mirror of https://github.com/FDOS/kernel.git
393 lines
12 KiB
NASM
393 lines
12 KiB
NASM
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; +--------+
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; | |
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; | |
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; |--------| 4000:0000
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; | |
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; | FAT |
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; | |
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; |--------| 2000:0000
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; |BOOT SEC|
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; |RELOCATE|
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; |--------| 1FE0:0000
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; | |
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; | |
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; | |
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; | |
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; |--------|
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; |BOOT SEC|
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; |ORIGIN | 07C0:0000
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; |--------|
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; | |
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; | |
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; | |
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; |--------|
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; |KERNEL |
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; |LOADED |
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; |--------| 0060:0000
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; | |
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; +--------+
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;%define MULTI_SEC_READ 1
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segment .text
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%define BASE 0x7c00
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org BASE
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Entry: jmp short real_start
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nop
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; bp is initialized to 7c00h
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%define bsOemName bp+0x03 ; OEM label
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%define bsBytesPerSec bp+0x0b ; bytes/sector
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%define bsSecPerClust bp+0x0d ; sectors/allocation unit
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%define bsResSectors bp+0x0e ; # reserved sectors
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%define bsFATs bp+0x10 ; # of fats
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%define bsRootDirEnts bp+0x11 ; # of root dir entries
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%define bsSectors bp+0x13 ; # sectors total in image
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%define bsMedia bp+0x15 ; media descrip: fd=2side9sec, etc...
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%define sectPerFat bp+0x16 ; # sectors in a fat
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%define sectPerTrack bp+0x18 ; # sectors/track
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%define nHeads bp+0x1a ; # heads
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%define nHidden bp+0x1c ; # hidden sectors
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%define nSectorHuge bp+0x20 ; # sectors if > 65536
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%define xsectPerFat bp+0x24 ; Sectors/Fat
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%define xrootClst bp+0x2c ; Starting cluster of root directory
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%define drive bp+0x40 ; Drive number
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times 0x5a-$+$$ db 0
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%define LOADSEG 0x0060
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%define FATSEG 0x2000
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%define fat_sector bp+0x48 ; last accessed sector of the FAT
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%define loadsegoff_60 bp+loadseg_off-Entry ; FAR pointer = 60:0
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%define loadseg_60 bp+loadseg_seg-Entry
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%define fat_start bp+0x5e ; first FAT sector
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%define data_start bp+0x62 ; first data sector
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%define fat_secmask bp+0x66 ; number of clusters in a FAT sector - 1
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%define fat_secshift bp+0x68 ; fat_secmask+1 = 2^fat_secshift
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;-----------------------------------------------------------------------
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; ENTRY
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;-----------------------------------------------------------------------
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real_start: cld
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cli
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sub ax, ax
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mov ds, ax
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mov bp, 0x7c00
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mov ax, 0x1FE0
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mov es, ax
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mov si, bp
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mov di, bp
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mov cx, 0x0100
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rep movsw ; move boot code to the 0x1FE0:0x0000
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jmp word 0x1FE0:cont
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loadseg_off dw 0
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loadseg_seg dw LOADSEG
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cont: mov ds, ax
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mov ss, ax
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lea sp, [bp-0x20]
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sti
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mov [drive], dl ; BIOS passes drive number in DL
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; call print
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; db "Loading ",0
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; Calc Params
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; Fat_Start
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mov si, word [nHidden]
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mov di, word [nHidden+2]
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add si, word [bsResSectors]
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adc di, byte 0
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mov word [fat_start], si
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mov word [fat_start+2], di
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; Data_Start
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mov al, [bsFATs]
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cbw
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push ax
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mul word [xsectPerFat+2]
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add di, ax
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pop ax
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mul word [xsectPerFat]
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add ax, si
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adc dx, di
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mov word[data_start], ax
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mov word[data_start+2], dx
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; fat_secmask
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mov ax, word[bsBytesPerSec]
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shr ax, 1
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shr ax, 1
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dec ax
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mov word [fat_secmask], ax
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; fat_secshift
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; cx = temp
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; ax = fat_secshift
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xchg ax, cx ; cx = 0 after movsw
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inc cx
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secshift: inc ax
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shr cx, 1
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cmp cx, 1
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jne secshift
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mov byte [fat_secshift], al
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dec cx
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; FINDFILE: Searches for the file in the root directory.
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;
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; Returns:
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; DX:AX = first cluster of file
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mov word [fat_sector], cx ; CX is 0 after "dec"
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mov word [fat_sector + 2], cx
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mov ax, word [xrootClst]
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mov dx, word [xrootClst + 2]
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ff_next_cluster:
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push dx ; save cluster
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push ax
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call convert_cluster
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jc boot_error ; EOC encountered
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ff_next_sector:
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push bx ; save sector count
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les bx, [loadsegoff_60]
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call readDisk
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push dx ; save sector
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push ax
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mov ax, [bsBytesPerSec]
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; Search for KERNEL.SYS file name, and find start cluster.
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ff_next_entry: mov cx, 11
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mov si, filename
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mov di, ax
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sub di, 0x20
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repe cmpsb
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jz ff_done
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sub ax, 0x20
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jnz ff_next_entry
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pop ax ; restore sector
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pop dx
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pop bx ; restore sector count
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dec bx
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jnz ff_next_sector
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ff_find_next_cluster:
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pop ax ; restore current cluster
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pop dx
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call next_cluster
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jmp short ff_next_cluster
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ff_done:
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mov ax, [es:di+0x1A-11] ; get cluster number
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mov dx, [es:di+0x14-11]
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c4:
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sub bx, bx ; ES points to LOADSEG
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c5: push dx
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push ax
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push bx
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call convert_cluster
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jc boot_success
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mov di, bx
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pop bx
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c6:
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call readDisk
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dec di
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jnz c6
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pop ax
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pop dx
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call next_cluster
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jmp short c5
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boot_error:
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xor ah,ah
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int 0x16 ; wait for a key
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int 0x19 ; reboot the machine
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; input:
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; DX:AX - cluster
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; output:
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; DX:AX - next cluster
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; CX = 0
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; modify:
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; DI
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next_cluster:
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push es
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mov di, ax
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and di, [fat_secmask]
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mov cx, [fat_secshift]
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cn_loop:
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shr dx,1
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rcr ax,1
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dec cx
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jnz cn_loop ; DX:AX fat sector where our
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; cluster resides
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; DI - cluster index in this
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; sector
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shl di,1 ; DI - offset in the sector
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shl di,1
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add ax, [fat_start]
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adc dx, [fat_start+2] ; DX:AX absolute fat sector
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push bx
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mov bx, FATSEG
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mov es, bx
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sub bx, bx
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cmp ax, [fat_sector]
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jne cn1 ; if the last fat sector we
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; read was this, than skip
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cmp dx,[fat_sector+2]
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je cn_exit
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cn1:
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mov [fat_sector],ax ; save the fat sector number,
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mov [fat_sector+2],dx ; we are going to read
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call readDisk
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cn_exit:
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pop bx
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mov ax, [es:di] ; DX:AX - next cluster
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mov dx, [es:di + 2] ;
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pop es
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ret
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boot_success:
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mov bl, [drive]
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jmp far [loadsegoff_60]
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; Convert cluster to the absolute sector
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;input:
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; DX:AX - target cluster
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;output:
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; DX:AX - absoulute sector
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; BX - [bsSectPerClust]
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;modify:
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; CX
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convert_cluster:
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cmp dx,0x0fff
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jne c3
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cmp ax,0xfff8
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jb c3 ; if cluster is EOC (carry is set), do ret
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stc
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ret
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c3:
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mov cx, dx ; sector = (cluster - 2)*clussize +
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; + data_start
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sub ax, 2
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sbb cx, byte 0 ; CX:AX == cluster - 2
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mov bl, [bsSecPerClust]
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sub bh, bh
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xchg cx, ax ; AX:CX == cluster - 2
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mul bx ; first handle high word
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; DX must be 0 here
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xchg ax, cx ; then low word
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mul bx
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add dx, cx ; DX:AX target sector
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add ax, [data_start]
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adc dx, [data_start + 2]
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ret
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; prints text after call to this function.
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print_1char:
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xor bx, bx ; video page 0
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mov ah, 0x0E ; else print it
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int 0x10 ; via TTY mode
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print: pop si ; this is the first character
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print1: lodsb ; get token
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push si ; stack up potential return address
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cmp al, 0 ; end of string?
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jne print_1char ; until done
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ret ; and jump to it
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;input:
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; DX:AX - 32-bit DOS sector number
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; ES:BX - destination buffer
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;output:
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; ES:BX points one byte after the last byte read.
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; DX:AX - next sector
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;modify:
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; ES if DI * bsBytesPerSec >= 65536, CX
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readDisk:
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read_next: push dx
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push ax
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;
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; translate sector number to BIOS parameters
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;
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;
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; abs = sector offset in track
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; + head * sectPerTrack offset in cylinder
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; + track * sectPerTrack * nHeads offset in platter
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;
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xchg ax, cx
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mov al, [sectPerTrack]
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mul byte [nHeads]
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xchg ax, cx
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; cx = nHeads * sectPerTrack <= 255*63
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; dx:ax = abs
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div cx
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; ax = track, dx = sector + head * sectPertrack
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xchg ax, dx
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; dx = track, ax = sector + head * sectPertrack
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div byte [sectPerTrack]
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; dx = track, al = head, ah = sector
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mov cx, dx
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; cx = track, al = head, ah = sector
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; the following manipulations are necessary in order to
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; properly place parameters into registers.
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; ch = cylinder number low 8 bits
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; cl = 7-6: cylinder high two bits
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; 5-0: sector
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mov dh, al ; save head into dh for bios
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xchg ch, cl ; set cyl no low 8 bits
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ror cl, 1 ; move track high bits into
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ror cl, 1 ; bits 7-6 (assumes top = 0)
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inc ah ; sector offset from 1
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or cl, ah ; merge sector into cylinder
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mov ax, 0x0201
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mov dl, [drive]
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int 0x13
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pop ax
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pop dx
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jnc read_ok ; jump if no error
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xor ah, ah ; else, reset floppy
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int 0x13
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jmp short read_next
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read_ok:
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add bx, word [bsBytesPerSec]
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jnc no_incr_es ; if overflow...
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mov cx, es
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add ch, 0x10 ; ...add 1000h to ES
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mov es, cx
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no_incr_es:
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add ax,byte 1
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adc dx,byte 0
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ret
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times 0x01f1-$+$$ db 0
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filename db "KERNEL SYS",0,0
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sign dw 0xAA55
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