| Offset |
Size |
Description |
|
-24
|
WORD
|
(DOS 3.1+) contents of CX from INT 21/AX=5E01h |
|
-22
|
WORD
|
(DOS ???+) LRU counter for FCB caching |
|
-20
|
WORD
|
(DOS ???+) LRU counter for FCB opens |
|
-18
|
DWORD
|
(DOS ???+) address of OEM function handler (see INT 21/AH=F8h)
FFFFh:FFFFh if not installed or not available |
|
-14
|
WORD
|
(DOS ???+) offset in DOS CS of code to return from INT 21 call |
|
-12
|
WORD
|
(DOS 3.1+) sharing retry count (see AX=440Bh) |
|
-10
|
WORD
|
(DOS 3.1+) sharing retry delay (see AX=440Bh) |
|
-8
|
DWORD
|
(DOS 3.0+) pointer to current disk buffer |
|
-4
|
WORD
|
(DOS 3.0+) pointer in DOS data segment of unread CON input
when CON is read via a handle, DOS reads an entire line, and returns the
requested portion, buffering the rest for the next read. 0000h indicates no
unread input |
|
-2
|
WORD
|
segment of first memory control block (see #01628) |
|
00h
|
DWORD
|
pointer to first Drive Parameter Block (see #01395
at AH=32h) |
|
04h
|
DWORD
|
 first System
File Table (see #01639,#01640,#01641,#01642) |
|
08h
|
DWORD
|
pointer to active CLOCK$ device's header (most recently loaded
driver with CLOCK bit set) |
|
0Ch
|
DWORD
|
pointer to active CON device's header (most recently loaded
driver with STDIN bit set) |
|
---DOS 2.x---
|
|
10h
|
BYTE
|
number of logical drives in system |
|
11h
|
WORD
|
maximum bytes/block of any block device |
|
13h
|
DWORD
|
pointer to first disk buffer (see #01649,#01650) |
|
17h
|
18 BYTEs
|
actual NUL device driver header (not a pointer!)
NUL is always the first device on DOS's linked list of device drivers. (see #01646) |
|
---DOS 3.0---
|
|
10h
|
BYTE
|
number of block devices |
|
11h
|
WORD
|
maximum bytes/block of any block device |
|
13h
|
DWORD
|
pointer to first disk buffer (see #01650,#01652) |
|
17h
|
DWORD
|
pointer to array of current directory structures (see #01643) |
|
1Bh
|
BYTE
|
value of LASTDRIVE command in CONFIG.SYS (default 5) |
|
1Ch
|
DWORD
|
pointer to STRING= workspace area |
|
20h
|
WORD
|
size of STRING area (the x in STRING=x from CONFIG.SYS) |
|
22h
|
DWORD
|
pointer to FCB table |
|
26h
|
WORD
|
the y in FCBS=x,y from CONFIG.SYS |
|
28h
|
18 BYTEs
|
actual NUL device driver header (not a pointer!)
NUL is always the first device on DOS's linked list of device drivers. (see #01646) |
|
---DOS 3.1-3.3---
|
|
10h
|
WORD
|
maximum bytes per sector of any block device |
|
12h
|
DWORD
|
pointer to first disk buffer in buffer chain (see #01650) |
|
16h
|
DWORD
|
pointer to array of current directory structures (see #01643) |
|
1Ah
|
DWORD
|
pointer to system FCB tables (see #01640,#01641,#01642) |
|
1Eh
|
WORD
|
number of protected FCBs (the y in the CONFIG.SYS FCBS=x,y) |
|
20h
|
BYTE
|
number of block devices installed |
|
21h
|
BYTE
|
number of available drive letters (largest of 5, installed
block devices, and CONFIG.SYS LASTDRIVE=). Also size of current directory
structure array. |
|
22h
|
18 BYTEs
|
actual NUL device driver header (not a pointer!)
NUL is always the first device on DOS's linked list of device drivers. (see #01646) |
|
34h
|
BYTE
|
number of JOIN'ed drives |
|
---DOS 4.x---
|
|
10h
|
WORD
|
maximum bytes per sector of any block device |
|
12h
|
DWORD
|
pointer to disk buffer info record (see #01652,#01653)
| Note: |
although the initialization code in IO.SYS uses this pointer,
MSDOS.SYS does not, instead using the hardcoded address of the info
record |
|
|
16h
|
DWORD
|
pointer to array of current directory structures (see #01643,#01644) |
|
1Ah
|
DWORD
|
pointer to system FCB tables (see #01640,#01641,#01642) |
|
1Eh
|
WORD
|
number of protected FCBs (the y in the CONFIG.SYS FCBS=x,y)
(always 00h for DOS 5.0) |
|
20h
|
BYTE
|
number of block devices installed |
|
21h
|
BYTE
|
number of available drive letters; also size of current
directory structure array.
For DOS 4.0-6.0: largest of 5, installed block devices, and CONFIG.SYS
LASTDRIVE=
For DOS 7.x (Windows9X), set to 32 if no LASTDRIVE= or LASTDRIVEHIGH=, else
set to larger of installed block devices and LASTDRIVE(HIGH)= |
|
22h
|
18 BYTEs
|
actual NUL device driver header (not a pointer!)
NUL is always the first device on DOS's linked list of device drivers. (see #01646) |
|
34h
|
BYTE
|
number of JOIN'ed drives |
|
35h
|
WORD
|
pointer within IBMDOS code segment to list of special program
names (see #01662)
(always 0000h for DOS 5.0) |
|
37h
|
DWORD
|
pointer to FAR routine for resident IFS utility functions (see
#01658)
may be called by any IFS driver which does not wish to service functions 20h
or 24h-28h itself |
|
3Bh
|
DWORD
|
pointer to chain of IFS (installable file system) drivers |
|
3Fh
|
WORD
|
the x in BUFFERS x,y (rounded up to multiple of 30 if in EMS) |
|
41h
|
WORD
|
number of lookahead buffers (the y in BUFFERS x,y) |
|
43h
|
BYTE
|
boot drive (1=A:) |
|
44h
|
BYTE
|
flag:
| 01h |
= |
to use DWORD moves (80386+) |
| 00h |
= |
otherwise |
|
|
45h
|
WORD
|
extended memory size in KB |
|
---DOS 5.0-6.0---
|
|
10h
|
39 BYTEs
|
maximum bytes per sector of any block device |
|
37h
|
DWORD
|
pointer to SETVER program list or 0000h:0000h |
|
3Bh
|
WORD
|
(DOS=HIGH) offset in DOS CS of function to fix A20 control
when executing special .COM format |
|
3Dh
|
WORD
|
PSP of most-recently EXECed program if DOS in HMA, 0000h if
low
used for maintaining count of INT 21 calls which disable A20 on return |
|
3Fh
|
8 BYTEs
|
the x in BUFFERS x,y (rounded up to multiple of 30 if in EMS) |
|
---Windows NT DOS Box---
|
|
10h
|
6 BYTEs
|
??? |
|
16h
|
DWORD
|
pointer to array of current directory structures (see #01645) |
|
1Ah
|
6 BYTEs
|
??? |
|
20h
|
BYTE
|
number of block devices installed |
|
21h
|
BYTE
|
number of local drive letters (= installed block devices)
Also size of current directory structure array. |
|
22h
|
18 BYTEs
|
actual NUL device driver header (not a pointer!)
NUL is always the first device on DOS's linked list of device drivers. (see #01646) |
|
---DOS 7.x---
|
|
10h
|
55 BYTEs
|
maximum bytes per sector of any block device |
|
47h
|
25 BYTEs
|
??? |
|
60h
|
BYTE
|
"DOS_FLAG" (see also INT 26) |