267 lines
10 KiB
NASM
267 lines
10 KiB
NASM
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; loader.asm
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PartEntry STRUC
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Bootable db ? ;80h = bootable, 00h = nonbootable
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BeginHead db ? ;beginning head
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BeginSector db ? ;beginning sector
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BeginCylinder db ? ;beginning cylinder
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FileSystem db ? ;name of file system
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EndHead db ? ;ending head
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EndSector db ? ;ending sector
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EndCylinder db ? ;ending cylinder
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StartSector dd ? ;starting sector (relative to beg. of disk)
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PartSectors dd ? ;number of sectors in partition
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PartEntry ENDS
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BootSector STRUC
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bsJump db 0EBh, (extra - bsJump), 090h
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; E9 XX XX or EB xx 90
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OemName db 8 dup (?) ; OEM name and version
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; start of BIOS parameter block
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BytesPerSec dw ? ; bytes per sector
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SecPerClust db ? ; sectors per cluster
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ResSectors dw ? ; number of reserved sectors
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FATs db ? ; number of FATs
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RootDirEnts dw ? ; number of root directory entries
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Sectors dw ? ; total number of sectors (see HugeSectors)
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Media db ? ; media descriptor byte (0f0h for floppies)
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FATsecs dw ? ; number of sectors per FAT
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SecPerTrack dw ? ; sectors per track
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Heads dw ? ; number of heads
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HiddenSecs dd ? ; number of hidden sectors
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HugeSectors dd ? ; number of sectors if Sectors equals 0
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; end of BIOS parameter block
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DriveNumber db ? ;
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Reserved1 db ? ;
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BootSignature db ? ;
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VolumeID dd ? ;
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VolumeLabel db 11 dup (?)
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FileSysType db 8 dup (?)
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extra dw ?
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BootSector ENDS
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DirEntry STRUC
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FileName db '????????' ;name
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Extension db '???' ;extension
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Attributes db ? ;attributes
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Reserved db 10 dup (?) ;reserved
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Time dw ? ;time stamp
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Date dw ? ;date stamp
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StartCluster dw ? ;starting cluster
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FileSize dd ? ;file size
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DirEntry ENDS
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CR EQU 0DH
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LF EQU 0AH
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yonder segment para public use16 at 2000h
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org 0h
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destination proc far
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destination endp
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yonder ends
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code segment para public use16 '_CODE'
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.386
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assume cs:code, ds:code, es:code, ss:code
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org 7c00h
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main PROC
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MBR:
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Boot bootsector < ,'BEROSET ',512,1,1,2,224,2880,0f0h,9,18,2,\
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0,0,0,0,29h,02a04063ch,'BEROSET 001',\
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'FAT12 ',07df1h>
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over:
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mov ax,cs ;
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cli
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mov ss,ax ; point ss:sp to CS:7c00h
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mov sp,7c00h ; which sets up a stack in first 64K
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sti
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mov ds,ax
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mov es,ax
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;****************************************************************************
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;
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; CalcClustOff - calculates the starting logical sector number of
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; cluster 0, which isn't really a cluster, but the
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; number returned is useful for calculations converting
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; cluster number to logical sector
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;
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; INPUT: ResSectors, FATsecs, FATs
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; OUTPUT: dx:ax contains the starting logical sector number
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; DESTROYED: none
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;
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;****************************************************************************
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CalcClustOff PROC
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xor dh,dh
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mov ax,[Boot.FatSecs]
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mov dl,[Boot.FATs]
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mul dx
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add ax,[Boot.ResSectors]
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adc dx,0
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; now dx:ax = FATs * FATsecs + ResSectors
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mov word ptr [ClustOffs],ax
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mov word ptr [ClustOffs+2],dx
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mov dx,20h ; bytes per dir entry
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mov ax,[Boot.RootDirEnts]
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mul dx ; multiply 'em out
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div word ptr [Boot.BytesPerSec] ; and divide by bytes/sec
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add word ptr [ClustOffs],ax
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adc word ptr [ClustOffs+2],dx ; create the aggregate
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mov al,[Boot.SecPerClust] ;
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xor ah,ah ;
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shl ax,1 ; AX = SecPerClust * 2
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sub word ptr [ClustOffs],ax ;
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sbb word ptr [ClustOffs+2],0 ; propagate carry flag
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; mov ax,word ptr [ClustOffs] ;
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; mov dx,word ptr [ClustOffs+2] ;
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; ret
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CalcClustOff ENDP
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; mov WORD ptr [ClustOffs],ax
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; mov WORD ptr [ClustOffs+2],dx
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mov bx,offset Boot
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call CalcClust2 C, \
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WORD ptr [(BootSector PTR bx).ResSectors], \
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WORD ptr [(BootSector PTR bx).FATsecs], \
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WORD ptr [(BootSector PTR bx).FATs]
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; now dx:ax contains the logical sector for cluster 2
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call LsectToGeom C, \
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WORD ptr [(BootSector PTR bx).HiddenSecs] , \
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WORD ptr [((BootSector PTR bx).HiddenSecs)+2],\
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[(BootSector PTR bx).Heads], \
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[(BootSector PTR bx).SecPerTrack]
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mov dl,[(BootSector PTR bx).DriveNumber]
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mov bx,offset buff
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retry1:
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mov al,[(BootSector PTR MBR).SecPerClust]
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mov ah,2h ; get ready to read
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int 13h
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jc retry1
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; now find our desired filename within buffer (which has the root dir)
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call FindFile C, \
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bx, 200h * 40h, offset BootFileName
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xor dh,dh
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mov dl,[(BootSector PTR MBR).SecPerClust]
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mov si,ax
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mov ax,[(DirEntry PTR si).StartCluster]
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mul dx
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add ax,WORD ptr [ClustOffs]
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adc dx,WORD ptr [ClustOffs+2]
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; now dx:ax contains logical sector number for start of file
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call LsectToGeom C, \
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WORD ptr [(BootSector PTR MBR).HiddenSecs] , \
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WORD ptr [((BootSector PTR MBR).HiddenSecs)+2],\
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[(BootSector PTR MBR).Heads], \
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[(BootSector PTR MBR).SecPerTrack]
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retry2:
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mov si,offset Boot
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mov dl,[(BootSector PTR si).DriveNumber]
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mov ah,2h
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; read in a cluster's worth of data
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mov al,[(BootSector PTR si).SecPerClust]
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; point to our magic location
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mov bx,seg destination
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mov es,bx
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mov bx,offset destination
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int 13h
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jc retry2
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@@exit:
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jmp destination
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ENDP main
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;****************************************************************************
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;
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; LsectToGeom - converts from logical sector number to the physical
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; geometry (head, cylinder, track) in the form required
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; by the BIOS (Int 13h) disk read and write calls.
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;
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; INPUT: dx:ax=lsect, HiddenSecs, Heads, SecPerTrack
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; OUTPUT: cx, dx are set with cylinder/track, and head respectively
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; DESTROYED: none
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;****************************************************************************
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LsectToGeom PROC C lHiddenSecs:DWORD, \
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lHeads:WORD, lSecPerTrack:WORD, buffer:DWORD
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USES ax ;save registers we'll use
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stc ;add one additional
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adc ax, WORD ptr [lHiddenSecs] ;add starting sector
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adc dx, WORD ptr [lHiddenSecs+2] ;
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div [lSecPerTrack] ;
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mov cl,dl ;store sector in cl
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xor dx,dx ;
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div [lHeads] ;
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mov dh,dl ;store head in dh
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mov ch,al ;store low 8 bits of cylinder in ch
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shr ax,1 ;
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shr ax,1 ;
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and al,0c0h ;pass through two hi bits only
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or cl,ah ;mov bits into location
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ret ;
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LsectToGeom ENDP
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;****************************************************************************
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;
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; CalcClust2 - calculates the starting logical sector number of
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; cluster 2, (the beginning of data space for
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; partitions).
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;
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; INPUT: ResSectors, FATsecs, FATs
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; OUTPUT: dx:ax contains the starting logical sector number
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; DESTROYED: none
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;
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;****************************************************************************
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CalcClust2 PROC C cResSectors:WORD, cFATsecs:WORD, cFATs:BYTE
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xor dx,dx ;
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mov ax,[cFATsecs] ;
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mul [cFATs] ;
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add ax,[cResSectors] ;
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adc dx,0 ;
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ret
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CalcClust2 ENDP
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;****************************************************************************
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;
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; FindFile - given a memory buffer containing the directory data
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; and a static file name for which to search, this routine
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; finds the file and returns a pointer to its directory
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; entry in ds:si
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;
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; INPUT: dirbuffer, filespec
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; OUTPUT: ax contains pointer to directory entry (or NULL)
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; DESTROYED: none
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;****************************************************************************
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FindFile PROC C dirbuffer:WORD, limit:WORD, filespec:WORD
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USES cx, dx, di, si, es
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mov cx,ds ;
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mov es,cx ; es and ds point to same segment
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cld ; always count forward
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mov ax,[dirbuffer] ; load 'em up
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add [limit],ax
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mov dx,[filespec] ;
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keepsearching:
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mov cx,11 ; size of dos filename (8.3)
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mov si,dx ;
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mov di,ax ;
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repe cmpsb ; compare 'em
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jz foundit ;
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add ax,20h ; size of directory entry
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cmp ax,[limit]
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jb keepsearching
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xor ax,ax
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foundit:
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ret
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FindFile ENDP
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BootFileName db "BEROSET SYS" ;the boot loader for this OS
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; MBR db 0200h DUP (?)
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buff db 0200h * 40h DUP (?)
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ClustOffs dd ?
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org 7dfeh
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dw 0AA55h ; signature byte
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code ends
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END
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