vbr: work in progress on block reading
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184
boot/vbr.asm
184
boot/vbr.asm
@ -1,57 +1,104 @@
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org 0x7C00
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bpb:
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jmp _startup ; 2 bytes
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nop ; 1 byte
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fdc: ; FDC Descriptor as per ECMA-107
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jmp _startup
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times (0x03 - ($-$$)) nop
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.oem:
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db "OEM " ; OEM
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.bps:
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dw 0x200 ; bytes per sector
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.spc:
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db 1 ; sectors per cluster
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.rsectors:
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dw 2 ; number of reserved sectors
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.fats:
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db 2 ; number of file allocation tables
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.dirents:
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dw 2 ; Number of directory entries
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.sectors:
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dw 720 ; total number of sectors
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.mtd: ; media descriptor type
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db "2B" ; creating system identifier
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times (0x0B - ($-$$)) db " " ; space-padded
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.ss:
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dw 0x200 ; sector size
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.sc:
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db 0 ; sectors per cluster
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.rsc:
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dw 2 ; reserved sector count
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.fn:
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db 0 ; number of file allocation tables
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.rde:
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dw 0 ; number of root directory entries
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.ts_old:
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dw 0 ; legacy: total number of sectors
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.mi: ; medium identifier
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db 0xFD ; 5.25-inch Double sided, 40 tracks per side, 9 sectors per track (360 KB)
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.spfat: ; sectors per fat
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.sf: ; sectors per fat
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dw 0
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.spt:
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dw 9 ; sectors per track
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.heads:
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dw 2 ; number of heads
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.hidsecs:
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dd 0 ; number of hidden sectors
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.lrgsectors:
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dd 0 ; large sector count
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.drivenum:
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.nos:
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dw 2 ; number of sides (heads)
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.po:
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dd 0 ; partition offset (in LBA blocks)
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.ts:
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dw 720
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.drv:
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db 0 ; drive number
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.winntflags:
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db 0
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.signature:
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db 0x28
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times (0x5A - ($-$$)) nop
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db 0x29 ; efdc signature
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.vid:
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dd 0 ; volume id
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.vlabel:
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db "2B"
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times (54 - ($-$$)) db " "
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.fstype:
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db "FAT12"
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times (62 - ($-$$)) db " "
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_startup:
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call adjust_bpb
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mov [fdc.drv], dl ; backup driver number
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mov ah, 0x08
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int 0x13 ; required, QEMU detects 360kB as 1200kB with CHS 80 2 15
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jc .noadjust ; skip if function does not exist
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inc dh
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mov [fdc.nos], dh
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mov ax, cx
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and ax, 0x003F
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mov [fdc.spt], ax ; no adjustment because sectors are 1-indexed
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mov ax, cx
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xchg al, ah
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mov cl,6
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shr ah,cl
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inc ax ; convert from maximum number (0-based) to total number (1-based) of cylinders
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mul word [fdc.nos] ; number of tracks = number of cylinders * heads
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mul word [fdc.spt] ; number of sectors = number of tracks * sectors per track
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mov [fdc.ts], ax
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.noadjust:
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mov ax, 0x0500
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mov es, ax
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mov si, 1
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mov di, 0x7D00
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mov cx, 0x10
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mov bx, 0x0000
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mov ax, 1
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call loadblk
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call dump
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mov al, [fdc.sc]
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call print8
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.end:
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hlt
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jmp .end
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; Load a cluster from cluster offset
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loadcluster:
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ret
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; Load a single cluster into memory
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; in ax sector number
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; es:bx buffer
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; out al error code,
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; carry set if error
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loadblk:
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xor dx, dx
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div word [fdc.spt] ; 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 [fdc.nos] ; 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, [fdc.drv] ; driver number
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mov ax, 0x0201 ; ah=0x02 al=0x01
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int 0x13
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ret
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dump:
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mov si, 0x0000
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mov cx, 0x10
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@ -80,73 +127,6 @@ dump:
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ret
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; Adjust the bdp data from BIOS-supplied data (if)
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; No args, no output
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adjust_bpb:
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push ax
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push cx
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push dx
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mov ah, 0x08
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int 0x13 ; required, QEMU detects 360kB as 1200kB with CHS 80 2 15
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jc .end ; skip if function does not exist
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inc dh
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mov [bpb.heads], dh
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mov ax, cx
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and ax, 0x003F
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mov [bpb.spt], ax ; no adjustment because sectors are 1-indexed
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mov ax, cx
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xchg al, ah
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mov cl,6
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shr ah,cl
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inc ax ; convert from maximum number (0-based) to total number (1-based) of cylinders
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mul word [bpb.heads] ; number of tracks = number of cylinders * heads
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mul word [bpb.spt] ; number of sectors = number of tracks * sectors per track
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mov [bpb.sectors], ax
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.end:
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pop dx
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pop cx
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pop ax
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ret
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; in si sector number,
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; dl drive number
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; es:0000 buffer
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; out al error code,
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; carry set if error
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loadblk:
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push bx
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push cx
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push dx
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xor dx, dx
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mov ax, si
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div word [bpb.spt] ; ax:temp = (lba / spt)
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inc dx ; dx:sector = (lba % spt) + 1
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mov cl, dl ; for int 0x13
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xor dx, dx
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div word [bpb.heads] ; ax:cylinder = (tmp / heads)
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; dx:head = (tmp % heads)
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mov ch, al ; for int 0x13
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mov ax, dx
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pop dx ; drive number comes back into dl
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mov dh, al ; for int 0x13
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mov bx, 0x0000
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mov ax, 0x0201 ; ah=0x02 al=0x01
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int 0x13
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call print16
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lahf
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xchg ah, al
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call print8
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mov al, 0x0A
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call printc
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mov al, 0x0D
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call printc
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pop cx
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pop bx
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ret
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%include "print.asm"
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times (0x1FE - ($-$$)) nop
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