Function 4400h Get Device Data
Returns information about a handle, such as whether it identifies
a file or a device.
Entry AX = 4400h
BX = Device or file handle
Return DX = Device information
AX changed
or
AX = Error code, if CF is set
| 01h Invalid function
| 05h Access denied
| 06h Invalid handle
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The information returned in the DX register is:
Disk file (DX bit 7 = 0)
Bit Description
0-5 Drive number (0 = A, 1 = B, etc.)
6 1 = File has not been written to
7 0 = Handle identifies a file (not a device)
8-15 Zero
Device (DX bit 7 = 1)
Bit Description
0 1 = Console input device
1 1 = Console output device
2 1 = Null device
3 1 = Clock device
4 1 = Special device
5 1 = Binary mode, 0 = ASCII mode
6 0 = EOF returned if device is read
7 1 = Handle identifies a device (not a file)
8-15 Identical to high 8 bits of the dhAttribute field
in the DeviceHeader structure for the device
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Input and Output Modes
I/O modes determine how character devices process input and
output. DOS has two I/O modes: ASCII and binary (sometimes called
"cooked" and "raw", respectively).
The chief difference between these two modes is the way in which
DOS processes control characters. In ASCII mode, DOS checks for
control characters as it processes input or output for a device.
If it encounters a control character, it removes the character
from the input and carries out its corresponding action:
Ctrl-C Passes control to the Ctrl-C exception handler.
Subsequent actions depend on the current handler;
the default handler terminates the program.
Ctrl-P Copies all subsequent input characters, up to the
next Ctrl-P, to the printer device.
Ctrl-S Suspends further output to the device. The next input
character restores output.
Ctrl-Z Marks the end of the file. Subsequent calls to Int
21h function 3Fh (Read File or Device) return zero
bytes.
In ASCII mode, using function 3Fh (Read File or Device) to read
from the standard input device, DOS reads characters from the
keyboard and copies the characters to standard output. It checks
for control characters (ctrl-c etc.) and for the special line
editing keys (F1 thru F6, backspace, left, right, del, ins, esc,
and enter); it one is found, the corresponding (editing) action is
carried out and the control character/editing key code is removed
from the input.
In binary mode, DOS reads the exact number of characters requested
by the program. It does not copy characters to the screen, nor
does it process editing keys and control characters. Instead, it
reads all characters as input.
In ASCII mode, using function 40h (Write File or Device) to write
to the standard output device, DOS sends all characters to the
screen, checks at the keyboard for control characters as it writes
and, if it finds one, carries out its corresponding action. Tab
characters (ASCII 09h) are expanded to space characters based on
8-space tab settings. DOS continues to write characters to the
screen until it has sent the requested number of characters or
reached an end-of-file character (ASCII 1Ah).
In binary mode, DOS writes the exact number of characters
requested by the program. It does not process control characters
(except CR and LF characters), expand tab characters, or stop
writing at an end-of-file character.
By default, the DOS I/O mode is ASCII. A program can determine or
change the current I/O mode for a device by using functions 4400h
or 4401h.
The I/O mode is a property of the device handle and affects the
input and output only of those programs that own the handle. Note,
however, that the standard devices are shared by all programs. If
a program needs to change the device attributes of a standard
device, it should open the device using function 3Dh (Open File
with Handle) using its device name and an appropriate access code.
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Example
; Test redirection of standard input/output
; Entry : Handle = 0 (StdIn) or 1 (StdOut)
; Exit : cf = 1 Error on DOS call
; else zf=0 Device is redirected
; zf=1 Device is not redirected
; Used : ax,bx,dx,flags
MACRO isDeviceRedirected Handle
local ahead
mov bx,Handle ; StdIn (Stdout)
mov ax,4400h ; Get Device Data
int 21h ; using DOS
jc ahead ; cf set on error
mov al,81h + Handle ; 81h (82h)
and dl,al ; Isolate bits 7,0 (7,1)
cmp al,dl ; zf=1 if original device
ahead: ; attributes, cf=0
ENDM