DOS Variables Block (List of Lists)
This is the undocumented format of the structure whose address is returned
in ES:BX by the undocumented DOS fn 52H. This data can be used to learn
low-level information such as how many drive IDs are recognized and the
address of the base MemBlockRec.
Offset Size Contents
▀▀▀▀▀▀ ▀▀▀▀ ▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀
-2 2 wBaseMemSeg ES:[BX-2] is seg of first Memory Control Block
+0 4 prBaseDPB address of first Drive Parameter Block (DPB)
+4 4 pHandleTbl address of first system file table
+8 4 prClockDev address of CLOCK device header
+0cH 4 prConDev address of CON device header
+10H 2 wMaxSect maximum sector size for block devices
+12H 4 pbCache address of cache head buffer
+16H 4 parDirs address of the CurDirRec structure array
+1aH 4 prFcbTbl address of system FCB file table
+1eH 2 wFcbCnt size of FCB table file table
+20H 1 bDriveCnt number of drives in system
+21H 1 bLastDrive corresponds to Config.sys LASTDRIVE
34 size of a DOSVarsRec
+22H 18 rNulDevHdr the DevHdrRec for the NUL device (the start of
device driver chain).
wBaseMemSeg The segment address of the "base MemBlockRec" This is the
lowest allocation in memory. It may be owned by DOS or
Note: This value is located two bytes before the structure
itself; i.e., after calling fn 52H, it is at ES:[BX-2].
prBaseDPB The first Drive Parameter Block. You can trace the DPB chain
by accessing the pfrNextDPB field of each structure.
pHandleTbl Internal info on each open file (?)
prClockDev Address of the DevHdrRec for the CLOCK device
prConDev Address of the DevHdrRec for the CON device
wMaxSect Largest allowed sector size for block devices (?)
pbCache FAR address of DOS's internal disk buffers
parDirs FAR address of an array of 88-byte CurDirRec structures. The
first item in each structure is an ASCIIZ▲ string that names
the current default directory for one drive. There are
bDriveCnt such records, placed sequentially in memory.
prFcbTbl Address of the system FCBRec file table (?)
wFcbCnt Number of FCBRecs in the system FCB table
bDriveCnt Number of drives (including logical drives) known to the
system (does not include network drives).
bLastDrive A count of the elements in the list pointed to by pparDirs.
For instance, if you specified LASTDRIVE=Z in your CONFIG.SYS
file, then this value will be 24 (18H).
rNulDevHdr is the actual 18-byte DevHdrRec of the NUL device. This is
the start of the device driver chain. You can locate all
device drivers by examining the chain of far pointers whose
first link is at ES:[BX+22H]. It is a 4-byte pointer to the
Notes: ■ You can use the wBaseMemSeg as the segment address to start
tracing the memory allocation chain. You can recognize a PSP
since it begins with an INT 20H (ie, cd 20). At offset 2cH of
a PSP is the address of the DOS environment segment for that
process (from which you may be able to obtain the fill filespec
of the program which owns it).
■ This structure has apparently been intact since DOS 2.0, but
Microsoft continues to leave it undocumented as late as 6.0.
See Also: System Information Functions
fn 52H (get DOS vars address)