v3] section 3.7.6 for a description of the bits in eax.
-
-# A bug in some versions of GAS prevents us from using the straightforward
-# mov es:[di + LOADER_PHYS_BASE / 1024 / 1024], eax
-# so we calculate the displacement in bx instead.
-
- mov eax, 0x11007
- mov ecx, 0x11
- sub di, di
- mov ebx, LOADER_PHYS_BASE
- shr ebx, 20
-1: mov es:[di], eax
- mov es:[bx + di], eax
- add di, 4
- add eax, 0x1000
- loop 1b
-
-# Set up one-to-map linear to physical map for the first 64 MB of RAM.
-# See [IA32-v3] section 3.7.6 for a description of the bits in eax.
-
- mov ax, 0x1100
+ call scan_partitions
+ popa
+ jmp next_parttbl_entry
+
+found_boot_partition:
+ mov ebx, [es:si+8] # EBX = first sector
+ mov cx, [es:si+12] # CX = number of sectors
+ mov ax, 0x1000 # ES:0000 -> load address
+ mov es, ax
+1: call read_sector
+ add ax, 0x20
mov es, ax
- mov eax, 0x7
- mov cx, 0x4000
- sub di, di
-1: mov es:[di], eax
- add di, 4
- add eax, 0x1000
loop 1b
-# Set page directory base register.
-
- mov eax, 0x10000
- mov cr3, eax
-
-#### Switch to protected mode.
-
-# Then we point the GDTR to our GDT. Protected mode requires a GDT.
-# We need a data32 prefix to ensure that all 32 bits of the GDT
-# descriptor are loaded (default is to load only 24 bits).
-
- data32 lgdt gdtdesc
-
-# Then we turn on the following bits in CR0:
-# PE (Protect Enable): this turns on protected mode.
-# PG (Paging): turns on paging.
-# WP (Write Protect): if unset, ring 0 code ignores
-# write-protect bits in page tables (!).
-# EM (Emulation): forces floating-point instructions to trap.
-# We don't support floating point.
-
- mov eax, cr0
- or eax, CR0_PE + CR0_PG + CR0_WP + CR0_EM
- mov cr0, eax
-
-# We're now in protected mode in a 16-bit segment. The CPU still has
-# the real-mode code segment cached in cs's segment descriptor. We
-# need to reload cs, and the easiest way is to use a far jump.
-# Because we're not in a 32-bit segment the data32 prefix is needed to
-# jump to a 32-bit offset.
-
- data32 ljmp SEL_KCSEG, 1f + LOADER_PHYS_BASE
-
-# We're now in protected mode in a 32-bit segment.
+ ljmp 0x1000, 0
- .code32
-
-# Reload all the other segment registers and the stack pointer to
-# point into our new GDT.
-
-1: mov ax, SEL_KDSEG
- mov ds, ax
- mov es, ax
- mov fs, ax
- mov gs, ax
- mov ss, ax
- mov esp, LOADER_PHYS_BASE + 0x30000
-
-#### Load kernel starting at physical address LOADER_KERN_BASE by
-#### frobbing the IDE controller directly.
-
- mov ebx, 1
- mov edi, LOADER_KERN_BASE + LOADER_PHYS_BASE
+ # ebx: sector number
+ # dl: drive number
+ # es:0000: destination buffer
+ # returns error flag in CF
read_sector:
+ pusha
+ or dl, dl # Floppy drives: DL < 0
+ js read_floppy_sector
-# Poll status register while controller busy.
-
- mov edx, 0x1f7
-1: in al, dx
- test al, 0x80
- jnz 1b
-
-# Read a single sector.
-
- mov edx, 0x1f2
- mov al, 1
- out dx, al
-
-# Sector number to write in low 28 bits.
-# LBA mode, device 0 in top 4 bits.
-
- mov eax, ebx
- and eax, 0x0fffffff
- or eax, 0xe0000000
-
-# Dump eax to ports 0x1f3...0x1f6.
-
- mov ecx, 4
-1: inc dx
- out dx, al
- shr eax, 8
- loop 1b
-
-# READ command to command register.
-
- inc dx
- mov al, 0x20
- out dx, al
-
-# Poll status register while controller busy.
-
-1: in al, dx
- test al, 0x80
- jnz 1b
-
-# Poll status register until data ready.
-
-1: in al, dx
- test al, 0x08
- jz 1b
-
-# Transfer sector.
-
- mov ecx, 256
- mov edx, 0x1f0
- rep insw
-
-# Next sector.
-
- inc ebx
- cmp ebx, KERNEL_LOAD_PAGES*8 + 1
- jnz read_sector
-
-#### Jump to kernel entry point.
-
- mov eax, LOADER_PHYS_BASE + LOADER_KERN_BASE
- call eax
- jmp panic
-
-#### GDT
-
-gdt:
- .quad 0x0000000000000000 # null seg
- .quad 0x00cf9a000000ffff # code seg
- .quad 0x00cf92000000ffff # data seg
+read_harddrv_sector:
+ sub eax, eax
+ push eax # LBA sector number [32:63]
+ push ebx # LBA sector number [0:31]
+ mov ax, es
+ shl eax, 4
+ push eax # Buffer linear address
+ push 1 # Transfer one sector
+ push 16 # Packet size
+ mov ah, 0x42 # Extended read
+ mov si, sp # DS:SI -> packet
+ int 0x13 # Error code in CF
+success:
+ popa
+ ret # Error code in CF
+
+read_floppy_sector:
+ #define HEADS 2
+ #define SECTORS 36
+
+ # In: BX = LBA sector number, DL = drive.
+ # Out: BL = drive, DX = cylinder, AL = head, AH = sector.
+ sub ax, ax
+ xchg dx, bx # DX = LBA sector number, BL = drive
+ mov cx, HEADS * SECTORS
+ div cx # AX = cyl, DX = hd + (sec-1) * SECTORS
+ xchg ax, dx # DX = cyl, AX = hd + (sec-1) * SECTORS
+ mov cl, SECTORS
+ div cl # AL = head, AH = sector - 1
+ inc ah
+
+ # Read sector.
+ mov ch, dl # CH = cylinder
+ mov cl, ah # CL = sector
+ mov dh, al # DH = head
+ mov dl, bl # DL = drive
+ mov ax, 0x0201 # AH = function, AL = sectors to read
+ sub bx, bx # ES:BX -> buffer
+ pusha
+ int 0x13
+ popa
+ jnc success
+
+ # Reset floppy drive motor, try again.
+ sub ah, ah
+ int 0x13
+ popa
+ jmp read_sector
-gdtdesc:
- .word 0x17 # sizeof (gdt) - 1
- .long gdt + LOADER_PHYS_BASE # address gdt
-
-#### Fatal error.
-#### Print panicmsg (with help from the BIOS) and spin.
-
-panic: .code16 # We only panic in real mode.
- mov si, offset panicmsg
- mov ah, 0xe
- sub bh, bh
-1: lodsb
- test al, al
-2: jz 2b # Spin.
- int 0x10
- jmp 1b
-
-panicmsg:
- .ascii "Panic!"
- .byte 0
-
-#### Memory size in 4 kB pages.
- .org LOADER_RAM_PAGES - LOADER_BASE
-ram_pages:
- .long 0
-
-#### Command-line arguments inserted by another utility.
-#### The loader doesn't use these, but we note their
-#### location here for easy reference.
- .org LOADER_CMD_LINE - LOADER_BASE
-cmd_line:
- .fill 0x80, 1, 0
+#### The partition table goes here.
+ .org 446
+part_tbl:
#### Boot-sector signature for BIOS inspection.
- .org LOADER_BIOS_SIG - LOADER_BASE
+ .org 510
.word 0xaa55
projects / pintos-anon / blobdiff