Online 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
[helppc.bosbyte.nl] INVARS