An Overview of Disk I/O

A.     The difficulty of doing I/O

1. synchronizing a fast processor with a slow I/O device
2. converting different representation of data (e.g. ASCII digit to twos complement binary)
3. security issues
4. tedious status and error checking makes writing I/O routines tedious and error prone

B.    Sequential Files : sequence of (fixed or variable length) logical records; only current record is accessible
         (accessing record n required accessing previous n-1 records)

1. virtual read and write : need file "handle" and memory address for “record”; uses "pointer” to current record
2. virtual rewind instructions
3. open permits user access to file (i.e create file handle); close breaks connection to file

1. read and write : need file "handle", memory address,  position of record
2. Index Sequential File : separate key to position table (index) maintained

1. physical setup : sectors/tracks/surfaces (r/w heads);  cylinders : same track across all surfaces
2. allocation unit = sector or cluster = adjacent sectors
3. consecutative vs non-consecutive storage allocation units;  non-consecutive allocation requires some sort of file index or linked list system
4. free list of allocation units maintained

1. grouping of files in directories
2. create, delete, rename, change protection operations

MS-DOS  3-1/4 inch Floppy Disk Organization

Hardware Setup :

    80 (40) circular tracks - numbered 0 - 79 (39)

    9  sectors per track (Double Density) numbered 1 - 9
    18 sectors per track (High Density) numbered 1 - 18

        Each sector is 512 bytes = 0.5 Kbytes

    2 heads or sides numbered 0 - 1

    A high-density diskette has 80 x 18 x 2 = 2880 sectors x 0.5 Kbytes/sector = 1440 Kbytes

Logical Setup :

    Boot Sector : Sector 1 - Track 0 - Head 0

        Contains "boot program" and disk characteristics (# of bytes/sector, # of sectors, sector/track, # of heads)

     Note -  A system disk contains 3 files :

          command.com <- resident part of MS-DOS
          ibmbios.com
          ibmdos.dos  ? hidden files - must be first 2 files on disk

    F.A.T. (File Allocation Table) One entry per cluster (1 cluster equals 3 sectors)

                Pointers to clusters containing file. Allows a file to be stored in non-contiguous locations on disk.

    Root Directory 32 byte entries

        filename          8 bytes
        extension         3 bytes
        attribute         1 byte
        reserved         10 bytes
        time              2 bytes
        date              2 bytes
        starting F.A.T.  2 bytes
        file size         4 bytes

        DD diskettes can have 112 entries; HD can have 224

    Data Area (including subdirectories)

A Short List of MS-DOS Int 21h Virtual I/O Interrupt Services

1. Path name to file given as ASCIIZ Strings - strings terminating in Null character (ASCII 0)
2. File Handles (16 bit) - used by MS-DOS to identify file
3. File Pointers  (maintained by operating system) - points to next byte in file
4. Error Return Codes : CF set on error; AX contains code

INT 21h Service 3Ch - Create a File Handle

 Initialize :   AH <- 3Ch
                CX : file attributes
                    00 - normal attributes
                    01 - read only
                    02 - hidden
                    04 - system
                    08 - volume ID name
                    10h - subdirectory name
                    20h - archive
                DS:DX <- pointer to ASCIIZ path name

 Returns:  if CF = 0 then AX = file handle
           if CF = 1 then AX = error code
                    03 - invalid path
                    04 - too many open files
                    05 - access denied
                        (directory full, matching file exists with restrictive attributes)

 Note : If file with same name exits, it is overwritten

INT 21h Service 3Dh - Open a File

        Initialize: AH <- 3Dh
                    AL : Access Code
                        00 - read
                        01 - write
                        02 - read/write
                    DS:DX <- pointer to ASCIIZ path name

        Returns:    if CF = 0 then AX = file handle
                    if CF = 1 then AX = error code
                        01 - invalid function code
                        02 - file not found
                        03 - invalid path
                        04 - too many open files
                        05 - access denied
                        0Ch - invalid access

 INT 21h Service 3Eh - Close a File

        Initialize: AH <- 3Eh
                    BX <- file handle

        Returns:    if CF = 0 success
                    if CF = 1 then AX = error code
                        06 - invalid handle

INT 21h Service 3Fh - Read from a File or Device

        Initialize: AH <- 3Fh
                    BX <- file handle
                    CX <- number of bytes to read
                    DS:DX <- pointer to read buffer

        Returns:    if CF = 0 then AX = number of bytes read
                    if CF = 1 then AX = error code
                        05 - read access denied
                        06 - invalid handle

    Note: Automatically updates File Pointer

 INT 21h Service 40h - Write to a File or Device

        Initialize: AH <- 40h
                    BX <- file handle
                    CX <- number of bytes to write
                    DS:DX <- pointer to write buffer

        Returns:    if CF = 0 then AX = number of bytes written
                    if CF = 1 then AX = error code
                        05 - write access denied
                        06 - invalid handle

 Note: If CF = 0 on return, AX should equal CX; otherwise an error has occurred.

 INT 21h Service 42h - Move File Read/Write Pointer

        Initialize: AH <- 42h
                    AL <- origin of move
                        00 - beginning of file + offset
                        01 - current location of file pointer + offset
                        02 - end of file + offset
                    BX <- file handle
                    CX:DX <- distance to move (in bytes) - offset

        Returns:    if CF = 0 then DX:AX = new pointer location
                    if CF = 1 then AX = error code
                        01 - AL out of range
                        06 - invalid handle

Int 21h DOS Services for Directory Management

    See Services 39h, 3Ah, 3Bh, 41H, 47h, 4Eh, 4Fh, etc.

Other DOS Based Low-Level Disk I/O Interrupts

Absolute Disk I/O: Sectors accessed by relative position where

relative sector number = (track x sectors/track) + (sector - 1)

INT 25h - Absolute Read

 =< 32M: [MS-DOS 3.3 or earlier)

    Initialize: AL <- drive number ( 0 = A, 1 = B, etc)
                CX <- number of sectors to read
                DX <- relative sector number
                DS:BX <- pointer to Disk Transfer Area

    Returns:    if CF = 0 then success
                if CF = 1 then AX = error code

 > 32M: [MS-DOS 4.0+]

    Initialize  AL <- drive number
                BX <- pointer to 10 byte parameter list
                    32 bit dword specifices 1st sector
                    16 bit word specifices number of sectors to read
                    32 bit dword points to DTA
                CX = -1 (indicates extended >32M) format
 

 INT 26h - Absolute Write

        counter part to INT 25h

BIOS Based Disk I/O - INT 13h - handles all Disk I/O in terms of track and sector numbers

Int 13h Service 02 - Read Floppy Disk Sectors

        Initialize  AH <- 02h
                    AL <- number of sectors to read (1 - 9)
                    CH <- track number (0 - 39)
                    CL <- starting sector number (1 - 9)
                    DH <- head number (0 - 1)
                    DL <- drive number (0 - 3)
                    ES:BX <- pointer to disk buffer

 Int 13h Service 03 - Write Floppy Disk Sectors

        Initialize  AH <- 03h
                    AL <- number of sectors to write (1 - 9)
                    CH <- track number (0 - 39)
                    CL <- starting sector number (1 - 9)
                    DH <- head number (0 - 1)
                    DL <- drive number (0 - 3)
                    ES:BX <- pointer to disk buffer

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