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~