CHAPTER 8 Programming Technical Reference - IBM Copyright 1988, Dave Williams DOS DISK INFORMATION THE DOS AREA All disks and diskettes formatted by DOS are created with a sector size of 512 bytes. The DOS area (entire area for a diskette, DOS partition for hard disks) is formatted as follows: Boot record - 1 sector First copy of the FAT - variable size Second copy of the FAT - same size as first copy Root directory - variable size Data area The following sections describe each of the allocated areas: THE BOOT RECORD The boot record resides on track 0, sector 1, side 0 of every diskette formatted by the DOS FORMAT program. It is put on all disks to provide an error message is you try to start up with a nonsystem disk in drive A:. For hard disks the boot record resides on the first sector of the DOS partition. THE DOS FILE ALLOCATION TABLE (FAT) This section explains how DOS uses the FAT to convert the clusters of a file into logical sector numbers. We recommend that system utilities use the DOS handle calls rather than interpreting the FAT. The FAT is used by DOS to allocate disk space for files, one cluster at a time. The FAT consists of a 12 bit entry (1.5 bytes) for each cluster on the disk or a 16 bit (2 bytes) entry when a hard disk has more than 20740 sectors as is the case with fixed disks larger than 10Mb. The first two FAT entries map a portion of the directory; these FAT entries contain indicators of the size and format of the disk. The FAT can be in a 12 or 16 bit format. DOS determines whether a disk has a 12 or 16 bit FAT by looking at the total number of allocation units on a disk. For all diskettes and hard disks with DOS partitions less than 20,740 sectors, the FAT uses a 12 bit value to map a cluster. For larger partitions, DOS uses a 16 bit value. The second, third, and fourth bit applicable for 16 bit FAT bytes always contains 0FFFFh. The first byte is used as follows: hex value meaning normally used 0F8h hard disk bootable hard disk at C:800 double sided 18 sector diskette PS/2 1.44 meg DSQD 0F9h double sided 15 sector diskette AT 1.2 meg DSQD double sided 9 sector diskette Convertible 720k DSHD 0FCh single sided 9 sector diskette DOS 2.0, 180k SSDD 0FDh double sided 9 sector diskette DOS 2.0, 360k DSDD 0FEh single sided 8 sector diskette DOS 1.0, 160k SSDD 0FFh double sided 8 sector diskette DOS 1.1, 320k SSDD The third FAT entry begins mapping the data area (cluster 002). NOTE: These values are provided as a reference. Therefore, programs should not make use of these values. Each entry contains a hexadecimal character (or 4 for 16 bit FATs). () indicates the high order four bit value in the case of 16 bit FAT entries. They can be: (0)000h if the cluster is unused and availible (0F)FF8h - (0F)FFFh to indicate the last cluster of a file (X)XXXh any other hexadecimal numbers that are the cluster number of the next cluster in the file. The cluster number is the first cluster in the file that is kept in the file's directory entry. The values (0F)FF0h - (0F)FF7h are used to indicate reserved clusters. (0F)FF7h indicates a bad cluster if it is not part of the allocation chain. (0F)FF8h - (0F)FFFh are used as end of file markers. The file allocation table always occupies the sector or sectors immediately following the boot record. If the FAT is larger than 1 sector, the sectors occupy consecutive sector numbers. Two copies of the FAT are written, one following the other, for integrity. The FAT is read into one of the DOS buffers whenever needed (open, allocate more space, etc). USE OF THE 12 BIT FILE ALLOCATION TABLE Obtain the starting cluster of the file from the directory entry. Now, to locate each subsequent sector of the file: 1. Multiply the cluster number just used by 1.5 (each FAT entry is 1.5 bytes long). 2. The whole part of the product is offset into the FAT, pointing to the entry that maps the cluster just used. That entry contains the cluster number of the next cluster in the file. 3. Use a MOV instruction to move the word at the calculated FAT into a register. 4. If the last cluster used was an even number, keep the low order 12 bits of the register, otherwise, keep the high order 12 bits. 5. If the resultant 12 bits are (0FF8h-0FFFh) no more clusters are in the file. Otherwise, the next 12 bits contain the cluster number of the next cluster in the file. To convert the cluster to a logical sector number (relative sector, such as that used by int 25h and 26h and DEBUG): 1. Subtract 2 from the cluster number 2. Multiply the result by the number of sectors per cluster. 3. Add the logical sector number of the beginning of the data area. USE OF THE 16 BIT FILE ALLOCATION TABLE Obtain the starting cluster of the file from the directory entry. Now to locate each subsequent cluster of the file: 1. Multiply the cluster number used by 2 (each FAT entry is 2 bytes long). 2. Use the MOV word instruction to move the word at the calculated FAT offset into a register. 3. If the resultant 16 bits are (0FF8h-0FFFFh) no more clusters are in the file. Otherwise, the 16 bits contain the cluster number of the next cluster in the file. Compaq DOS makes availible a new disk type (6) with 32 bit partition values, allowing 512 megabytes per hard disk (Compaq DOS 3.3.1) DOS DISK DIRECTORY The FORMAT command initially builds the root directory for all disks. Its location (logical sector number) and the maximum number of entries are availible through the device driver interfaces. DIRECTORY ENTRIES Since directories other than the root directory are actually files, there is no limit to the number of entries that they may contain. All directory entries are 32 bytes long, and are in the following format (byte and offset are decimal). The following paragraphs describe the directory entry bytes: *BYTES 0-7 Bytes 0-7 represent the filename. The first byte of the filename indicates the status of the filename. The status of a filename can contain the following values: 00h Filename never used. This is used to limit the length of directory searches, for performance reasons. 05h Indicates that the first character of the filename actually has an 0Edh character. 0E5h Filename has been used but the file has been erased. 2Eh This entry is for a directory. If the second byte is also 2Eh, the cluster field contains the cluster number of this directory's parent directory. (0000h if the parent directory is the root directory). Any other character is the first character of a filename. *BYTES 8-10 These bytes indicate the filename extension. *BYTE 11 This byte indicates the file's attribute. The attribute byte is mapped as follows (values are in hexadecimal): NOTE: Attributes 08h and 10h cannot be changed using function call 43h (CHMOD). The system files IBMBIO.COM and IBMDOS.COM (or customized equivalent) are marked as read-only, hidden, and system files. Files can be marked hidden when they are created. Also, the read-only, hidden, and system and archive attributes may be changed through the CHMOD function call. 01h Indicates that the file is marked read-only. An attempt to open the file for output using function call 3Dh results in an error code being returned. This value can be used with other values below. 02h Indicates a hidden file. The file is excluded from normal directory searches. 04h Indicates a system file. This file is excluded from normal directory searches. 08h Indicates that the entry contains the volume label in the first 11 bytes. The entry contains no other usable information and may exist only in the root directory. 20h Indicates an archive bit. This bit is set on whenever the file is written to and closed. It is used by BACKUP and RESTORE. All other bits are reserved, and must be 0. *BYTES 12-21 reserved by DOS *BYTES 22-23 These bytes contain the time when the file was created or last updated. The time is mapped in the bits as follows: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴쩡컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � B Y T E 23 � B Y T E 22 � 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � F E D C B A 9 8 � 7 6 5 4 3 2 1 0 � 쳐컴컴컴컴컴컴컴컴컴쩡컴컴컴컴컴좔컴컴컴컴컴쩡컴컴컴컴컴컴컴컴컴� � H H H H H � M M M M M M � D D D D D � 쳐컴컴컴컴컴컴컴컴컴탠컴컴컴컴컴컴컴컴컴컴컴탠컴컴컴컴컴컴컴컴컴� � binary # hrs 0-23 � binary # minutes 0-59 � bin. # 2-sec incr � 읕컴컴컴컴컴컴컴컴컴좔컴컴컴컴컴컴컴컴컴컴컴좔컴컴컴컴컴컴컴컴컴� NOTE: The time is stored with the least significant byte first. *BYTES 24-25 This area contains the date when the file was created or last updated. The mm/dd/yy are mapped in the bits as follows: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴쩡컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � B Y T E 25 � B Y T E 24 � 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � F E D C B A 9 8 � 7 6 5 4 3 2 1 0 � 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴쩡컴좔컴컴컴컴컴쩡컴컴컴컴컴컴컴컴컴� � Y Y Y Y Y Y Y � M M M M � D D D D D � 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴탠컴컴컴컴컴컴컴탠컴컴컴컴컴컴컴컴컴� � 0-119 (1980-2099) � 1-12 � 1-31 � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴좔컴컴컴컴컴컴컴좔컴컴컴컴컴컴컴컴컴� NOTE: The date is stored with the least significant byte first. *BYTES 26-27 This area contains the starting cluster number of the first cluster in the file. The first cluster for data space on all fixed disks and floppy disks is always cluster 002. The cluster number is stored with the least significant byte first. *BYTES 28-31 This area contains the file size in bytes. The first word contains the low order part of the size. Both words are stored with the least significant byte first. File Allocation Table offset size description 3 8 bytes system id (such as IBM 3.3) 11 2 bytes number of bytes per sector, ie 512 bytes is 200h 13 1 byte sectors per cluster (1 or 2) 14 2 bytes number of reserved sectors at beginning, 1 for floppies 16 1 byte number of copies of FAT, 2 for floppies 17 2 bytes number of root directory entries (64, 112, 256,etc) 19 2 bytes total sectors per disk 21 1 byte format ID (F8, F9, FC, FF, etc) 22 2 bytes number of sectors per FAT (1 or 2) 24 2 bytes number of sectors per track (8 or 9, 17) 26 2 bytes number of sides, heads, or cylinders (1 or 2 for floppy) 28 2 bytes number of special reserved sectors THE DATA AREA Allocation of space for a file (in the data area) is done only when needed (it is not preallocated). The space is allocated one cluser (unit allocation) at a time. A cluster is always one or more consecutive sector numbers, and all of the clusters in a file are "chained" together in the FAT. The clusters are arranged on disk to minimize head movement for multisided media. All of the space on a track (or cylinder) is allocated before moving on to the next track. This is accomplished by using the sequential sector numbers on the lowest-numbered head, then all the sector numbers on the next head, and so on until all sectors of all heads of the track are used. Then the next sector used will be sector 1 of head 0 on the next track. An interesting innovation that was introduced in MS-DOS 3.0: disk space that is freed by erasing a file is not re-used immediately, unlike earlier versions of DOS. Instead, free space is obtained from the area not yet used during the current session, until all of it is used up. Only then will space that is freed during the current session be re-used. This feature minimizes fragmentation of files, since never-before-used space is always contiguous. However, once any space has been freed by deleting a file, that advantage vanishes at the next system boot. The feature also greatly simplifies un-erasing files, provided that the need to do an un-erase is found during the same session and also provided that the file occupies contiguous clusters. However, when one is using programs which make extensive use of temporary files, each of which may be created and erased many times during a session, the feature becomes a nuisance; it forces the permanent files to move farther and farther into the inner tracks of the disk, thus increasing rather than decreasing the amount of fragmentation which occurs. The feature is implemented in DOS by means of a single 16-bit "last cluster used" (LCU) pointer for each physical disk drive; this pointer is a part of the physical drive table maintained by DOS. At boot time, the LCU pointer is zeroed. Each time another cluster is obtained from the free-space pool (the FAT), its number is written into the LCU pointer. Each time a fresh cluster is required, the FAT is searched to locate a free one; in older versions of DOS this search always began at Cluster 0000, but in 3.x it begins at the cluster pointed to by the LCU pointer. For hard disks, the size of the file allocation table and directory are determined when FORMAT initializes it and are based on the size of the DOS partition. The following table gives the specifications for floppy disk formats: # of sectors FAT size DIR DIR sectors total disk DOS ver sides /track (sectors) (sectors) (entries) /cluster sectors (5-1/4 inch) 160k (DOS 1.0) 1 8 (40) 1 4 64 1 320 320k (DOS 1.1) 2 8 (40) 1 7 112 2 360 180k (DOS 2.0) 1 9 (40) 2 4 64 1 640 360k (DOS 2.0) 2 9 (40) 2 7 112 2 720 1.2M (DOS 3.0) 2 15 (80) 7 14 224 1 2400 (3-1/2 inch) 720k (DOS 3.2) 2 9 (80) 3 7 112 2 1440 1.44M(DOS 3.3) 2 18 (80) 9 14 224 1 2880 Files in the data area are not nescessarily written sequentially on the first. The data area space is allocated one cluster at a time, skipping over clusters already allocated. The first free cluster found is the next cluster allocated, regardless of its physical location on the disk. This permits the most efficient utilization of disk space because clusters freed by erasing files can be allocated for new files. Refer back to the description of the DOS FAT in this chapter for more information. Hard Disk Layout The DOS hard disk routines perform the following services: 1) Allow multiple operating systems to utilize the hard disk without the need to backup and restore files when changing operating systems. 2) Allow a user-selected operating system to be started from the hard disk. I) In order to share the hard disk among operating systems, the disk may be logically divided into 1 to 4 partitions. The space within a given partition is contiguous, and can be dedicated to a specific operating system. Each operating system may "own" only one partition in DOS versions 2.0 through 3.2. PCDOS 3.3 introduced the "Extended DOS Partition" which allows multiple DOS partitions on the same hard disk. The FDISK.COM (or similar program from other DOS vendors) utility allows the user to select the number, type, and size of each partition. The partition information is kept in a partition table that is embedded in the master fixed disk boot record on the first sector of the disk. The format of this table varies from version to version of DOS. II) An operating system must consider its partition to be the entire disk, and must ensure that its functions and utilities do not access other partitions on the disk. III) Each partition may contain a boot record on its first sector, and any other programs or data that you choose - including a copy of an operating system. For example, the DOS FORMAT command may be used to format and place a copy of DOS in the DOS partition in the same manner that a diskette is formatted. With the FDISK utility, you may designate a partition as "active" (bootable). The master hard disk boot record causes that partition's boot record to receive control when the system is started or reset. Additional disk partitions could be FORTH, UNIX, Pick, CP/M-86, or the UCSD p-System. SYSTEM INITIALIZATION The boot sequence is as follows: 1. System initialization first attempts to load an operating system from diskette drive A. If the drive is not ready or a read error occurs, it then attempts to read a master hard disk boot record on the first sector of the first hard disk in the system. If unsuccessful, or if no hard disk is present, it invokes ROM BASIC in an IBM PC or displays a disk error message on most compatibles. 2. If initialization is successful, the master hard disk boot record is given control and it examines the partition table embedded within it. If one of the entries indicates an active (bootable) partition, its boot record is read from the partition's first sector and given control. 3. If none of the partitions is bootable, ROM BASIC is invoked on an IBM PC or a disk error on most compatibles. 4. If any of the boot indicators are invalid, or if more than one indicator is marked as bootable, the message INVALID PARTITION TABLE is displayed and the system stops. 5. If the partition's boot record cannot be successfully read within five retries due to read errors, the message ERROR LOADING OPERATING SYSTEM appears and the system stops. 6. If the partition's boot record does not contain a valid "signature", the message MISSING OPERATING SYSTEM appears, and the system stops. NOTE: When changing the size or location of any partition, you must ensure that all existing data on the disk has been backed up. The partitioning program will destroy the data on the disk. BOOT RECORD/PARTITION TABLE A boot record must be written on the first sector of all hard disks, and must contain the following: 1. Code to load and give control to the boot record for one of four possible operating systems. 2. A partition table at the end of the boot record. Each table entry is 16 bytes long, and contains the starting and ending cylinder, sector, and head for each of four possible partitions, as well as the number of sectors preceding the partition and the number of sectors occupied by the partition. The "boot indicator" byte is used by the boot record to determine if one of the partitions contains a loadable operating system. FDISK initialization utilities mark a user-selected partition as "bootable" by placing a value of 80h in the corresponding partition's boot indicator (setting all other partitions' indicators to 0 at the same time). The presence of the 80h tells the standard boot routine to load the sector whose location is contained in the following three bytes. That sector is the actual boot record for the selected operating system, and it is responsible for the remainder of the system's loading process (as it is from the diskette). All boot records are loaded at absolute address 0:7C00. The partition table with its offsets into the boot record is: (except for Wyse DOS 3.2 with 32 bit allocation table, and DOS 3.3-up) 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컫컴컴컴컴컴쩡컴컴컴컴컫컴컴컴컴컴쩡컴컴컴컴커 � Offset / Purpose � � Head � Sector � Cylinder � 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컵컴컴컴컴컴탠컴컴컴컴컵컴컴컴컴컴탠컴컴컴컴캑 � 1BEh partition 1 begin � boot ind � H � S � cyl � 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컵컴컴컴컴컴탠컴컴컴컴컵컴컴컴컴컴탠컴컴컴컴캑 � 1C2h partition 1 end � syst ind � H � S � cyl � 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컵컴컴컴컴컴좔컴컴컴컴컵컴컴컴컴컴좔컴컴컴컴캑 � 1C6h partition 1 relative sector � low word � high word � 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컵컴컴컴컴컴컴컴컴컴컴컵컴컴컴컴컴컴컴컴컴컴캑 � 1CAh partition 1 # sectors � low word � high word � 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컵컴컴컴컴컴쩡컴컴컴컴컵컴컴컴컴컴쩡컴컴컴컴캑 � 1CEh partition 2 begin � boot ind � H � S � cyl � 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컵컴컴컴컴컴탠컴컴컴컴컵컴컴컴컴컴탠컴컴컴컴캑 � 1D2h partition 2 end � syst ind � H � S � cyl � 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컵컴컴컴컴컴좔컴컴컴컴컵컴컴컴컴컴좔컴컴컴컴캑 � 1D6h partition 2 relative sector � low word � high word � 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컵컴컴컴컴컴컴컴컴컴컴컵컴컴컴컴컴컴컴컴컴컴캑 � 1DAh partition 2 # sectors � low word � high word � 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컵컴컴컴컴컴쩡컴컴컴컴컵컴컴컴컴컴쩡컴컴컴컴캑 � 1DEh partition 3 begin � boot ind � H � S � cyl � 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컵컴컴컴컴컴탠컴컴컴컴컵컴컴컴컴컴탠컴컴컴컴캑 � 1E2h partition 3 end � syst ind � H � S � cyl � 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컵컴컴컴컴컴좔컴컴컴컴컵컴컴컴컴컴좔컴컴컴컴캑 � 1E6h partition 3 relative sector � low word � high word � 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컵컴컴컴컴컴컴컴컴컴컴컵컴컴컴컴컴컴컴컴컴컴캑 � 1EAh partition 3 # sectors � low word � high word � 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컵컴컴컴컴컴쩡컴컴컴컴컵컴컴컴컴컴쩡컴컴컴컴캑 � 1EEh partition 4 begin � boot ind � H � S � cyl � 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컵컴컴컴컴컴탠컴컴컴컴컵컴컴컴컴컴탠컴컴컴컴캑 � 1F2h partition 4 end � syst ind � H � S � cyl � 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컵컴컴컴컴컴좔컴컴컴컴컵컴컴컴컴컴좔컴컴컴컴캑 � 1F6h partition 4 relative sector � low word � high word � 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컵컴컴컴컴컴컴컴컴컴컴컵컴컴컴컴컴컴컴컴컴컴캑 � 1FAh partition 4 # sectors � low word � high word � 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컵컴컴컴컴컴쩡컴컴컴컴컵컴컴컴컴컴컴컴컴컴컴켸 � 1FEh signature � hex 55 � hex AA � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컨컴컴컴컴컴좔컴컴컴컴켸 HARD DISK TECHNICAL INFORMATION Boot indicator (boot ind): The boot indicator byte must contain 0 for a non- bootable partition or 80h for a bootable partition. Only one partition can be marked as bootable at a time. System Indicator (sys ind): The sys ind field contains an indicator of the operating system that "owns" the partition. The system indicators are: 00h unknown (unspecified or non-DOS) 01h DOS 12 bit FAT 02h DOS 16 bit FAT Cylinder (CYL) and Sector (S): The 1 byte fields labelled CYL contain the low order 8 bits of the cylinder number - the high order 2 bits are in the high order 2 bits of the sector (S) field. This corresponds with the ROM BIOS interrupt 13h (disk I/O) requirements, to allow for a 10 bit cylinder number. The fields are ordered in such a manner that only two MOV instructions are required to properly set up the DX and CX registers for a ROM BIOS call to load the appropriate boot record (hard disk booting is only possible from the first hard disk in the system, where a BIOS drive number of 80h corresponds to the boot indicator byte). All partitions are allocated in cylinder multiples and begin on sector 1, head 0. EXCEPTION: The partition that is allocated at the beginning of the disk starts at sector 2, to account for the hard disk's master boot record. Relative Sector (rel sect): The number of sectors preceding each partition on the disk is kept in the 4 byte field labelled "rel sect". This value is obtained by counting the sectors beginning with cylinder 0, sector 1, head 0 of the disk, and incrementing the sector, head, and then track values up to the beginning of the partition. This, if the disk has 17 sectors per track and 4 heads, and the second partition begins at cylinder 1, sector 1, head 0,and the partition's starting relative sector is 68 (decimal) - there were 17 sectors on each of 4 heads on 1 track allocated ahead of it. The field is stored with the least significant word first. Number of sectors (#sects): The number of sectors allocated to the partition is kept in the "# of sects" field. This is a 4 byte field stored least significant word first. Signature: The last 2 bytes of the boot record (55AAh) are used as a signature to identify a valid boot record. Both this record and the partition boot record are required to contain the signature at offset 1FEh. The master disk boot record invokes ROM BASIC if no indicator byte reflects a bootable system. When a partition's boot record is given control. It is passed its partition table entry address in the DS:SI registers. DETERMINING FIXED DISK ALLOCATION DOS determines disk allocation using the following formula: D * BPD TS - RS - 컴컴컴컴컴� BPS SPF = 컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴 BPS * SPC CF + 컴컴컴컴컴컴컴 BPC where: TS total sectors on disk RS the number of sectrs at the beginning of the disk that are reserved for the boot record. DOS reserves 1 sector. D The number of directory entries in the root directory. BPD the number of bytes per directory entry. This is always 32. BPS the number of bytes per logical sector. Typically 512, but you can specify a different number with VDISK. CF The number of FATS per disk. Usually 2. VDISK is 1. SPF the number of sectors per FAT. Maximum 64. SPC The number of sectors per allocation unit. BPC the number of bytes per FAT entry. BPC is 1.5 for 12 bit FATs. 2 for 16 bit FATS. To calculate the minimum partition size that will force a 16-bit FAT: CYL = (max clusters * 8)/(HEADS * SPT) where: CYL number of cylinders on the disk max clusters 4092 (maximum number of clusters for a 12 bit FAT) HEADS number of heads on the hard disk SPT sectors per track (normally 17 on MFM) note: DOS 2.0 uses a "first fit" algorithm when allocating file space on the hard disk. Each time an application requests disk space, it will scan from the beginning of the FAT until it finds a contiguous peice of storage large enough for the file. DOS 3.0 keeps a pointer into the disk space, and begins its search from the point it last left off. This pointer is lost when the system is rebooted. This is called the "next fit" algorithm. It is faster than the first fit and helps minimize fragmentation. In either case, if the FCB function calls are used instead of the handle function calls, the file will be broken into peices starting with the first availible space on the disk. Comment to 826. Comment(s). ---------- Better late than never... A partition table entry for the IBM AT is set up as follows: DB drive ; 0 or 80H, 80H marks a bootable, active partition DB head1 ; starting heads DW trksec1 ; starting track/sector (CX value for INT 13) DB system ; see below DB head2 ; ending head DW trksec2 ; ending track/sector DD sector1 ; absolute # of starting sector DD sector2 ; absolute # of last sector The system byte is different for different O/S entries: 1 DOS, 12-bit FAT entries 4 DOS, 16-bit FAT entries DB Concurrent DOS F2 2nd partition for Sperry machines with large disks And so on. There are bytes for XENIX, Prologue and lots of other O/S. Many manufacturers diddle with these system bytes to implement more than 1 DOS partition per disk. The only one I know about who violates the rule that only one DOS partition (1 or 4) per disk may exist is Tandon.