;=-------------------------------------------------------------------------= ; ; MZ Loader 2.0 FAT12 Boot Sector ; ; Copyright (c) 1999-2000 by Thomas Kjoernes, all rights reserved. ; ; ; Boot Sector Overview: ; --------------------- ; ; - Support for multiple boot images. You can add more images ; between the szImageListDef/End labes. The first image found ; in the root directory will be loaded. ; ; - Support for "MZ" executables as well as plain binaries. ; ; ; Known limitations: ; ------------------ ; ; - Does not support FAT12 partitions beyond 8GB (INT13 extensions). ; - Does not provide any error messages; calls INT18 on any errors. ; ; ; Memory Map: ; ----------- ; ; 旼컴컴컴컴컴컴컴컴컴컴컴커 ; � Interrupt Vector Table � 0000 ; 쳐컴컴컴컴컴컴컴컴컴컴컴캑 ; � BIOS Data Area � 0040 ; 쳐컴컴컴컴컴컴컴컴컴컴컴캑 ; � PrtScr Status / Unused � 0050 ; 쳐컴컴컴컴컴컴컴컴컴컴컴캑 ; � Image Load Address � 0060 ; 쳐컴컴컴컴컴컴컴컴컴컴컴캑 ; � Available Memory � nnnn ; 쳐컴컴컴컴컴컴컴컴컴컴컴캑 ; � 2KB Boot Stack � A000 - SizeOf(BootSector) - 2KB ; 쳐컴컴컴컴컴컴컴컴컴컴컴캑 ; � Boot Sector � A000 - SizeOf(BootSector) ; 쳐컴컴컴컴컴컴컴컴컴컴컴캑 ; A000 ; ; ; Boot Image Startup (register values): ; ------------------------------------- ; ; ds = load segment, this is where the executable is loaded. ; es = free segment, very first available paragraph above image. ; ss = this is the stack segment somewhere near top of memory. ; ; dl = boot drive number ; ; dh = ??? (undoc) ; cx = ??? (undoc) ; si = ??? (undoc) ; di = ??? (undoc) ; ; ax:bx = CS:IP used to RETF to the image. ; ; ss:sp = pointer to 2KB stack. ; ss:bp = pointer to boot sector, with STD_VARS located below it. ; ; The STD_VARS structure includes the relative LBA address of the ; disk data area and the image load and FAT segment addresses. ; ;=-------------------------------------------------------------------------= .386p INCLUDE fat.inc codeSegment SEGMENT USE16 'boot' ASSUME cs:codeSegment, ds:codeSegment, ss:codeSegment FAT12_BOOT <"MZLDR2.0"> ;=----------------------------------------------------------= ; Detect memory, setup stack, buffers and relocate ourself ;=----------------------------------------------------------= int 12h ; get conventional memory size shl ax, 6 ; and convert it to paragraphs sub ax, 512 / 16 ; es = ax - 512 mov es, ax ; ; setup stack and base pointers sub ax, 2048 / 16 ; ss = ax - 2KB mov ss, ax ; mov sp, 2048 ; 2KB stack mov bp, sp ; mov ax, 0060h ; ax = "load" segment ; copy ourself to top of memory mov cx, 256 mov si, 7C00h xor di, di rep movs WORD PTR es:[di], cs:[si] ; jump to relocated code push es push OFFSET mainEntryPoint retf mainEntryPoint: mov BS.bsDriveNumber, dl ;=----------------------------------------------------------= ; Load entire FAT (maximum 6KB for FAT12) ;=----------------------------------------------------------= push ax ; ax = "load" segment mov ax, BPB.bpbBytesPerSector mov cx, BPB.bpbSectorsPerFAT mul cx shr ax, 4 ; convert to paragraphs mov di, ss ; sub di, ax ; es = di = FAT buffer mov es, di ; mov ax, BPB.bpbReservedSectors call int13ReadSector pop es ; es = "load" segment ;=----------------------------------------------------------= ; Load entire Root Directory (maximum 16KB for FAT12/FAT16) ;=----------------------------------------------------------= mov ax, 32 mov si, BPB.bpbRootEntries mul si div BPB.bpbBytesPerSector xchg ax, cx ; cx = root sectors, ax = zero mov al, BPB.bpbNumberOfFATs mul BPB.bpbSectorsPerFAT add ax, BPB.bpbReservedSectors call int13ReadSector push dx ; stdDataAreaHi push ax ; stdDataAreaLo push di ; stdFATSegment push es ; stdBufSegment ;=----------------------------------------------------------= ; Look through list of "images" and load the first match ;=----------------------------------------------------------= xchg ax, si ; ax = root entries mov si, OFFSET szImageListDef bootFindNext: call fat12FindFile ; found? jnc bootLoadFile ; add si, cx cmp si, OFFSET szImageListEnd jb bootFindNext int 18h ; ToDo: add buffer overrun checking bootLoadFile: call fat12ReadCluster jnc bootLoadFile ;=----------------------------------------------------------= ; Do some special checks on the image and transfer control ;=----------------------------------------------------------= mov ax, VAR.stdBufSegment mov ds, ax xor bx, bx xor si, si ; check for "MZ" executable signature cmp WORD PTR ds:[si], "ZM" jne bootExecFile add ax, ds:[si][08h] ; ax = image base mov cx, ds:[si][06h] ; cx = reloc items mov bx, ds:[si][18h] ; bx = reloc table pointer jcxz bootHackDone bootHackFile: mov di, ds:[bx][0] ; di = item ofs mov dx, ds:[bx][2] ; dx = item seg (rel) add dx, ax ; dx = item seg (abs) push ds ; mov ds, dx ; ds = dx add ds:[di], ax ; fixup pop ds ; add bx, 4 ; point to next entry dec cx ; more? jnz bootHackFile ; bootHackDone: ; adjust CS:IP according to header mov bx, ds:[si][14h] ; ip add ax, ds:[si][16h] ; cs bootExecFile: mov dl, BS.bsDriveNumber push ax push bx retf ;--------------------------------------------------------------------------- ; fat12ReadCluster ; ; Entry: ; ch = 0 ; si = cluster number to read ; es = segment of read buffer ; Exit: ; si = next cluster number in chain (if CF == 0) ; es = next segment ; fat12ReadCluster PROC NEAR lea ax, [si-2] ; cx = sectors per cluster mov cl, BPB.bpbSectorsPerCluster mul cx ; add start of dataarea add ax, VAR.stdDataAreaLo adc dx, VAR.stdDataAreaHi call int13ReadSector shr bx, 4 ; convert bytes into paragraphs mov ax, es ; ax = our buffer add ax, bx ; ax = old buffer + bytes read mov es, ax ; es = new buffer mov ds, VAR.stdFATSegment mov ax, 3 ; mul si ; ax = si * 3 / 2 shr ax, 1 ; xchg ax, si ; mov si, ds:[si] ; get next cluster jnc fat12ReadClusterEven shr si, 4 ; keep upper 4 bits fat12ReadClusterEven: and si, 0FFFh ; mask off junk, EOF? cmp si, 0FF8h ; cmc ret fat12ReadCluster ENDP ;--------------------------------------------------------------------------- ; fat12FindFile ; ; Entry: ; ax = number of directory entries ; cs:si = pointer to filename string ; es = segment of root directory ; Exit: ; cx = 0Bh ; si = initial cluster number ; es:di = pointer to matching entry (if CF == 0) ; fat12FindFile PROC NEAR mov cx, 11 mov dx, ax xor di, di fat12FindFileNext: pusha rep cmps BYTE PTR cs:[si], es:[di] popa je fat12FindFileDone add di, 32 dec dx jnz fat12FindFileNext stc ret fat12FindFileDone: mov si, es:[di].dirCluster ret fat12FindFile ENDP ;--------------------------------------------------------------------------- ; int13ReadSector ; ; Entry: ; ax = sector number (16-bit LBA) ; cx = sector count ; es = sector buffer segment ; Exit: ; bx = number of bytes read ; cx = zero ; dx:ax = sector number (32-bit LBA) of next sector ; int13ReadSector PROC NEAR push di xor bx, bx xor dx, dx int13ReadNext: mov di, 5 ; five retries int13ReadMore: pusha ; add start of partition (harddisks only) add ax, WORD PTR BPB.bpbHiddenSectors[0] adc dx, WORD PTR BPB.bpbHiddenSectors[2] xor cx, cx ; cx = null xchg ax, cx ; ax = null, cx = LBA[0] xchg ax, dx ; ax = LBA[2], dx = null div BPB.bpbSectorsPerTrack xchg ax, cx ; ax = LBA[0], cx = Result[2] div BPB.bpbSectorsPerTrack xchg dx, cx ; dx = Result[2], cx = Sector# inc cx ; cx = one-based Sector# div BPB.bpbHeadsPerCylinder mov ch, al ; ch = Cylinder# [7:0] shl ah, 6 ; ah = Cylinder# [9:8] or cl, ah ; combine Cylinder & Sector fields mov dh, dl ; dh = Head# mov dl, BS.bsDriveNumber mov ax, 0201h int 13h jnc int13ReadDone ;* mov ah, 00h ; uncomment if you want DISK RESETs ;* int 13h ; popa dec di jnz int13ReadMore int 18h ;* int13ReadDone: popa add bx, BPB.bpbBytesPerSector inc ax jnz int13ReadSkip inc dx int13ReadSkip: dec cx jnz int13ReadNext pop di ret int13ReadSector ENDP ; list of possible boot images szImageListDef LABEL BYTE DB "LOADER EXE" szImageListEnd LABEL BYTE BOOT_MAGIC_HERE codeSegment ENDS END