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Format of DOS 3.10-3.30 Swappable Data Area

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Offset Size Description

-34

BYTE

(DOS 3.10+) printer echo flag
00h = off
FFh = active

-31

BYTE

(DOS 3.30) current switch character

-30

BYTE

current memory allocation strategy (see AH=58h)

-28

BYTE

(DOS 3.30) incremented on each INT 21/AX=5E01h call

-27

16 BYTEs

(DOS 3.30) machine name set by INT 21/AX=5E01h

-11

5 WORDs

zero-terminated list of offsets which need to be patched to enable critical-section calls (see INT 2A/AH=80h)

-1

BYTE

unused padding

---start of actual SDA---

00h

BYTE

critical error flag ("ErrorMode")

01h

BYTE

InDOS flag (count of active INT 21 calls)

02h

BYTE

drive on which current critical error occurred, or FFh (DR DOS sets to drive number during INT 24, 00h otherwise)

03h

BYTE

locus of last error

04h

WORD

extended error code of last error

06h

BYTE

suggested action for last error

07h

BYTE

class of last error

08h

DWORD

ES:DI pointer for last error

0Ch

DWORD

current DTA (Disk Transfer Address)
Note: may point into SDA during the DOS EXEC function (see AH=4Bh), so programs which swap the SDA must be prepared to move the DTA to a private buffer if they might be invoked during an EXEC

10h

WORD

current PSP

12h

WORD

stores SP across an INT 23

14h

WORD

return code from last process termination (zerod after reading with AH=4Dh)

16h

BYTE

current drive

17h

BYTE

extended break flag

---remainder need only be swapped if in DOS---

18h

WORD

value of AX on call to INT 21

1Ah

WORD

PSP segment for sharing/network

1Ch

WORD

network machine number for sharing/network (0000h = us)

1Eh

WORD

first usable memory block found when allocating memory

20h

WORD

best usable memory block found when allocating memory

22h

WORD

last usable memory block found when allocating memory

24h

WORD

memory size in paragraphs (used only during initialization)

26h

WORD

last entry checked during directory search

28h

BYTE

flag: INT 24 returned Fail

29h

BYTE

flags: allowable INT 24 actions (passed to INT 24 in AH)

2Ah

BYTE

directory flag
00h = directory
01h = file

2Bh

BYTE

flag:
FFh = Ctrl-Break termination
00h = otherwise

2Ch

BYTE

flag: allow embedded blanks in FCB

2Dh

BYTE

padding (unused)

2Eh

BYTE

day of month

2Fh

BYTE

month

30h

WORD

year - 1980

32h

WORD

number of days since 01jan1980

34h

BYTE

day of week
0 - Sunday
1 - Monday
2 - Tuesday
3 - Wednesday
4 - Thursday
5 - Friday
6 - Saturday

35h

BYTE

flag: console swapped during read from device

36h

BYTE

flag: safe to call INT 28 if nonzero

37h

BYTE

flag: if nonzero, INT 24 Abort turned into INT 24 Fail (set only during process termination)

38h

26 BYTEs

device driver request header (see #02597 at INT 2F/AX=0802h)

52h

DWORD

pointer to device driver entry point (used in calling driver)

56h

22 BYTEs

device driver request header for I/O calls

6Ch

14 BYTEs

device driver request header for disk status check

7Ah

DWORD

pointer to device I/O buffer???

7Eh

WORD

???

80h

WORD

???

82h

BYTE

type of PSP copy
00h = simple for INT 21/AH=26h
FFh = make child

83h

BYTE

padding (unused)

84h

3 BYTEs

24-bit user number (see AH=30h)

87h

BYTE

OEM number (see #01394 at AH=30h)

88h

WORD

offset to error code conversion table for INT 25/INT 26

8Ah

6 BYTEs

CLOCK$ transfer record (see #01688)

90h

BYTE

device I/O buffer for single-byte I/O functions

91h

BYTE

padding??? (unused)

92h

128 BYTEs

buffer for filename

112h

128 BYTEs

buffer for filename

192h

21 BYTEs

findfirst/findnext search data block (see #01626 at AH=4Eh)

1A7h

32 BYTEs

directory entry for found file (see #01352 at AH=11h)

1C7h

81 BYTEs

copy of current directory structure for drive being accessed

218h

11 BYTEs

FCB-format filename for device name comparison

223h

BYTE

terminating NUL for above filename

224h

11 BYTEs

wildcard destination specification for rename (FCB format)

22Fh

BYTE

terminating NUL for above spec

230h

BYTE

???

231h

WORD

destination file/directory starting sector

233h

5 BYTEs

???

238h

BYTE

extended FCB file attribute

239h

BYTE

type of FCB
00h = regular
FFh = extended

23Ah

BYTE

directory search attributes

23Bh

BYTE

file open/access mode

23Ch

BYTE

file found/delete flag
bit 0 : file found
bit 4 : file deleted

23Dh

BYTE

flag: device name found on rename, or file not found

23Eh

BYTE

splice flag (file name and directory name together)

23Fh

BYTE

flag indicating how DOS function was invoked
00h = direct INT 20/INT 21
FFh = server call AX=5D00h

240h

BYTE

sector position within cluster

241h

BYTE

flag: translate sector/cluster
00h = no
01h = yes

242h

BYTE

flag:
00h = read
01h = write

243h

BYTE

current working drive number

244h

BYTE

cluster factor

245h

BYTE

flag: cluster split mode

246h

BYTE

line edit (AH=0Ah) insert mode flag (nonzero = on)

247h

BYTE

canonicalized filename referred to existing file/dir if FFh

248h

BYTE

volume ID flag

249h

BYTE

type of process termination (00h-03h) (see AH=4Dh)

24Ah

BYTE

file create flag (00h = no, search only)

24Bh

BYTE

value with which to replace first byte of deleted file's name (normally E5h, but 00h as described under INT 21/AH=13h)

24Ch

DWORD

pointer to Drive Parameter Block for critical error invocation
temp: used during process termination

250h

DWORD

pointer to stack frame containing user registers on INT 21

254h

WORD

stores SP across INT 24

256h

DWORD

pointer to DOS Drive Parameter Block for ???

25Ah

WORD

saving partial cluster number

25Ch

WORD

temp: sector of work current cluster

25Eh

WORD

high part of cluster number (only low byte referenced)

260h

WORD

??? temp

262h

BYTE

Media ID byte returned by AH=1Bh,1Ch

263h

BYTE

padding (unused)

264h

DWORD

pointer to device header when filename is character device

268h

DWORD

pointer to current SFT

26Ch

DWORD

pointer to current directory structure for drive being accessed

270h

DWORD

pointer to caller's FCB

274h

WORD

number of SFT to which file being opened will refer

276h

WORD

temporary storage for file handle

278h

DWORD

pointer to a JFT entry in process handle table (see #01378 at AH=26h)

27Ch

WORD

offset in DOS DS of first filename argument

27Eh

WORD

offset in DOS DS of second filename argument

280h

WORD

offset of last component in pathname or FFFFh

282h

WORD

offset of transfer address to add

284h

WORD

last relative cluster within file being accessed

286h

WORD

temp: absolute cluster number being accessed

288h

WORD

directory sector number

28Ah

WORD

??? current cluster number

28Ch

WORD

current relative sector number within file

28Eh

WORD

current sector number

290h

WORD

current byte offset within sector

292h

DWORD

current offset in file

296h

DWORD

temp: file byte count

29Ah

WORD

temp: file byte count

29Ch

WORD

free file cluster entry

29Eh

WORD

last file cluster entry

2A0h

WORD

next file cluster number

2A2h

DWORD

number of bytes appended to file

2A6h

DWORD

pointer to current work disk buffer

2AAh

DWORD

pointer to working SFT

2AEh

WORD

used by INT 21 dispatcher to store caller's BX

2B0h

WORD

used by INT 21 dispatcher to store caller's DS

2B2h

WORD

temporary storage while saving/restoring caller's registers

2B4h

DWORD

pointer to prev call frame (offset 250h) if INT 21 reentered
also switched to for duration of INT 24

2B8h

21 BYTEs

FindFirst search data for source file(s) of a rename operation (see #01626 at AH=4Eh)

2CDh

32 BYTEs

directory entry for file being renamed (see #01352 at AH=11h)

2EDh

331 BYTEs

critical error stack

403h

35 BYTEs

scratch SFT

438h

384 BYTEs

disk stack (functions greater than 0Ch, INT 25,INT 26)

5B8h

384 BYTEs

character I/O stack (functions 01h through 0Ch)

---DOS 3.2, 3.3x only---

738h

BYTE

device driver lookahead flag (usually printer) (see AH=64h"DOS 3.2+")

739h

BYTE

volume change flag

73Ah

BYTE

flag: virtual open

73Bh

BYTE

???

Note:

The only fields which remain valid BETWEEN calls to INT 21h are those in the initial "swap-always" portion of the SDA.

See Also:

#01690