%if 0 File system boot sector loader code for FAT32 Adapted from 2002-11-26 fatboot.zip/fat12.asm, released as public domain by Chris Giese Public domain by C. Masloch, 2012 %endif %include "lmacros2.mac" %ifndef _MAP %elifempty _MAP %else ; defined non-empty, str or non-str [map all _MAP] %endif defaulting strdef OEM_NAME, " lDOS" strdef OEM_NAME_FILL, '_' strdef DEFAULT_LABEL, "lDOS" numdef VOLUMEID, 0 strdef FSIBOOTNAME, "FSIBOOT4" ; used to set experimental name ; strdef FSIBOOTNAME, "FSIBEX02" strdef LOAD_NAME, "LDOS" strdef LOAD_EXT, "COM" ; name of file to load numdef LOAD_ADR, 02000h ; where to load numdef LOAD_MIN_PARA, paras(1536) numdef EXEC_SEG_ADJ, 0 ; how far cs will be from _LOAD_ADR numdef EXEC_OFS, 400h ; what value ip will be numdef CHECKOFFSET, 1020 numdef CHECKVALUE, "lD" numdef LOAD_DIR_SEG, 0 ; => where to store dir entry (0 if nowhere) numdef ADD_SEARCH, 0 ; whether to search second file strdef ADD_NAME, "" strdef ADD_EXT, "" ; name of second file to search numdef ADD_DIR_SEG, 0 ; => where to store dir entry (0 if nowhere) numdef QUERY_GEOMETRY, 1 ; query geometry via 13.08 (for CHS access) numdef QUERY_GEOMETRY_DISABLED, 0 numdef USE_PART_INFO, 1 ; use ds:si-> partition info from MBR, if any numdef USE_PART_INFO_DISABLED, 0 numdef USE_AUTO_UNIT, 1 ; use unit passed from ROM-BIOS in dl numdef RPL, 1 ; support RPL and do not overwrite it numdef CHS, 1 ; support CHS (if it fits) numdef LBA, 1 ; support LBA (if available) numdef LBA_33_BIT, 1 ; support 33-bit LBA numdef LBA_CHECK_NO_33, 1 ; else: check that LBA doesn't carry numdef RELOCATE, 0 ; relocate the loader to top of memory numdef SET_DL_UNIT, 0 ; if to pass unit in dl numdef SET_BL_UNIT, 0 ; if to pass unit in bl as well numdef SET_AXBX_DATA, 0 ; if to pass first data sector in ax:bx numdef SET_DSSI_DPT, 0 ; if to pass DPT address in ds:si numdef PUSH_DPT, 0 ; if to push DPT address numdef MEMORY_CONTINUE, 1 ; if to just execute when memory full numdef SET_SIDI_CLUSTER,0 ; if to pass first load file cluster in si:di numdef TURN_OFF_FLOPPY, 0 ; if to turn off floppy motor after loading numdef DATASTART_HIDDEN,0 ; if to add hidden sectors to data_start numdef LBA_SET_TYPE, 0 ; if to set third byte to LBA partition type numdef SET_LOAD_SEG, 1 ; if to set load_seg (word [ss:bp - 6]) numdef SET_FAT_SEG, 1 ; if to set fat_seg (word [ss:bp - 8]) numdef SET_CLUSTER, 1 ; if to set first_cluster (dword [ss:bp - 16]) numdef ZERO_ES, 0 ; if to set es = 0 before jump numdef ZERO_DS, 0 ; if to set ds = 0 before jump numdef FIX_SECTOR_SIZE, 0 ; fix sector size (0 = disable, else = sector size) numdef FIX_SECTOR_SIZE_SKIP_CHECK, 0 ; don't check sector size numdef FIX_CLUSTER_SIZE, 0 ; fix cluster size numdef FIX_CLUSTER_SIZE_SKIP_CHECK, 0 ; don't check cluster size numdef NO_LIMIT, 0 ; allow using more memory than a boot sector ; also will not write 0AA55h signature! numdef WARN_PART_SIZE, 0 numdef LBA_SKIP_CHECK, 1 ; don't use proper LBA extensions check numdef LBA_SKIP_CY, 1 ; skip check: set up CY before 13.42 numdef LBA_SKIP_ANY, 0 ; skip check: try CHS on any error incdef _LBA_SKIP_ANY, LBA_SKIP_CY numdef LBA_RETRY, 0 ; retry LBA reads one time numdef CHS_RETRY, 1 ; retry CHS reads one time numdef CHS_RETRY_REPEAT,16 ; retry CHS reads multiple times ; (value of the def is used as count) numdef CHS_RETRY_NORMAL,1 ; do not use aggressive optimisation numdef RETRY_RESET, 1 ; call reset disk system 13.00 on retries ; Unlike the 1440 KiB diskette image defaults for the FAT12 ; loader we just fill the BPB with zeros by default. numdef MEDIAID, 0 ; media ID numdef UNIT, 0 ; load unit in BPB numdef CHS_SECTORS, 0 ; CHS geometry field for sectors numdef CHS_HEADS, 0 ; CHS geometry field for heads numdef HIDDEN, 0 ; number of hidden sectors numdef SPI, 0 ; sectors per image numdef BPS, 0 ; bytes per sector numdef SPC, 0 ; sectors per cluster numdef SPF, 0 ; sectors per FAT numdef SECTOR_FSINFO, 0 ; FSINFO sector numdef SECTOR_BACKUP, 0 ; backup boot sector numdef CLUSTER_ROOT, 0 ; root directory first cluster numdef NUMFATS, 0 ; number of FATs numdef NUMRESERVED, 0 ; number of reserved sectors numdef COMPAT_FREEDOS, 0 ; partial FreeDOS load compatibility numdef COMPAT_IBM, 0 ; partial IBMDOS load compatibility numdef COMPAT_MS7, 0 ; partial MS-DOS 7 load compatibility numdef COMPAT_LDOS, 0 ; lDOS load compatibility %if (!!_COMPAT_FREEDOS + !!_COMPAT_IBM + !!_COMPAT_MS7 + !!_COMPAT_LDOS) > 1 %error At most one set must be selected. %endif %if _COMPAT_FREEDOS strdef LOAD_NAME, "KERNEL" strdef LOAD_EXT, "SYS" numdef LOAD_ADR, 00600h numdef LOAD_MIN_PARA, paras(512) numdef EXEC_SEG_ADJ, 0 numdef EXEC_OFS, 0 numdef CHECKVALUE, 0 numdef SET_BL_UNIT, 1 numdef MEMORY_CONTINUE, 0 numdef RELOCATE, 1 ; The FreeDOS load protocol mandates that the entire file be loaded. %endif %if _COMPAT_IBM strdef LOAD_NAME, "IBMBIO" strdef LOAD_EXT, "COM" numdef LOAD_ADR, 00700h numdef LOAD_MIN_PARA, paras(512) numdef EXEC_SEG_ADJ, 0 numdef EXEC_OFS, 0 numdef CHECKVALUE, 0 numdef LOAD_DIR_SEG, 50h numdef ADD_SEARCH, 1 strdef ADD_NAME, "IBMDOS" strdef ADD_EXT, "COM" numdef ADD_DIR_SEG, 52h ; Note: The IBMBIO.COM directory entry must be stored at ; 0:500h, and the IBMDOS.COM directory entry at 0:520h. numdef SET_DL_UNIT, 1 numdef MEMORY_CONTINUE, 1 ; 3 sectors * 512 BpS should suffice. We load into 700h--7A00h, ; ie >= 6000h bytes (24 KiB), <= 7300h bytes (28.75 KiB). numdef SET_AXBX_DATA, 1 numdef DATASTART_HIDDEN,1 numdef SET_DSSI_DPT, 1 numdef PUSH_DPT, 1 %endif %if _COMPAT_MS7 strdef LOAD_NAME, "IO" strdef LOAD_EXT, "SYS" numdef LOAD_ADR, 00700h numdef LOAD_MIN_PARA, paras(1024) numdef EXEC_SEG_ADJ, 0 numdef EXEC_OFS, 200h numdef CHECKVALUE, 0 numdef SET_DL_UNIT, 1 numdef SET_DSSI_DPT, 0 numdef PUSH_DPT, 1 numdef MEMORY_CONTINUE, 1 ; 4 sectors * 512 BpS should suffice. We load into 700h--7A00h, ; ie >= 6000h bytes (24 KiB), <= 7300h bytes (28.75 KiB). numdef SET_SIDI_CLUSTER,1 numdef DATASTART_HIDDEN,1 numdef LBA_SET_TYPE, 1 %endif %if _COMPAT_LDOS strdef LOAD_NAME, "LDOS" strdef LOAD_EXT, "COM" numdef LOAD_ADR, 02000h numdef LOAD_MIN_PARA, paras(1536) numdef EXEC_SEG_ADJ, 0 numdef EXEC_OFS, 400h numdef CHECKOFFSET, 1020 numdef CHECKVALUE, "lD" numdef SET_DL_UNIT, 0 numdef SET_CLUSTER, 1 numdef SET_FAT_SEG, 1 numdef SET_LOAD_SEG, 1 numdef MEMORY_CONTINUE, 1 numdef DATASTART_HIDDEN,0 %endif %if 0 Notes about partial load compatibilities * FreeDOS: * Relocates to an address other than 27A00h (1FE0h:7C00h) * IBMDOS: * Does not actually relocate DPT, just provide its address * MS-DOS 7: * Does not actually relocate DPT, just provide its address * Does not contain message table used by loader %endif %if _SET_BL_UNIT && _SET_AXBX_DATA %error Cannot select both of these options! %endif %assign LOADLIMIT 0A0000h %assign POSITION 07C00h %if _FIX_SECTOR_SIZE %assign i 5 %rep 13-5 %if (1 << i) != (_FIX_SECTOR_SIZE) %assign i i+1 %endif %endrep %if (1 << i) != (_FIX_SECTOR_SIZE) %error Invalid sector size _FIX_SECTOR_SIZE %endif %endif %if _FIX_CLUSTER_SIZE %if _FIX_CLUSTER_SIZE > 256 %error Invalid cluster size _FIX_CLUSTER_SIZE %endif %assign i 0 %rep 8-0 %if (1 << i) != (_FIX_CLUSTER_SIZE) %assign i i+1 %endif %endrep %if (1 << i) != (_FIX_CLUSTER_SIZE) %warning Non-power-of-two cluster size _FIX_CLUSTER_SIZE %endif %endif %if (_LOAD_ADR & 0Fh) %error Load address must be on a paragraph boundary %endif %if _LOAD_ADR > LOADLIMIT %error Load address must be in LMA %elif _LOAD_ADR < 00500h %error Load address must not overlap IVT or BDA %endif %if ! _RELOCATE %if _LOAD_ADR > (POSITION-512) && _LOAD_ADR < (POSITION+1024) %error Load address must not overlap loader %endif %endif %if ((_EXEC_SEG_ADJ<<4)+_EXEC_OFS) < 0 %error Execution address must be in loaded file %elif ((_EXEC_SEG_ADJ<<4)+_EXEC_OFS+_LOAD_ADR) > LOADLIMIT %error Execution address must be in LMA %endif %if (_EXEC_OFS & ~0FFFFh) %error Execution offset must fit into 16 bits %endif %if (_EXEC_SEG_ADJ > 0FFFFh || _EXEC_SEG_ADJ < -0FFFFh) %error Execution segment adjustment must fit into 16 bits %endif %if !_CHS && _QUERY_GEOMETRY %warning No CHS support but querying geometry anyway %endif %if !_CHS && !_LBA %error Either CHS or LBA or both must be enabled %endif %if 0 There is some logic inside MS-DOS's hard disk partition initialisation code that sets up several requirements for us to fulfil. Otherwise, it will not accept the information given in the BPB (using default information based on the length as specified by MBR/EPBR instead) or make the whole file system inaccessible except for formatting. Both of those are undesirable of course. Some/all(?) checks are documented on pages 601,602 in "DOS Internals", Geoff Chappell 1994, as follows: * First three bytes contain either "jmp sho xx\ nop" or "jmp ne xx". * Media descriptor field >= 0F0h. * Bytes per sector field == 512. * Sectors per cluster field a power of 2 (1,2,4,8,16,32,64,128). * OEM name "version" (last three to four characters) * must be "20.?", "10.?" (? = wildcard), but no other with "0.?", * otherwise, must be "2.0", or * 2nd-to-last,3rd-to-last character codes together > "3.", or * those == "3.", last character code > "0" To stay compatible to those, display a warning here if the name itself would disqualify our boot sector already. %endif %push %strlen %$len _OEM_NAME_FILL %if %$len != 1 %error Specified OEM name fill must be 1 character %endif %strlen %$len _OEM_NAME %define %$nam _OEM_NAME %if %$len > 8 %error Specified OEM name is too long %else %assign %$warn 0 %rep 8 - %$len %strcat %$nam %$nam,_OEM_NAME_FILL %endrep %substr %$prefix %$nam 5 ; "wvxyZa.b", get "Z" %substr %$major %$nam 6,7 ; "wvxyzA.b", get "A." %substr %$minor %$nam 8 ; "wvxyza.B", get "B" %if %$major == "0." %ifn %$prefix == "1" || %$prefix == "2" %assign %$warn 1 %endif %elifn %$major == "2." && %$minor == "0" %if %$major < "3." %assign %$warn 1 %elif %$major == "3." && %$minor < "1" %assign %$warn 1 %endif %endif %if %$warn %warning Specified OEM name fails MS-DOS's validation %endif %endif %pop ; 512-byte stack (minus the variables). ADR_STACK_LOW equ 7C00h - 200h ; 07A00h ADR_FSIBOOT equ end -start+7C00h ; 07E00h %define _AFTER (ADR_FSIBOOT + 512) %if _RELOCATE ; dynamic allocation of FAT buffer %else %if _LOAD_ADR < 7C00h ADR_FATBUF equ _AFTER %define _AFTER (ADR_FATBUF + 8192) %else ADR_FATBUF equ 2000h %endif %endif %if _ADD_SEARCH %if _RELOCATE ; dynamic allocation of dir buffer %elif _LOAD_ADR < 7C00h ADR_DIRBUF equ _AFTER %define _AFTER (ADR_DIRBUF + 8192) %else ADR_DIRBUF equ 4000h %endif %else %if _RELOCATE ADR_DIRBUF equ _LOAD_ADR ; 00600h %elif _LOAD_ADR < 7C00h ; one-sector directory buffer. Assumes sectors are no larger than 8 KiB ADR_DIRBUF equ _AFTER ; 0A000h %define _AFTER (ADR_DIRBUF + 8192) %else ADR_DIRBUF equ 4000h %endif %endif ADR_FREE equ _AFTER ; 08000h %if ! _RELOCATE %if ((ADR_FATBUF + 8192 - 1) & ~0FFFFh) != (ADR_FATBUF & ~0FFFFh) %warning Possibly crossing 64 KiB boundary while reading FAT %endif %endif %ifn _RELOCATE && _ADD_SEARCH %if ((ADR_DIRBUF + 8192 - 1) & ~0FFFFh) != (ADR_DIRBUF & ~0FFFFh) %warning Possibly crossing 64 KiB boundary while reading directory %endif %endif %if _LOAD_ADR >= ADR_FREE || _RELOCATE ; If reading on a sector size boundary, no crossing can occur. ; Check for all possible sector sizes (32 B to 8 KiB). If one ; of them fails display a warning, including the minimum size. %assign SECSIZECHECK 32 %assign EXITREP 0 %rep 256 %ifn EXITREP %if _LOAD_ADR & (SECSIZECHECK - 1) %warning Possibly crossing 64 KiB boundary while reading file (sector size >= SECSIZECHECK) %assign EXITREP 1 %exitrep %endif %if SECSIZECHECK == 8192 %assign EXITREP 1 %exitrep %endif %assign SECSIZECHECK SECSIZECHECK * 2 %endif %endrep %else ; If loading below the boot sector, address 1_0000h is never reached. %endif struc FSINFO ; FAT32 FSINFO sector layout .signature1: resd 1 ; 41615252h ("RRaA") for valid FSINFO .reserved1: ; former unused, initialized to zero by FORMAT .fsiboot: resb 480 ; now used for FSIBOOT .signature2: resd 1 ; 61417272h ("rrAa") for valid FSINFO .numberfree: resd 1 ; FSINFO: number of free clusters or -1 .nextfree: resd 1 ; FSINFO: first free cluster or -1 .reserved2: resd 3 ; unused, initialized to zero by FORMAT .signature3: resd 1 ; AA550000h for valid FSINFO or FSIBOOT endstruc struc FSIBOOTG ; FSIBOOT general layout .signature: resq 1 ; 8 bytes that identify the FSIBOOT type .fsicode: resb 470 ; 470 bytes FSIBOOT type specific data or code .dirsearch: resw 1 ; 1 word -> directory search entrypoint endstruc struc DIRENTRY deName: resb 8 deExt: resb 3 deAttrib: resb 1 dePlusSize: resb 1 resb 7 deClusterHigh: resw 1 deTime: resw 1 deDate: resw 1 deClusterLow: resw 1 deSize: resd 1 endstruc ATTR_READONLY equ 1 ATTR_HIDDEN equ 2 ATTR_SYSTEM equ 4 ATTR_VOLLABEL equ 8 ATTR_DIRECTORY equ 10h ATTR_ARCHIVE equ 20h ; use byte-offset addressing from BP for smaller code %define VAR(x) ((x) - start) + bp cpu 8086 ; bootsector loaded at address 07C00h, addressable using 0000h:7C00h org POSITION start: %define _LASTVARIABLE start %macro nextvariable 2.nolist %1 equ (_LASTVARIABLE - %2) %define _LASTVARIABLE %1 %endmacro ; Variables ; (dword) sector where the first cluster's data starts nextvariable data_start, 4 ; (word) current load segment (points behind last loaded data) nextvariable load_seg, 2 ; (word) segment of FAT buffer ; for FAT12 this holds the entire FAT ; for FAT16 this holds the sector given by wo[fat_sector] ; for FAT32 this holds the sector given by dwo[fat_sector] nextvariable fat_seg, 2 ; (dword for FAT32) currently loaded sector-in-FAT, -1 if none nextvariable fat_sector, 4 ; (dword for FAT32) first cluster of load file nextvariable first_cluster, 4 ADR_STACK_START equ _LASTVARIABLE -start+POSITION ; (word) number of 16-byte paragraphs per sector nextvariable para_per_sector, 2 ; (word) number of 32-byte directory entries per sector nextvariable entries_per_sector, 2 ; (word) segment of last available memory for sector nextvariable last_available_sector, 2 ; (word) actual sectors per cluster nextvariable adj_sectors_per_cluster, 2 ; (word) paragraphs left to read nextvariable paras_left, 2 lowest_variable equ _LASTVARIABLE jmp strict short skip_bpb %if !_CHS && _LBA_SET_TYPE db 0Ch ; LBA-enabled FAT32 FS partition type %else nop ; default: no LBA %endif ; BIOS Parameter Block (BPB) ; ; Installation will use the BPB already present in your file system. ; These values must be initialised when installing the loader. oem_id: ; offset 03h (03) - not used by this code fill 8,_OEM_NAME_FILL,db _OEM_NAME bytes_per_sector: ; offset 0Bh (11) - refer to _FIX_SECTOR_SIZE ! dw _BPS sectors_per_cluster: ; offset 0Dh (13) - refer to _FIX_CLUSTER_SIZE ! db _SPC & 255 fat_start: num_reserved_sectors: ; offset 0Eh (14) dw _NUMRESERVED num_fats: ; offset 10h (16) db _NUMFATS num_root_dir_ents: ; offset 11h (17) dw 0 total_sectors: ; offset 13h (19) - not used by this code %if _SPI < 1_0000h dw _SPI %else dw 0 %endif media_id: ; offset 15h (21) - not used by this code db _MEDIAID sectors_per_fat: ; offset 16h (22) dw 0 sectors_per_track: ; offset 18h (24) dw _CHS_SECTORS heads: ; offset 1Ah (26) dw _CHS_HEADS hidden_sectors: ; offset 1Ch (28) dd _HIDDEN total_sectors_large: ; offset 20h (32) - not used by this code %if _SPI >= 1_0000h dd _SPI %else dd 0 %endif sectors_per_fat_large: ; offset 24h (36) dd _SPF fsflags: ; offset 28h (40) dw 0 fsversion: ; offset 2Ah (42) dw 0 root_cluster: ; offset 2Ch (44) dd _CLUSTER_ROOT fsinfo_sector: ; offset 30h (48) dw _SECTOR_FSINFO backup_sector: ; offset 32h (50) dw _SECTOR_BACKUP times 12 db 0 ; offset 34h (52) reserved ; Extended BPB ; offset 40h (64) boot_unit: db _UNIT db 0 ext_bpb_signature: db 29h serial_number: dd _VOLUMEID volume_label: fill 11,32,db _DEFAULT_LABEL filesystem_identifier: fill 8,32,db "FAT32" ; Initialised data align 2 fsiboot_table: ; this table is used by the FSIBOOT stage .error: dw error ; INP: al = error condition letter ; ('B' = bad chain / FS error, 'F' = file not found, ; 'R' = disk read error, 'M' = not enough memory, ; 'E' = not enough data in file) .success: dw load_finish ; INP: dword [ss:sp] = first cluster ; Note: The first cluster dword is always filled in ; by FSIBOOT; the option _SET_SIDI_CLUSTER only ; affects usage in the primary loader. %if _MEMORY_CONTINUE .memory_full: dw load_finish_mc ; INP: al = error condition letter ('M'), ; dword [ss:sp] = current cluster number, ; dword [ss:sp + 4] = first cluster, refer to .success %else .memory_full: dw error ; refer to previous .memory_full comment %endif .read_sector: dw read_sector ; INP: dx:ax = sector number within partition, ; bx => buffer, (_LBA) ds = ss ; OUT: dx:ax incremented, bx => incremented, ; es = input bx, does not return if error occurred ; CHG: none %if _RELOCATE && _ADD_SEARCH .dirbuf: dw 8192 >> 4 ; initialised to segment displacement from FAT buffer %else .dirbuf: dw ADR_DIRBUF>>4 ; => directory sector buffer (one sector) %endif .writedirentry: dw writedirentry.loaddir ; INP: es:bx -> found dir entry in dir sector buffer ; si:di = loaded sector-in-FAT ; CHG: ax, cx, dx ; STT: ss:bp -> boot sector ; ds = ss ; UP ; OUT: directory entry copied if so desired ; (is a no-op if not to copy dir entry) .filename: dw .load_name ; -> name to search .minpara: dw _LOAD_MIN_PARA .loadseg: dw _LOAD_ADR>>4 ; => where to load .load_name: ; = blank-padded 11-byte filename to search for fill 8,32,db _LOAD_NAME fill 3,32,db _LOAD_EXT fsiboot_name: fill 8, 32, db _FSIBOOTNAME %if _WARN_PART_SIZE %assign num $ - start %warning BPB + data size is num bytes %endif ; Code ; Note that this may be entered with cs:ip = 07C0h:0 ! skip_bpb: cli cld xor cx, cx mov bp, start ; magic bytes - checked by instsect mov ss, cx mov sp, ADR_STACK_START %if _USE_AUTO_UNIT mov [VAR(boot_unit)], dl; magic bytes - checked by instsect %else mov dl, [VAR(boot_unit)]; magic bytes - checked by instsect %endif ; Note: es is left uninitialised here until the first call to ; read_sector if the below conditional is false. %if _USE_PART_INFO ; +19 bytes mov es, cx ; Note: Award Medallion BIOS v6.0 (ASUS MED 2001 ACPI BIOS Revision 1009) ; loads from a floppy disk drive with ds:si = 0F000h:0A92Dh -> ; FF FF FF FF 08 00 08 01 FF FF FF FF FF FF FF FF, which was detected ; as a valid partition table entry by this handling. Therefore, we ; only accept partition information when booting from a hard disk now. ; start of magic byte sequence for instsect test dl, dl ; floppy ? jns @F ; don't attempt detection --> ; Check whether an MBR left us partition information. ; byte[ds:si] bit 7 means active and must be set if valid. cmp byte [si], cl ; flags for xx-00h (result is xx), SF = bit 7 jns @F ; xx < 80h, ie info invalid --> ; byte[ds:si+4] is the file system type. Check for valid one. cmp byte [si+4], cl ; is it zero? je @F ; yes, info invalid --> ; Info valid, trust their hidden sectors over hardcoded. ; Assume the movsw instructions won't run with si = FFFFh. mov di, hidden_sectors ; -> BPB field add si, 8 ; -> partition start sector in info %if _USE_PART_INFO_DISABLED nop nop ; size has to match enabled code %else movsw movsw ; overwrite BPB field with value from info %endif @@: ; end of magic byte sequence for instsect %endif mov ds, cx sti %if _QUERY_GEOMETRY ; +27 bytes ; start of magic byte sequence for instsect ; test dl, dl ; floppy? ; jns @F ; don't attempt query, might fail --> ; Note that while the original PC BIOS doesn't support this function ; (for its diskettes), it does properly return the error code 01h. ; https://sites.google.com/site/pcdosretro/ibmpcbios (IBM PC version 1) mov ah, 08h ; xor cx, cx ; initialise cl to 0 ; Already from prologue cx = 0. stc ; initialise to CY %if _QUERY_GEOMETRY_DISABLED nop nop ; size has to match enabled code %else int 13h ; query drive geometry %endif jc @F ; apparently failed --> and cx, 3Fh ; get sectors jz @F ; invalid (S is 1-based), don't use --> mov [VAR(sectors_per_track)], cx mov cl, dh ; cx = maximum head number inc cx ; cx = number of heads (H is 0-based) mov [VAR(heads)], cx @@: ; end of magic byte sequence for instsect %endif ; 16-byte paragraphs per sector mov ax, [VAR(bytes_per_sector)] mov cl, 4 shr ax, cl push ax ; push into word [VAR(para_per_sector)] ; 32-byte FAT directory entries per sector shr ax, 1 ; /2 = 32-byte entries per sector push ax ; push into word [VAR(entries_per_sector)] load_fsiboot: cmp ax, 1024 >> 5 ; sector size is at least 1 KiB (large) ? jae .loaded ; already loaded as part of the boot sector --> ; If this code runs, then ax was below 1024 >> 5, ; therefore this cwd instruction always zeros dx. cwd mov ax, [VAR(fsinfo_sector)] ; inc ax ; jz error_fsiboot ; was FFFFh, invalid --> ; dec ax ; FFFFh always fails the < num_reserved_sectors check test ax, ax jz @F ; is 0 ? invalid --> (ZR, NC) cmp ax, [VAR(num_reserved_sectors)] @@: ; (ZR, NC if ax == 0) jae error_fsiboot ; (jump if NC) ; dx:ax = FSINFO sector (dx = 0 from cwd) mov cx, 512 mov bx, ADR_FSIBOOT >> 4 @@: call read_sector sub cx, [VAR(bytes_per_sector)] ja @B ; read 512 bytes --> .loaded: push ds pop es mov di, fsiboot.signature mov si, fsiboot_name mov cx, 4 ; size of fsiboot_name repe cmpsw jne error_fsiboot ; Note that now es:di -> fsiboot.start %if _LOAD_ADR >= ADR_FREE || _RELOCATE ; Get conventional memory size and store it int 12h mov cl, 6 shl ax, cl %if _RPL ; +33 bytes xchg dx, ax lds si, [4 * 2Fh] add si, 3 lodsb cmp al, 'R' jne .no_rpl lodsb cmp al, 'P' jne .no_rpl lodsb cmp al, 'L' jne .no_rpl mov ax, 4A06h int 2Fh .no_rpl: push ss pop ds xchg ax, dx %endif %if _RELOCATE %if _ADD_SEARCH sub ax, (20 * 1024 + 8192 + 8192 + (8192-16) + 1024 + 7C00h) >> 4 %else sub ax, (20 * 1024 + 8192 + (8192-16) + 1024 + 7C00h) >> 4 %endif ; 20 KiB: reserved for iniload ; 8 KiB: dir buffer (only if _ADD_SEARCH) ; 8 KiB: FAT buffer ; 8 KiB - 16 B: to allow rounding down position of buffers ; 1 KiB: sector + FSIBOOT ; 7C00h: stack and to allow addressing with 7C00h in bp ; ; Note also that by addressing the stack and sector and FSIBOOT ; with bp at 7C00h, and insuring this subtraction doesn't ; underflow, makes sure that we do not overwrite the IVT or ; BDA. (However, we assume that ax is at least 60h or so.) ; ; The FAT buffer segment is masked so that the actual buffer ; is stored on an 8 KiB boundary. This is to ensure that ; the buffer doesn't possibly cross a 64 KiB DMA boundary. jc error_fsiboot cmp ax, (end_after_fsiboot -start+7C00h - ADR_STACK_LOW + 15) >> 4 ; This check is to ensure that the start of the destination ; for the relocation (stack, sector, FSIBOOT) is ; above-or-equal the end of the source for the relocation. ; That is, to avoid overwriting any of the source with the ; string move instruction (which for simplicity is always UP). jb error_fsiboot ; With _RELOCATE enabled, the FAT buffer segment ; is fixed up by the primary loader here before ; FSIBOOT is executed. mov bx, ((8192 - 16) + 1024 + 7C00h)>>4 ; bx is initialised to the value ; ((8192 - 16) + 1024 + 7C00h)>>4 ; so this is like calculating the following for its value: ; ((LMA_top - 20 KiB - 8 KiB - 8 KiB{AS} - (8 KiB - 16 B) - 1 KiB - 7C00h) + \ ; ((8 KiB - 16 B) + 1 KiB + 7C00h))>>4 ; == (LMA_top - 20 KiB - 8 KiB - 8 KiB{AS})>>4 ; {AS} = only if _ADD_SEARCH add bx, ax and bx, ~ ((8192>>4) - 1) ; => FAT sector buffer (one sector) %if _ADD_SEARCH ; .dirbuf is initialised to 8192 >> 4 add word [VAR(fsiboot_table.dirbuf)], bx ; => dir sector buffer (one sector) %endif push ax push di ; -> reloc destination of fsiboot.start mov es, ax ; => destination mov si, sp ; ds:si = ss:entries_per_sector - 4 mov di, si ; es:di -> destination for stack low mov cx, (end_after_fsiboot - (entries_per_sector - 4)) >> 1 ; end_after_fsiboot is the top of used memory ; entries_per_sector is the lowest filled stack frame slot ; 4 is for the additional slots taken by the return address rep movsw ; relocate stack, sector, and FSIBOOT mov ss, ax mov ds, ax ; relocate these add ax, (ADR_STACK_LOW) >> 4 ; (rounding down) => behind available retf ; jump to relocated FSIBOOT %else sub ax, (20 * 1024) >> 4 ; 20 KiB reserved for iniload jc error_fsiboot mov bx, ADR_FATBUF >> 4 ; => FAT sector buffer (one sector) push es push di ; -> fsiboot.start retf ; Do a far return here to ensure that cs is zero. %endif %elif _LOAD_ADR < ADR_STACK_LOW mov ax, (ADR_STACK_LOW >> 4) mov bx, ADR_FATBUF >> 4 ; => FAT sector buffer (one sector) push es push di ; -> fsiboot.start retf ; Do a far return here to ensure that cs is zero. %else %error Load address within used memory %endif %if _WARN_PART_SIZE %assign num $ - skip_bpb %warning init size is num bytes %endif finish_start: ; INP: es:bx -> found dir entry in dir sector buffer ; si:di = loaded sector-in-FAT ; CHG: ax, cx, dx ; STT: ss:bp -> boot sector ; ds = ss ; UP ; OUT: directory entry copied if so desired ; (is a no-op if not to copy dir entry) writedirentry: %if _LOAD_DIR_SEG .loaddir: mov ax, _LOAD_DIR_SEG %else .loaddir: equ read_sector.retn %endif %if _LOAD_DIR_SEG || (_ADD_DIR_SEG && _ADD_SEARCH) ; INP: ax => where to store directory entry .ax: push ds push si push di push es pop ds mov si, bx ; ds:si -> directory entry mov cx, DIRENTRY_size >> 1 mov es, ax xor di, di ; es:di -> where to store directory entry rep movsw ; move to here (one directory entry) push ds pop es ; es:bx -> dir entry in dir sector buffer pop di pop si pop ds retn %endif %if _MEMORY_CONTINUE load_finish_mc: pop bx pop cx %endif load_finish: %if _ADD_SEARCH mov word [VAR(fsiboot_table.filename)], add_name call near word [dirsearch_entrypoint] %if _ADD_DIR_SEG mov ax, _ADD_DIR_SEG call writedirentry.ax %endif %endif %if _TURN_OFF_FLOPPY ; turn off floppy motor mov dx,3F2h mov al,0 out dx,al %endif ; Set-up registers for and jump to loaded file ; Already: ss:bp-> boot sector containing BPB %if _CHECKVALUE CHECKLINEAR equ _LOAD_ADR + _CHECKOFFSET %if CHECKLINEAR <= (64 * 1024 - 2) && ! _RELOCATE cmp word [CHECKLINEAR], _CHECKVALUE %else mov ax, CHECKLINEAR >> 4 mov es, ax cmp word [es:CHECKLINEAR & 15], _CHECKVALUE %endif mov al, 'V' ; check 'V'alue mismatch jne error %endif %if _SET_SIDI_CLUSTER && (_PUSH_DPT || _SET_DSSI_DPT) pop cx pop dx %endif %if _PUSH_DPT || _SET_DSSI_DPT %ifn _SET_DSSI_DPT ; (implying that only _PUSH_DPT is set) %if _RELOCATE xor ax, ax mov es, ax ; => IVT mov di, 1Eh*4 les si, [es:di] ; -> original (also current) DPT push es push si ; original (also current) DPT address push ax push di ; 0000h:0078h (address of 1Eh IVT entry) %else ; If not _RELOCATE, ds = 0000h (=> IVT) mov di, 1Eh*4 les si, [di] ; -> original (also current) DPT push es push si ; original (also current) DPT address ; If not _RELOCATE, ss = 0000h (=> IVT) push ss push di ; 0000h:0078h (address of 1Eh IVT entry) %endif %else %if _RELOCATE xor ax, ax mov ds, ax ; => IVT %endif ; If not _RELOCATE, ds = 0000h (=> IVT) mov di, 1Eh*4 lds si, [di] ; -> original (also current) DPT %if _PUSH_DPT push ds push si ; original (also current) DPT address %if _RELOCATE push ax push di ; 0000h:0078h (address of 1Eh IVT entry) %else ; If not _RELOCATE, ss = 0000h (=> IVT) push ss push di ; 0000h:0078h (address of 1Eh IVT entry) %endif %endif %endif %endif %if _ZERO_ES || _ZERO_DS %if _RELOCATE %ifn _PUSH_DPT || _SET_DSSI_DPT xor ax, ax %endif %if _ZERO_ES mov es, ax %endif %if _ZERO_DS mov ds, ax %endif %else %if _ZERO_ES push ss pop es %endif %if _ZERO_DS && _SET_DSSI_DPT push ss pop ds %endif %endif %endif %if _DATASTART_HIDDEN mov bx, [VAR(hidden_sectors + 0)] mov ax, [VAR(hidden_sectors + 2)] add word [VAR(data_start + 0)], bx adc word [VAR(data_start + 2)], ax %endif %if _SET_AXBX_DATA mov bx, [VAR(data_start)] mov ax, [VAR(data_start+2)] %endif %if _SET_SIDI_CLUSTER %if _SET_DSSI_DPT %error Cannot select both of these. %endif %if _PUSH_DPT || _SET_DSSI_DPT mov di, cx mov si, dx %else pop di pop si %endif %endif %if _SET_DL_UNIT mov dl, [VAR(boot_unit)]; set dl to unit %if _SET_BL_UNIT mov bl, dl ; set bl to unit, too %endif %elif _SET_BL_UNIT mov bl, [VAR(boot_unit)]; set bl to unit %endif ; ss:bp-> boot sector with BPB jmp (_LOAD_ADR>>4)+_EXEC_SEG_ADJ:_EXEC_OFS %if _WARN_PART_SIZE %assign num $ - finish_start %warning finish size is num bytes %endif error_start: error_fsiboot: mov al,'I' db __TEST_IMM16 ; (skip mov) read_sector.err: mov al, 'R' ; Disk 'R'ead error error: %if _RELOCATE mov bx, 7 mov ds, bx mov bh, [462h - 70h] %else mov bh, [462h] mov bl, 7 %endif mov ah, 0Eh int 10h ; display character mov al, 07h int 10h ; beep! xor ax, ax ; await key pressed int 16h int 19h ; re-start the boot process %if _WARN_PART_SIZE %assign num $ - error_start %warning error size is num bytes %endif ; Read a sector using Int13.02 or Int13.42 ; ; INP: dx:ax = sector number within partition ; bx => buffer ; (_LBA) ds = ss ; OUT: If unable to read, ; ! jumps to error instead of returning ; If sector has been read, ; dx:ax = next sector number (has been incremented) ; bx => next buffer (bx = es+word[para_per_sector]) ; es = input bx ; CHG: - read_sector: push dx push cx push ax push si mov es, bx ; => buffer ; DX:AX==LBA sector number ; add partition start (= number of hidden sectors) add ax,[VAR(hidden_sectors + 0)] adc dx,[VAR(hidden_sectors + 2)] %if (!_LBA || !_LBA_33_BIT) && _LBA_CHECK_NO_33 jc .err %endif %if _LBA ; +70 bytes (with CHS, +63 bytes without CHS) %if _LBA_33_BIT sbb si, si ; -1 if was CY, 0 else neg si ; 1 if was CY, 0 else %endif xor cx, cx push cx ; highest word = 0 %if _LBA_33_BIT push si ; bit 32 = 1 if operating in 33-bit space %else push cx ; second highest word = 0 %endif push dx push ax ; = qword sector number push bx push cx ; bx => buffer inc cx push cx ; word number of sectors to read mov cl, 10h push cx ; word size of disk address packet mov si, sp ; ds:si -> disk address packet (on stack) %if _LBA_SKIP_CHECK ; -14 bytes mov dl, [VAR(boot_unit)] %else mov ah, 41h mov dl, [VAR(boot_unit)] mov bx, 55AAh stc int 13h ; 13.41.bx=55AA extensions installation check jc .no_lba cmp bx, 0AA55h jne .no_lba shr cl, 1 ; support bitmap bit 0 jnc .no_lba %endif %if _LBA_RETRY %if _LBA_SKIP_CHECK && _LBA_SKIP_CY stc %endif mov ah, 42h int 13h ; 13.42 extensions read jnc .lba_done %if _RETRY_RESET xor ax, ax int 13h ; reset disk %endif ; have to reset the LBAPACKET's lpCount, as the handler may ; set it to "the number of blocks successfully transferred". ; (in any case, the high byte is still zero.) mov byte [si + 2], 1 %endif %if _LBA_SKIP_CHECK && _LBA_SKIP_CY stc %endif mov ah, 42h int 13h %if _LBA_SKIP_CHECK && _CHS %if _LBA_SKIP_ANY jc .no_lba .err_CY: equ .err .err_2: equ .err %else jnc .lba_done cmp ah, 1 ; invalid function? je .no_lba ; try CHS instead --> .err_CY: .err_2: jmp .lba_error %endif %else .err_CY: jc .lba_error .err_2: equ .err %endif .lba_done: %if _CHS && _LBA_SET_TYPE mov byte [bp + 2], 0Ch ; LBA-enabled FAT32 FS partition type %endif add sp, 10h %if _CHS jmp short .done %endif .lba_error: equ .err %if !_CHS .no_lba: equ .err %else .no_lba: add sp, 8 pop cx ; cx = low word of sector pop ax pop dx ; dx:ax = middle two words of sector ; Here dx <= 1 if _LBA_33_BIT, else zero. ; If dx is nonzero then the CHS calculation ; should fail. If CHS sectors is equal to 1 ; (very unusual) then the div may fail. Else, ; we will detect a cylinder > 1023 eventually. pop si ; discard highest word of qword %endif %else .err_2: equ .err %endif %if _CHS ; +70 bytes ; dx:ax = LBA sector number, (if _LBA) cx = 0 ; divide by number of sectors per track to get sector number ; Use 32:16 DIV instead of 64:32 DIV for 8088 compatability ; Use two-step 32:16 divide to avoid overflow %if !_LBA xchg cx, ax ; cx = low word of sector, clobbers ax xchg ax, dx ; ax = high word of sector, clobbers dx xor dx, dx ; dx:ax = high word of sector %else ; from the .no_lba popping we already have: ; cx = low word of sector ; dx:ax = high word of sector %endif div word [VAR(sectors_per_track)] xchg cx,ax div word [VAR(sectors_per_track)] xchg cx,dx ; DX:AX=quotient, CX=remainder=sector (S) - 1 ; divide quotient by number of heads xchg bx, ax ; bx = low word of quotient, clobbers ax xchg ax, dx ; ax = high word of quotient, clobbers dx xor dx, dx ; dx = 0 div word [VAR(heads)] ; ax = high / heads, dx = high % heads xchg bx, ax ; bx = high / heads, ax = low quotient div word [VAR(heads)] ; bx:ax=quotient=cylinder (C), dx=remainder=head (H) ; move variables into registers for INT 13h AH=02h mov dh, dl ; dh = head inc cx ; cl5:0 = sector xchg ch, al ; ch = cylinder 7:0, al = 0 shr ax, 1 shr ax, 1 ; al7:6 = cylinder 9:8 ; bx has bits set iff it's > 0, indicating a cylinder >= 65536. or bl, bh ; collect set bits from bh or cl, al ; cl7:6 = cylinder 9:8 ; ah has bits set iff it was >= 4, indicating a cylinder >= 1024. or bl, ah ; collect set bits from ah mov dl,[VAR(boot_unit)] ; dl = drive .nz_err: jnz .err_2 ; error if cylinder >= 1024 --> ; ! bx = 0 (for 13.02 call) ; we call INT 13h AH=02h once for each sector. Multi-sector reads ; may fail if we cross a track or 64K boundary %if _CHS_RETRY_REPEAT mov si, _CHS_RETRY_REPEAT + 1 %if _CHS_RETRY_NORMAL && _RETRY_RESET db __TEST_IMM16 ; (skip int 13h) .loop_chs_retry_repeat: int 13h ; reset disk %elif _RETRY_RESET .loop_chs_retry_repeat: xor ax, ax int 13h ; reset disk %else .loop_chs_retry_repeat: %endif dec si ; another attempt ? js .nz_err ; no --> mov ax, 0201h int 13h ; read one sector %if _CHS_RETRY_NORMAL && _RETRY_RESET mov ax, bx ; ax = 0 %endif jc .loop_chs_retry_repeat ; fall through to .done %else mov ax, 0201h %if _CHS_RETRY %if _RETRY_RESET ; In this case we cannot store to the stack and ; pop the value at the right moment for both ; cases of the "jnc .done" branch. So use the ; original code to re-init ax to 0201h. int 13h ; read one sector jnc .done ; reset drive xor ax, ax int 13h mov ax, 0201h %else push ax int 13h ; read one sector pop ax ; restore ax = 0201h jnc .done %endif %endif ; try read again int 13h jc .err_CY %endif %if ! _LBA .err_CY: equ .err %endif %endif ; _CHS .done: ; increment segment mov bx, es %if _FIX_SECTOR_SIZE add bx, _FIX_SECTOR_SIZE >> 4 %else add bx, [VAR(para_per_sector)] %endif pop si pop ax pop cx pop dx ; increment LBA sector number inc ax jne @F inc dx @@: .retn: retn %if _WARN_PART_SIZE %assign num $ - read_sector %warning read_sector size is num bytes %endif %if _ADD_SEARCH add_name: ; = blank-padded 11-byte filename to search for fill 8,32,db _ADD_NAME fill 3,32,db _ADD_EXT %endif %if !_NO_LIMIT available: _fill 508,38,start signatures: dw 0 ; 2-byte magic bootsector signature dw 0AA55h %assign num signatures-available %warning FAT32: num bytes still available. %else ; for testing align 4 signatures: dw 0 ; 2-byte magic bootsector signature dw 0AA55h %endif align 16, nop end: fsiboot: dd "RRaA" .signature: fill 8, 32, db _FSIBOOTNAME %if ($ - .signature) != 8 %error Unexpected name size %endif .start: ; INP: ax => after last segment to be used for loading ; bx => FAT buffer (8 KiB) ; ss:bp -> boot sector, with EBPB ; dwo [ss:bp - 4] = data_start (uninit) ; wo [ss:bp - 6] = load_seg (uninit) ; wo [ss:bp - 8] = fat_seg (uninit) ; dwo [ss:bp - 12] = fat_sector (uninit) ; dwo [ss:bp - 16] = first_cluster (uninit) ; wo [ss:bp - 18] = para_per_sector ; wo [ss:bp - 20] = entries_per_sector ; ss:sp = ss:bp - 20 ; (Note: The following stack frame entries are currently ; only used by FSIBOOT itself, so they may be ; considered implementation detail instead of ; part of the FSIBOOT protocol.) ; wo [ss:bp - 22] = last_available_sector (uninit) ; wo [ss:bp - 24] = adj_sectors_per_cluster (uninit) ; wo [ss:bp - 26] = paras_left (uninit) ; Stack layout has to match! ; ss = ds ; ss:bp + ((11 + ebpbNew + BPBN_size + 1) & ~1) ; = ss:fsiboot_table, refer to its comments ; cs set to address jump table offsets in fsiboot_table ; ds set to address fsiboot_table.load_name ; OUT: Jumps to fsiboot_table.error if error occurs: ; al = error condition letter ; Else: ; data_start (within partition) initialised ; load_seg => behind last loaded data ; fat_seg initialised (from bx) ; fat_sector initialised ; (-1 initially, loaded sector-in-FAT later) ; first_cluster initialised ; last_available_sector initialised ; (from input ax minus word [para_per_sector]) ; adj_sectors_per_cluster initialised (from BPB) ; paras_left initialised ; Jumps to fsiboot_table.success if loaded entirely: ; dword [ss:sp] = first cluster ; Jumps to fsiboot_table.memory_full if loaded partially: ; al = error condition letter ('M') ; dword [ss:sp] = current cluster number ; dword [ss:sp + 4] = first cluster mov word [VAR(fat_seg)], bx ; initialise => FAT buffer sub ax, word [VAR(para_per_sector)] jc .CY_fsiboot_error_badchain push ax ; push into word [VAR(last_available_sector)] mov bx, word [VAR(fsiboot_table.loadseg)] mov word [VAR(load_seg)], bx ; initialise => load address cmp ax, bx jb .CY_fsiboot_error_badchain ; adjusted sectors per cluster (store in a word, ; and decode EDR-DOS's special value 0 meaning 256) mov al, [VAR(sectors_per_cluster)] dec ax mov ah, 0 inc ax push ax ; push into word [VAR(adj_sectors_per_cluster)] dec ax ; ! ah = 0 mov al,[VAR(num_fats)] ; ! ah = 0, hence ax = number of FATs push ax mul word [VAR(sectors_per_fat_large)] ; ax = low word SpF*nF ; dx = high word xchg bx, ax xchg cx, dx ; cx:bx = first mul pop ax mul word [VAR(sectors_per_fat_large + 2)] ; ax = high word adjust ; dx = third word test dx, dx stc jnz .CY_fsiboot_error_badchain ; dx = zero xchg dx, ax ; dx = high word adjust ; ax = zero add dx, cx jc .CY_fsiboot_error_badchain ; dx:bx = result xchg ax, bx ; dx:ax = result ; bx = zero add ax,[VAR(num_reserved_sectors)] adc dx, bx ; bx is zero here .CY_fsiboot_error_badchain: jc ..@CY_2_fsiboot_error_badchain ; first sector of disk data area: mov [VAR(data_start)], ax mov [VAR(data_start+2)], dx mov di, -1 mov si, di mov word [VAR(fat_sector + 2)], si mov word [VAR(fat_sector + 0)], di call dirsearch found_load_file: call near word [VAR(fsiboot_table.writedirentry)] ; get starting cluster of file push word [es:bx + deClusterHigh] push word [es:bx + deClusterLow] ; check FAT+ size bits test byte [es:bx + dePlusSize], 0E7h ; test whether bits 7-5 and 2-0 NZ ; https://web.archive.org/web/20150219123449/http://www.fdos.org/kernel/fatplus.txt jnz .large_file ; yes, clamp to maximum paras --> mov ax, [es:bx + deSize + 2] mov bx, [es:bx + deSize] ; ax:bx = file size (non-FAT+) mov cx, [VAR(bytes_per_sector)] dec cx ; BpS - 1 add bx, cx adc ax, 0 ; large ? jc .large_file ; yes, clamp to maximum paras --> not cx ; ~ (BpS - 1) and bx, cx ; mask to limit to rounded-up sector ; (this also rounds up paragraphs) mov cx, 4 @@: shr ax, 1 rcr bx, 1 loop @B ; ax:bx = size in paragraphs ; bx = size in paragraphs if < 1_0000h test ax, ax ; > 0FFFFh paras ? jz @F ; no, take actual size --> .large_file: mov bx, 0FFFFh ; cx = clamp size to 0FFFFh paras @@: call check_enough.in_bx ; (CHG ax) pop ax pop dx ; dx:ax = first cluster push bx ; push into word [VAR(paras_left)] mov word [VAR(first_cluster + 0)], ax mov word [VAR(first_cluster + 2)], dx push dx push ax ; remember cluster for later call check_clust ..@CY_2_fsiboot_error_badchain: jc ..@CY_3_fsiboot_error_badchain next_load_cluster: push dx push ax ; preserve cluster number for later call clust_to_first_sector ; dx:ax = first sector of cluster ; cx = adjusted sectors per cluster ; xxx - this will always load an entire cluster (e.g. 64 sectors), ; even if the file is shorter than this @@: mov bx, [VAR(load_seg)] ; => where to read next sector cmp bx, [VAR(last_available_sector)] jbe @F call check_enough mov al, 'M' ; (! _MEMORY_CONTINUE: error code) jmp near word [VAR(fsiboot_table.memory_full)] @@: call near word [VAR(fsiboot_table.read_sector)] mov [VAR(load_seg)], bx ; => after last read data mov bx, [VAR(para_per_sector)] sub word [VAR(paras_left)], bx jbe @F ; read enough --> loop @BB pop ax pop dx call clust_next jnc next_load_cluster inc ax inc ax test al, 8 ; set in 0FFF_FFF8h--0FFF_FFFFh, ; clear in 0, 1, and 0FFF_FFF7h jz fsiboot_error_badchain db __TEST_IMM16 @@: pop bx pop cx call check_enough jmp near word [VAR(fsiboot_table.success)] dirsearch: mov ax, [VAR(root_cluster)] mov dx, [VAR(root_cluster + 2)] call check_clust ..@CY_3_fsiboot_error_badchain: jc fsiboot_error_badchain next_root_clust: push dx push ax call clust_to_first_sector ; dx:ax = first sector of cluster ; cx = adjusted sectors per cluster next_root_sect: push cx mov cx, [VAR(entries_per_sector)] ; Scan root directory for file. We don't bother to check for deleted ; entries (E5h) or entries that mark the end of the directory (00h). mov bx, [VAR(fsiboot_table.dirbuf)] call near word [VAR(fsiboot_table.read_sector)] push di xor di, di ; es:di-> first entry in this sector next_ent: test byte [es:di + deAttrib], ATTR_DIRECTORY | ATTR_VOLLABEL jnz @F ; directory, label, or LFN entry --> (NZ) push si push di push cx mov si, [VAR(fsiboot_table.filename)] ; ds:si-> name to match mov cx, 11 ; length of padded 8.3 FAT filename repe cmpsb ; check entry pop cx pop di pop si @@: ; ZR = match, NZ = mismatch je found_it ; found entry --> lea di, [di + DIRENTRY_size] loop next_ent ; count down sector's entries (jumps iff cx >0) pop di pop cx loop next_root_sect pop ax pop dx call clust_next jnc next_root_clust file_not_found: mov al, 'F' db __TEST_IMM16 fsiboot_error_badchain: mov al, 'B' fsiboot_error: jmp near word [VAR(fsiboot_table.error)] ; INP: dx:ax = cluster - 2 (0-based cluster) ; OUT: dx:ax = first sector of that cluster ; cx = adjusted sectors per cluster ; CHG: bx clust_to_first_sector: mov cx, word [VAR(adj_sectors_per_cluster)] push dx mul cx xchg bx, ax pop ax push dx mul cx test dx, dx jnz fsiboot_error_badchain xchg dx, ax pop ax add dx, ax jc ..@CY_fsiboot_error_badchain xchg ax, bx add ax, [VAR(data_start)] adc dx, [VAR(data_start + 2)] ..@CY_fsiboot_error_badchain: jc fsiboot_error_badchain ; dx:ax = first sector in cluster retn check_enough: mov bx, [VAR(load_seg)] ; => behind last read sector sub bx, word [VAR(fsiboot_table.loadseg)] .in_bx: cmp bx, word [VAR(fsiboot_table.minpara)] mov al, 'E' jb fsiboot_error retn found_it: ; es:di -> dir entry in dir sector buffer mov bx, di pop di ; restore si:di = loaded FAT sector pop cx ; (discard sectors per cluster counter) pop cx pop cx ; (discard current cluster number) ; es:bx -> dir entry retn ; INP: dx:ax = cluster (0-based) ; si:di = loaded FAT sector, -1 if none ; OUT: CY if no next cluster ; NC if next cluster found ; dx:ax = next cluster value (0-based) ; si:di = loaded FAT sector ; CHG: cx, bx, es clust_next: add ax, 2 adc dx, 0 add ax, ax adc dx, dx add ax, ax adc dx, dx ; * 4 = byte offset into FAT (0--4000_0000h) push ax xchg ax, dx xor dx, dx ; dx:ax = high word div word [VAR(bytes_per_sector)] xchg bx, ax ; bx = result high word, clobbers ax pop ax ; dx = remainder, ax = low word div word [VAR(bytes_per_sector)] xchg dx, bx ; dx:ax = result, bx = remainder ; dx:ax = sector offset into FAT (0--200_0000h) ; bx = byte offset into FAT sector (0--8190) cmp dx, si jne @F ; read sector cmp ax, di je @FF ; sector is already buffered @@: mov si, dx mov di, ax mov word [VAR(fat_sector + 2)], dx mov word [VAR(fat_sector + 0)], ax push bx add ax, [VAR(fat_start)] adc dx, 0 mov bx, [VAR(fat_seg)] call near word [VAR(fsiboot_table.read_sector)] pop bx @@: mov es, [VAR(fat_seg)] mov ax, [es:bx] mov dx, [es:bx + 2] ; INP: dx:ax = cluster value, 2-based ; OUT: dx:ax -= 2 (makes it 0-based) ; CY iff invalid cluster check_clust: and dh, 0Fh sub ax, 2 sbb dx, 0 cmp dx, 0FFFh jb @F ; CY here means valid ...- cmp ax, 0FFF7h - 2 @@: ; -... or if NC first, CY here also cmc ; NC if valid retn fsinfo_available: _fill 480 + 4 - 2,38,fsiboot %assign num $-fsinfo_available %warning FSINFO: num bytes still available. ; INP: si:di = loaded sector-in-FAT ; word [fsiboot_table.filename] -> 8.3 filename ; CHG: ax, cx, dx ; OUT: si:di updated, if so ; es:bx -> found directory entry ; jumps to error handler if an error occurs ; STT: stack variables as set up by main FSIBOOT entry dirsearch_entrypoint: dw dirsearch dd "rrAa" dd -1 ; number of free clusters dd -1 ; first free cluster times 3 dd 0 ; FSINFO.reserved2 dd 0_AA55_0000h ; FSINFO.signature3 end_after_fsiboot: %if $ - fsiboot != 512 %error Wrong FSIBOOT layout %endif