Use uppercase non-struct constants for BPB
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30
boot/fat.asm
30
boot/fat.asm
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@ -19,7 +19,7 @@ bootsect: equ 0x7C00 ; our origin before reloc
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times ( 0x0B - ($-$$) ) db 0
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params: times bpb_size db 0
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bpb: times BPBSIZ4 db 0
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; Area for BPB
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times ( 0x3E - ($-$$) ) db 0
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@ -54,15 +54,15 @@ read: push ax
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push dx
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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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mov al, byte [bp+BPBNOS]
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mul byte [bp+BPBSPT]
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xchg ax, bx
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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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div byte [bp+BPBSPT]
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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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@ -92,7 +92,7 @@ read: push ax
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loop read
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ret
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rootdirsects: mov bx, [bp+bpb.direntries]
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rootdirsects: mov bx, [bp+BPBRDE]
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mov cl, 4
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shr bx, cl ; 32 bytes per entry
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ret
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@ -103,16 +103,16 @@ offset_cluster: ; add offset of cluster data area to DX:AX
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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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mov cl, byte [bp+BPBFN]
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.loop: add ax, [bp+BPBFS]
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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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add ax, [bp+BPBRSC]
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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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add ax, [bp+BPBHS]
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add dx, [bp+BPBHS]
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ret
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resetbuf: ; DS = ES
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@ -127,7 +127,7 @@ resetbuf: ; DS = ES
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xor dx, dx
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ret
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main: mov bp, params
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main: mov bp, bpb
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mov [bp-1], dl
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; load root directory
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@ -140,7 +140,7 @@ main: mov bp, params
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; search for file
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pop bx
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mov cx, [bp+bpb.direntries]
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mov cx, [bp+BPBRDE]
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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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@ -156,7 +156,7 @@ main: mov bp, params
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call resetbuf
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push di
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call offset_fat
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mov cx, [bp+bpb.fatsectors]
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mov cx, [bp+BPBFS]
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call read
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; fetch cluster numbers (FAT12-specific)
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@ -189,10 +189,10 @@ readfat: stosw
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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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mul byte [bp+BPBSC]
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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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mov cl, [bp+BPBSC]
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call read
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jmp loadclus
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12
boot/mbr.asm
12
boot/mbr.asm
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@ -10,7 +10,7 @@
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%include "inc/bpb.asm"
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%define params bs+0x0B
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%define bpb bs+0x0B
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cpu 8086
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org 0x0600
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@ -72,11 +72,11 @@ main: ; find active partition
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; FAT bpb has "bytes per sector" at this place
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; we support only 512 byte sectors anyways
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cmp word [params+bpb.sectorsize], 512
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cmp word [bpb+BPBSS], 512
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jne chain
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; Media descriptor byte is never smaller than 0xF0
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cmp byte [params+bpb.mediadesc], 0xF0
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cmp byte [bpb+BPBMD], 0xF0
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jc chain
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; ask BIOS what our disk geometry is
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@ -92,17 +92,17 @@ main: ; find active partition
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; update sectors per track
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xor ax, ax
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mov al, cl
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mov [params+bpb.tracksectors], ax
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mov [bpb+BPBSPT], ax
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; update number of sides
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xor ax, ax
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mov al, dh
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mov [params+bpb.heads], ax
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mov [bpb+BPBNOS], ax
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; update partition offset
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push si
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add si, 0x08
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mov di, params+bpb.sectoroffset
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mov di, bpb+BPBHS
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movsw
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movsw
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pop si
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40
inc/bpb.asm
40
inc/bpb.asm
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@ -1,14 +1,26 @@
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struc bpb
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.sectorsize: resw 1 ; WORD sector size
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.clustersects: resb 1 ; BYTE sector / cluster
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.reservedsects: resw 1 ; WORD reserved sector count
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.fatnumber: resb 1 ; BYTE fat number
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.direntries: resw 1 ; WORD root dir entries
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.total: resw 1 ; WORD total sectors
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.mediadesc: resb 1 ; BYTE media descriptor
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.fatsectors: resw 1 ; WORD sectors / fat
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.tracksectors: resw 1 ; WORD sectors per track
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.heads: resw 1 ; WORD number of sides
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.sectoroffset: resd 1 ; DWORD partition offset
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.longtotal: resd 1 ; DWORD large total sectors
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endstruc
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BPBOFF equ 0xB
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; DOS 2.0 BPB
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BPBSS equ 0 ; word
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BPBSC equ 2 ; byte
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BPBRSC equ 3 ; word
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BPBFN equ 5 ; byte
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BPBRDE equ 6 ; word
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BPBTS equ 8 ; word
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BPBMD equ 10 ; byte
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BPBFS equ 11 ; word
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BPBSIZ2 equ 13 ; size constant
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; DOS 3.31 BPB
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BPBSPT equ 0x0D ; word
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BPBNOS equ 0x0F ; word
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BPBHS equ 0x11 ; dword
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BPBLTS equ 0x15 ; dword
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BPBSIZ3 equ 25 ; size constant
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; DOS 3.4 EBPB
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BPBDN equ 0x19 ; byte
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BPBFALG equ 0x1A ; byte
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BPBSIG equ 0x1B ; byte
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BPBSER equ 0x1C ; dword serial number
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BPBSIZ4 equ 32 ; size constant
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@ -1,60 +0,0 @@
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section .text
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load_bpb: mov bx, [drive_ptr]
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; read word for "sectors per fat"
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mov ax, [diskbuf+0x016]
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xor dx, dx
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; if zero, we need to check dword in BPB 7.1
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test ax, ax
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jnz .short_sf
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; fail if BPB 7.1 signature is not present
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mov cl, [diskbuf+0x042]
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or cl, 1
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cmp cl, 0x29
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jne .err
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; load dword
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mov ax, [diskbuf+0x024]
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mov dx, [diskbuf+0x026]
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.short_sf:
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; store local value
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mov [bx+drive.fat_size], ax
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mov [bx+drive.fat_size+2], dx
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; copy number of fat's
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xor ah, ah
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mov al, [diskbuf+0x010]
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mov [bx+drive.fat_num], ax
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; copy number of root directory entries
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mov ax, [diskbuf+0x011]
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mov cl, 4
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shr ax, cl
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mov [bx+drive.dir_size], ax
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; calculate offsets for everything
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xor ax, ax
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xor dx, dx
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; add reserved sectors
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add ax, [diskbuf+0x0B+bpb_rsc]
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adc ax, 0
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; save begin of FAT tables
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mov [bx+drive.fat_offset], ax
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mov [bx+drive.fat_offset+2], dx
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; add FAT size * FAT number
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mov cx, [bx+drive.fat_num]
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.floop: add ax, [bx+drive.fat_size]
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adc dx, [bx+drive.fat_size+2]
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loop .floop
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; save begin of root directories
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mov [bx+drive.dir_offset], ax
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mov [bx+drive.dir_offset+2], dx
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; add root directory size (might be zero)
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add ax, [bx+drive.dir_size]
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adc dx, 0
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; save start of data area
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mov [bx+drive.clus_offset], ax
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mov [bx+drive.clus_offset+2], dx
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ret
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.err: stc
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ret
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