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Table 1687

Format of DOS 3.10-3.30 Swappable Data Area:
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 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
SeeAlso: #01690


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  Copyright 2000   by Ralf Brown     Updated Jul 2000