Rework fat boot sector, load FAT only once instead of partial
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31eac279c2
commit
adf67d9723
325
boot/fat.asm
325
boot/fat.asm
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@ -19,7 +19,7 @@ params: times bpb_size db 0
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init: xor ax, ax
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mov ss, ax
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mov sp, 0x7C00
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mov sp, ax
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; save potential partition table entry
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push ds
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@ -36,232 +36,153 @@ init: xor ax, ax
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%include "inc/booterr.asm"
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; offsets relative to FS
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readd: ; add offset of cluster data area to DX:AX
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push cx
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mov di, [bp+bpb.direntries]
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mov cl, 4
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shr di, cl ; 32 bytes per entry
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add ax, di
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adc dx, 0
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pop cx
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readr: ; add offset to rootdir to DX:AX (FAT12/FAT16 only)
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push cx
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xor ch, ch
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mov cl, byte [bp+bpb.fatnumber]
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fatlp: add ax, [bp+bpb.fatsectors]
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adc dx, 0
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loop fatlp
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pop cx
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readf: ; add offset to FAT table to DX:AX
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add ax, word [bp+bpb.reservedsects]
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adc dx, 0
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readp: ; read sector DX:AX from partition
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add ax, word [bp+bpb.sectoroffset]
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adc dx, word [bp+bpb.sectoroffset+2]
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jnc ($+5)
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call errcll
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read_: ; read sector DX:AX from disk
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; uses basic CHS disk IO
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; this cant reliable read over track boundaries
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; so we read each single sector in loop over CX
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chslp: push ax
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; Read sectors from disk
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; IN ES:DI target buffer
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; DX:AX absulute 32-bit sector number
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; CX numeber of sectors
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read: push ax
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push cx
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push dx
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push bx
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; calculate CHS data
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div word [bp+bpb.tracksectors] ; ax:temp = (lba / spt)
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inc dx ; dx:sector = (lba % spt) + 1
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mov cl, dl ; sector number
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xor dx, dx
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div word [bp+bpb.heads] ; ax:cylinder = (tmp / heads)
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; dx:head = (tmp % heads)
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mov ch, al ; cylinder number
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mov dh, dl ; head number
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mov dl, [bp+bpb.drivenum] ; drive number
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xchg ax, bx
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mov al, byte [bp+bpb.heads]
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mul byte [bp+bpb.tracksectors]
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xchg ax, bx
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; set up read buffer
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mov es, bx
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xor bx, bx
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; call bios
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; dx:ax = lba
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div word bx
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xchg ax, dx
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; dx = cylinder, ax = head * tracksectors + sector
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div byte [bp+bpb.tracksectors]
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; dx = cylinder, al = head, ah = sector
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xchg dl, dh
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ror dl, 1
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ror dl, 1
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or dl, ah
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inc dx
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; al: head number
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; dh bit 0-7: cylinder bits 0-7
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; dl bit 0-5: sector bits 0-5
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; dl bit 6-7: cylinder bits 8-9
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mov cx, dx
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mov dh, al
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mov dl, [bp+bpb.drivenum]
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mov ax, 0x0201
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int 0x13
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jnc ($+5)
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call errcll
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mov bx, di
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int 0x13
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jc errjmp
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pop bx
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pop dx
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pop cx
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pop ax
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; increment sector num + write offset
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add ax, 1
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adc dx, 0
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add bx, 0x20
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; loop on CX
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loop chslp
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add di, 0x200
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loop read
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ret
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next: ; Advances DX:AX to next FAT entry
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push ax
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push bx
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; shift 2 left for dword-sized FAT entries
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; shift 9 right for sector size
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mov cl, 7
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shftl: clc
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rcr dx, 1
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rcr ax, 1
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loop shftl
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mov bx, 0xA0
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%ifdef FAT12
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; FAT item might span over two sectors
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mov cx, 2
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%else
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mov cx, 1
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%endif
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call readf
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pop bx
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; get address of cluster within sector
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pop si
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%ifdef FAT12
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push si
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; times 1.5
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mov ax, si
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shr ax, 1
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add si, ax
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add si, buf
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lodsw
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pop si
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test si, 1
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jz .odd
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.even: mov cl, 4
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shr ax, cl
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jmp .done
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.odd: and ax, 0x0FFF
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.done: xor dx, dx
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%endif
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%ifdef FAT16
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; double
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sal si, 1
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; make sure its within sector range
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and si, 0x1FF
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add si, buf
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; load word
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lodsw
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xor dx, dx
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%endif
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ret
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; reads current cluster into [dest]
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readc: ; load cluster number
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push ax
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push dx
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; subtract the two dummy entries from FAT start
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sub ax, 2
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sbb dx, 0
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; convert cluster number to sector number
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; this is some cheapo multiplication with 2^n
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mov cl, [bp+bpb.clustersects]
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l02: shr cl, 1
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jz l02e
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clc
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rcl ax, 1
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rcl dx, 1
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jmp l02
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l02e:
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; dx:ax is now sector num in data area
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mov cx, [bp+bpb.clustersects]
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xor ch, ch
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call readd
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xchg di, cx
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mov cl, 5
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sal di, cl
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add bx, di
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pop dx
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pop ax
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ret
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; Load root directory
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loadr: mov ax, [bp+bpb.direntries]
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rootdirsects: mov bx, [bp+bpb.direntries]
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mov cl, 4
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shr ax, cl
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mov cx, ax
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xor ax, ax
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xor dx, dx
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mov bx, 0x07C0
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jmp readr
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; Load file identified by fsattr
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; Directory data is expected at 0x07C00
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loadf: mov si, (0x7C00-0x20)
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sloop: add si, 0x20
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; check if entry is valid
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mov al, [si]
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; unallocated direntry
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test al, al
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jnz ($+5)
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call errcll
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; deleted files
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cmp al, 0xE2
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je sloop
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; check attr
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mov al, [si+11]
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and al, 0x5C
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cmp al, ah
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jne sloop
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mov ax, [si+0x1A]
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mov dx, [si+0x14]
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; Load cluster chain DX:AX to 0x07C00
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load: mov bx, 0x07C0
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lloop: call readc
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call next
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%ifdef FAT12
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cmp word ax, 0x0FF8
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%endif
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%ifdef FAT16
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cmp word ax, 0xFFF8
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%endif
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jc lloop
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shr bx, cl ; 32 bytes per entry
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ret
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main: ; Set up BP to point to BPB
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; Relative addressing saves one instruction byte per access
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mov bp, params
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; Save DL contents
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offset_cluster: ; add offset of cluster data area to DX:AX
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call rootdirsects
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add ax, bx
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adc dx, 0
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offset_rootdir: ; add length of fat table times number of fats
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xor ch, ch
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mov cl, byte [bp+bpb.fatnumber]
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.loop: add ax, [bp+bpb.fatsectors]
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adc dx, 0
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loop .loop
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offset_fat: ; add reserved sectors
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add ax, [bp+bpb.reservedsects]
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adc dx, 0
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offset_part: ; add partition offset
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add ax, [bp+bpb.sectoroffset]
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add dx, [bp+bpb.sectoroffset+2]
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ret
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resetbuf: xor ax, ax
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mov dx, ax
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mov di, buf
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ret
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main: mov bp, params
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mov [bp+bpb.drivenum], dl
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; load root directory
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call loadr
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call resetbuf
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call offset_rootdir
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call rootdirsects
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mov cx, bx
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call read
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; search for first system file
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mov ah, 0x04
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call loadf
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; search for file
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mov bx, buf
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mov cx, [bp+bpb.direntries]
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.loop: push cx
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mov si, filename
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mov di, bx
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add bx, 0x20
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mov cx, 0x0B
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repe cmpsb
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pop cx
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loopne .loop
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jne err_not_found
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push word [bx-0x20+0x1A]
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; restore drive number
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mov dl, [bp+bpb.drivenum]
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; load fat table
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call resetbuf
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call offset_fat
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mov cx, [bp+bpb.fatsectors]
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call read
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; restore potential partition table
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pop si
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; fetch cluster numbers (FAT12-specific)
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mov di, 0x0500
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pop ax
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readfat: stosw
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mov bx, ax
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shr bx, 1
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pushf
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add bx, ax
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mov ax, [buf+bx]
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popf ; CF: 0=even, 1=odd entry
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jc .odd
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and ax, 0x0FFF
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jmp .both
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.odd: mov cl, 4
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shr ax, cl
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.both: cmp ax, 0xFF8
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jc readfat
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xor ax, ax
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stosw
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; load clusters
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mov si, 0x500
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mov di, 0x7C00
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loadclus: lodsw
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sub ax, 2
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jc .jump
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mul byte [bp+bpb.clustersects]
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call offset_cluster
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xor cx, cx
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mov cl, [bp+bpb.clustersects]
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call read
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jmp loadclus
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.jump: pop si
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pop ds
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jmp 0x7C00
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; Padding and signature
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times (0x1FE - ($-$$)) db 0
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err_not_found:
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hlt: hlt
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jmp hlt
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times (0x1F3 - ($-$$)) db 0
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filename: db "KERNEL ", "BS "
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dw 0xAA55
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buf:
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