da Silva, Joe wrote:
>
> DOS reserves five special file handles for use by itself and
> applications programs. They are:
>
> |-----------------------------------------------------------------------|
> | 0000h | STDIN | standard input device (input can be redirected) |
> | 0001h | STDOUT | standard output device (output can be redirected) |
> | 0002h | STDERR | standard error output device (output cannot be |
> | | | redirected) |
> | | | NOTE: DOS opens STDERR for both writing and reading. |
> | | | Since STDIN can be redirected, using STDERR to read |
> | | | the keyboard is a reliable way to ensure that your |
> | | | program is actually |
> | | | reading the keyboard, if that's what you want to do. |
> | 0004h | STDAUX | standard auxiliary device |
> | 0005h | STDPRN | standard printer device (PRN, normally LPT1) |
> |-----------------------------------------------------------------------|
>
> These handles are predefined by DOS and can be used by an
> application program. They do not need to be opened by a program,
> although a program can close these handles. STDIN should be treated
> as a read-only file, and STDOUT and STDERR should be treated as write-
> only files. STDIN and STDOUT can be redirected. All handles
> inherited by a process can be redirected, but not at the command line.
>
> [BTW, I was puzzled by that last line at first, but all
> I think it is saying, is just that only STDIN and STDOUT
> can be redirected via the command line ... Joe]
I think it is saying they are re-directed by inheritance, and you can't
change it.
BTW, I thought it might be of interest to see the difference in overhead
required for handle console text output via a file handle:
Here is the "standard" character method:
mov dx,offset signon
mov ah,09h
int 21
Here is the file handle method as used by a respected co. we DON'T complain
about and dissassembled by me:
L0131H: MOV AX,OFFSET SIGNON ;Sign on
CALL L0209H ;Writes file or device
L0209H: MOV SI,AX
CALL L0234H
MOV BX,DS:[OFFSET WHANDLE]
MOV DX,SI
XCHG CX,AX
MOV AH,64H ;Write to file/device.
INT 21H
JMP L0252H
L0234H: CALL L0242H ;*WHY? Does everything get trashed ?
PUSH SI
XCHG SI,AX
MOV DX,0FFFFH
L0239H: LODSB ;Load AL with byte at (SI)+-.
INC DX ;Count the bytes
TEST AL,AL
JNZ L0239H
XCHG DX,AX
POP SI
RET
L0242H: POP DS:[OFFSET SAVERTN] ;SAVE RETURN ADDRESS
PUSH ES
PUSH BP
PUSH SI
PUSH DI
PUSH BX
PUSH CX
PUSH DS
POP ES
JMP DS:[OFFSET WORD PTR SAVERTN] ;YUP
L0252H: POP CX
POP BX
POP DI
POP SI
POP BP
POP ES
RET
Cheers.
- Clarence Verge
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