|
| |
Interrupt 27h Terminate And Stay Resident
|
Input |
DOS1 |
Y |
DOS2 |
Y |
DOS3 |
Y |
DOS5 |
Y |
Output |
|
AX |
|
|
|
|
AX |
|
BX |
|
|
|
|
BX |
|
CX |
|
|
|
|
CX |
|
DX |
Last
Program Byte + 1 |
|
|
DX |
|
CS |
Current
Program Segment |
|
CF |
This vector is used by programs that are to remain resident when
COMMAND.COM regains control.
After initializing itself, the program must set DX to its last
address plus one relative to the program's initial DS or ES value
(the offset at which other programs can be loaded), then execute
interrupt 27h. DOS then considers the program as an extension of
itself, so the program is not overlaid when other programs are
executed. This is useful for loading programs such as utilities
and interrupt handlers that must remain resident.
Notes:
1) This interrupt must not be used by .EXE programs that are
loaded into the high end of memory.
2) This interrupt restores the interrupt 22h, 23h, and 24h
vectors in the same manner as interrupt 20h. Therefore, it
cannot be used to install permanently resident Ctrl-Break or
critical error handler routines.
3) The maximum size of memory that can be made resident by this
method is 64K.
4) Memory can be more efficiently used if the block containing
a copy of the environment is deallocated before terminating.
This can be done by loading ES with the segment contained in
2Ch of the PSP, and issuing function call 49h (Free
Allocated Memory).
5) DOS function call 4Ch allows a program to pass a completion
code to DOS, which can be interpreted with processing (see
function call 31h).
6) Terminate and stay resident programs do not close files.
7) Int 21, function 31h is the preferred method to cause a
program to remain resident because this allows return
information to be passed and allows a program larger than
64K to remain resident.
See Also Func/31h Func/00h INT/20h Func/4Ch |