Int 21/AX=5D06h

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DOS 3.0+ internal - GET ADDRESS OF DOS SWAPPABLE DATA AREA

AX = 5D06h

Return:
CF set on error AX = error code (see #01680) CF clear if successful DS:SI -> nonreentrant data area (includes all three DOS stacks) (critical error flag is first byte) (see #01687) CX = size in bytes of area which must be swapped while in DOS DX = size in bytes of area which must always be swapped

Notes: The Critical Error flag is used in conjunction with the InDOS flag (see AH=34h) to determine when it is safe to enter DOS from a TSR. Setting CritErr flag allows use of functions 50h/51h from INT 28h under DOS 2.x by forcing use of correct stack. Swapping the data area allows reentering DOS unless DOS is in a critical section delimited by INT 2A/AH=80h and INT 2A/AH=81h,82h. Under DOS 4.0, AX=5D0Bh should be used instead of this function. SHARE and other DOS utilities consult the byte at offset 04h in the DOS data segment (see INT 2F/AX=1203h) to determine the SDA format

in use:
00h = DOS 3.x, 01h = DOS 4.0-6.0, other = error.. DR DOS 3.41+ supports this function, but the SDA format beyond the first 18h bytes is completely different from MS-DOS

BUG: Calling this function with certain values in DX crashes Novell DOS 7.0 prior to Update 14

See Also: AX=5D0Bh - INT 2A/AH=80h - INT 2A/AH=81h - INT

Format of DOS 3.10-3.30 Swappable Data Area: Offset Size Description (Table 01687) -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 if 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)

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 if read, 01h if 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

Format of CLOCK$ transfer record: Offset Size Description (Table 01688) 00h WORD number of days since 1-Jan-1980 02h BYTE minutes 03h BYTE hours 04h BYTE hundredths of second 05h BYTE seconds

Category: DOS Kernel - Int 21h - D

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