Interrupt 27h Terminate And Stay Resident (0:009Ch) (obsolete, see Int\21f\31) This vector is used by programs that are to remain resident on exit. 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 Int\27. 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. entry CS current program segment DX last program byte + 1 return none note 1) This interrupt must not be used by .EXE programs that are loaded into the high end of memory. 2) This interrupt restores the Int\22, Int\23, and Int\24 vectors in the same manner as Int\20. 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 int 27 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 Int\21f\49 (Free Allocated Memory). Interrupt 27h Terminate And Stay Resident 5) DOS call Int\21f\4C allows a program to pass a completion code to DOS, which can be interpreted with processing (see DOS call Int\21f\4D). 6) Terminate and stay resident programs do not close files. 7) Int\21f\31 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. o Ref: Programmers Problem Solver 1986 Robert Jourdain: At the time that int 27h or Int\20 is executed, CS must point to the start of the program segment prefix for this function to work properly. In COM programs, CS is initially set to this position, and so you need simply end the program with int 27h. In EXE programs, on the other hand, CS initially points to the first byte following the PSP (that is , to 100h). In the normal termination of an EXE program, the final RET instruction pops off the stack the first values pushed on to the stack: PUSH DS;MOV AX,0;PUSH AX. Since DS initially points to the bottom of the PSP, when these values are popped the instruction pointer is directed to offset 0 in the PSP, which is initialized to contain the code for int 20h. Int 20h is then executed, and it is the standard function for terminating programs and returning control to DOS. To make int 27h work in an EXE program poke 27h into the second byte of the PSP (the first holds the machine code for "int"), and end the program with the usual RET. For both kinds of file, before int 27h is executed DX must contain the offset of the end of the program, starting from the beginning of the PSP.
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