HelpPC 2.10 Quick Reference Utility Copyright 1991 David Jurgens

           INT 21,52 - Get Pointer to DOS "INVARS"  (Undocumented)

       AH = 52h

       on return:
       ES:BX = pointer to DOS "invars", a table of pointers used by DOS.
               Known "invars" fields follow (varies with DOS version):

       Offset Size              Description

        -12   word   sharing retry count (DOS 3.1-3.3)
        -10   word   sharing retry delay  (DOS 3.1-3.3)
         -8   dword  pointer to current disk buffer (DOS 3.x)
         -4   word   pointer in DOS code segment of unread CON input;
                     0 indicates no unread input (DOS 3.x)
         -2   word   segment of first Memory Control Block (MCB)
         00   dword  pointer to first DRIVE PARAMETER TABLE (A:) in chain
         04   dword  pointer to DOS System File Table (SFT)
         08   dword  pointer to $CLOCK device driver
         0C   dword  pointer to CON device driver
         10   byte   number of logical drives in system
         11   word   maximum bytes/block of any block device
         13   dword  pointer to DOS cache buffer header
         17 18bytes  NUL device header, first 4 bytes of device header
                     point to the next device in device chain


                         DOS 3.0 Specific Information

       Offset Size             Description

         10   byte   number of block devices
         11   word   max sector size used by DOS prior to loading block
                     device drivers;  limits allowable device sector size
         13   dword  pointer to DOS cache buffer header
         17   dword  pointer to drive information table
         1B   byte   value of LASTDRIVE command in CONFIG.SYS
         1C   dword  pointer to STRING= workspace area
         20   word   size of STRING area (CONFIG.SYS value of STRING=x)
         22   dword  FCB file table pointer
         26   word   number of protected FCBs
         28  18bytes NUL device header, first 4 bytes of device header
                     point to the next device in device chain


                      DOS 3.1-3.3  Specific Information

       Offset Size              Description

         10   word   max sector size used by DOS prior to loading block
                     device drivers;  limits allowable device sector size
         12   dword  pointer to DOS cache buffer header
         16   dword  pointer to drive information table
         1A   dword  FCB file table pointer
         1E   word   number of protected FCBs
         20   byte   number of block devices
         21   byte   value of LASTDRIVE command in CONFIG.SYS
         22  18bytes NUL device header, first 4 bytes of device header
                     point to the next device in device chain
         34   byte   number of JOIN'ed drives


                        DOS 4.x  Specific Information

       Offset Size             Description

         10   word   max sector size used by DOS prior to loading block
                     device drivers;  limits allowable device sector size
         12   dword  pointer to DOS cache buffer header
         16   dword  pointer to drive information table
         1A   dword  FCB file table pointer
         1E   word   number of protected FCBs
         20   byte   number of block devices
         21   byte   value of LASTDRIVE command in CONFIG.SYS (default 5)
         22  18bytes NUL device header, first 4 bytes of device header
                     point to the next device in device chain
         34   byte   number of joined drives
         35   word   pointer within IBMDOS code segment to list of special
                     program names
         37   dword  pointer to resident IFS utility function
         3B   dword  pointer to chain of installable file system drivers
         3F   word   the x in BUFFERS x,y (rounded up to multiple of 30
                     if EMS is used)
         41   word   the y in BUFFERS x,y
         43   byte   boot drive (1=A:)
         44   byte   ???
         45   byte   extended memory size in K bytes


       - see Bibliography references for Bernd Schemmer & "Undocumented DOS"
       - see   INDOS  INT 21,5D

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