add and update 21.4400, 21.4401, MS-DOS v4 Handle_Fail_I24

source/Interrupt List/INT 21 DOS Function Calls/INT 214400 DOS 2 IOCTL GET DEVICE INFORMATION.txt

@@ -9,11 +9,13 @@ INT 21 - DOS 2+ - IOCTL - GET DEVICE INFORMATION
 	BX = handle
 Return: CF clear if successful
 	    DX = device information word (see #01423)
-	    AX destroyed
+	    AX destroyed (= DX in MS-DOS v4.00, v4.01, v5, v7.10)
 	CF set on error
 	    AX = error code (01h,05h,06h) (see #01680 at AH=59h/BX=0000h)
 Notes:	value in DH corresponds to high byte of device driver's attribute word
-	  if handle refers to a character device
+	  if handle refers to a character device.
+	DH appears to be all-zeroes if handle refers to a disk file, at least
+	  on MS-DOS v4.00, v4.01, v5, and v7.10
 	Novell NetWare reportedly does not return a drive number in bits 5-0
 	  for a disk file
 	this function was not supported by Digital Research's DOS Plus
@@ -22,6 +24,7 @@ SeeAlso: AX=4401h,INT 2F/AX=122Bh
 Bitfields for device information word:
 Bit(s)	Description	(Table 01423)
  character device
+  15	set (indicates that a device is a character device)
   14	device driver can process IOCTL requests (see AX=4402h"DOS 2+")
   13	output until busy supported
   11	driver supports OPEN/CLOSE calls
@@ -34,14 +37,24 @@ Bit(s)	Description	(Table 01423)
    2	NUL device
    1	standard output
    0	standard input
- disk file
+ disk file (internal, not returned(?) by function AX=4400h)
   15	file is remote (DOS 3.0+)
   14	don't set file date/time on closing (DOS 3.0+)
   11	media not removable
-   8	(DOS 4 only) generate INT 24 if no disk space on write or read past
-	  end of file
+   8	(DOS 4 only) generate INT 24 if no disk space on write disk full
+	  or read past end of file, equate is called "Handle_Fail_I24"
+ disk file (returned by function AX=4400h)
    7	clear (indicates file)
    6	file has not been written
   5-0	drive number (0 = A:)
+Notes:	bit 8 can be set on MS-DOS v4, both for disk files or character
+	  devices, using AX=4401h but it apparently cannot be reset
+	  using any system call.
+	bit 8 seen set with no AX=4401h call on dosemu2 mfs redirector.
+	  this is considered a bug in dosemu2, reported on 2026-10-08 on
+	  a change added 2019-10-16.
+	bit 8 is never returned from the SFT entry on AX=4400h. for disk
+	  files, DH is always all-zeroes. for character devices, DH is
+	  always set from the device header's attributes, not the SFT.
+	  tested on MS-DOS v4.00, v4.01, v5, v7.10 as yet.
 SeeAlso: INT 29
-

source/Interrupt List/INT 21 DOS Function Calls/INT 214401 DOS 2 IOCTL SET DEVICE INFORMATION.txt

@@ -6,12 +6,25 @@
 
 INT 21 - DOS 2+ - IOCTL - SET DEVICE INFORMATION
 	AX = 4401h
-	BX = handle (must refer to character device)
+	BX = handle (must refer to character device on MS-DOS v5 and v7.10)
 	DX = device information word (see #01423)
-	    (DH must be zero for DOS version prior to 6.x)
+	    (DH must be zero for MS-DOS v5 and v7.10 at least)
+	    (DH may be zero or one for MS-DOS v4.00 and v4.01)
 Return: CF clear if successful
+	    (undocumented) AL = prior SFT flags low byte
 	CF set on error
 	    AX = error code (01h,05h,06h,0Dh) (see #01680 at AH=59h/BX=0000h)
 Notes:	this function was not supported by Digital Research's DOS Plus
+	calling this function on character devices will set the SFT flags
+	  low byte to whatever was passed in DL, except that bit 7 is always
+	  forced set so the SFT entry remains marked as a character device.
+	on MS-DOS v4.00 and v4.01, the handle may refer to either a disk
+	  file or a character device. disk files only use the low bit of DH,
+	  which can set "Handle_Fail_I24", ignoring what was passed in DL.
+	  for character devices, the low bit of DH can be set to set the
+	  "Handle_Fail_I24" flag. there's no way to clear "Handle_Fail_I24".
+	errors are checked as follows: error 06h if invalid handle, then
+	  error 0Dh if invalid bits set in DH, then error 01h if on MS-DOS
+	  v5 or v7.10 and a disk file handle was passed.
 SeeAlso: AX=4400h,INT 2F/AX=122Bh
-
+

source/Interrupt List/INT 21 DOS Function Calls/INT 2152 DOS 2 internal SYSVARS GET LIST OF LISTS.txt

@@ -596,6 +596,7 @@ Offset	Size	Description	(Table 01642)
 			bit 13 set if named pipe
 			bit 12 set if no inherit
 			bit 11 set if network spooler
+			bit 8  "Handle_Fail_I24", MS-DOS v4 AX=4401h can set
 			bit 7  set if device, clear if file (only if local)
 			bits 6-0 as for AX=4400h
 	 07h	DWORD	pointer to device driver header if character device