Function  4Bh   Load or Execute a Program  (EXEC) 
entry   AH      4Bh 
	AL      00h     load and execute program. A PSP is built for the program 
			the ctrl-break and terminate addresses are set to the 
			new PSP. 
	       *01h     load but don't execute  (note 1) 
	       *01h     load but don't execute (internal, DOS 3.x & DESQview)  
	       *02h     load but do not execute (internal, DOS 2.x only)  
		03h     load overlay (do not create PSP, do not begin execution) 
	DS:DX   points to the ASCIIZ string with the drive, path, and filename 
		to be loaded 
	ES:BX   points to a parameter block for the load 
	       (AL=00h) word    segment address of environment string to be 
			        passed 
		       dword    pointer to the command line to be placed at 
			        PSP+80h 
		       dword	pointer to default Struct -FCB to be passed
				at PSP+5Ch
		       dword	pointer to default Struct -FCB to be passed
				at PSP+6Ch
              (*AL=01h) word    segment of environment (0 = use current)  
		       dword    pointer to command line  
		       dword	pointer to Struct -FCB 1
		       dword	pointer to Struct -FCB 2
		       dword    will hold SS:SP on return  
		       dword    will hold program entry point (CS:IP) on return  
	      (*AL=02h) word    segment of environment (0 = use current)  
		       dword    pointer to command line  
		       dword	pointer to Struct -FCB 1
		       dword	pointer to Struct -FCB 2
	       (AL=03h) word    segment address where file will be loaded 
			word    relocation factor to be applied to the image 
return  CF      set     error 
			AX      error code (1, 2, 8, 0Ah, 0Bh) 
note 1) If you make this call with AL=1 the program will be loaded as if you
	made the call with AL=0 except that the program will not be executed.
	Additionally, with AL=1 the stack segment and pointer along with the
	program's CS:IP entry point are returned to the program which made the
	4B01h call. These values are put in the four words at ES:BX+0Eh. On
	entry to the call ES:BX points to the environment address, the command
	line and the two default Struct -FCBs. This form of EXEC is used by
	DEBUG.COM.
     2) Application programs may invoke a secondary copy of the command  
	processor (normally COMMAND.COM) by using the EXEC function.  Your  
	program may pass a DOS command as a parameter that the secondary  
	command processor will execute as though it had been entered from the  
	standard input device. 
	The procedure is: 
	 A. Assure that adequate free memory (17k for 2.x and 3.0, 23k for 3.1  
	    up) exists to contain the second copy of the command processor and  
	    the command it is to execute. This is accomplished by executing  
	    function call 4Ah to shrink memory allocated to that of your current 
	    requirements. Next, execute function call 48h with BX=0FFFFh. This  
	    returns the amount of memory availible. 
	B. Build a parameter string for the secondary command processor in the  
	   form: 
			 1 byte   length of parameter string 
			xx bytes  parameter string 
			 1 byte   0Dh (carriage return) 
	   For example, the assembly language statement below would build the  
	   string to cause execution of the command FOO.EXE: 
			      DB 19,"/C C:FOO",13 
	C. Use the EXEC function call (4Bh), function value 0 to cause execution 
	   of the secondary copy of the command processor. (The drive,  
	   directory, and name of the command processor can be gotten from the  
	   COMSPEC variable in the DOS environment passed to you at PSP+2Ch.) 
	D. Remember to set offset 2 of the EXEC control block to point to the  
	   string built above. 
     3) All open files of a process are duplicated in the newly created  
	process after an EXEC, except for files originally opened with the  
	inheritance bit set to 1. 
     4) The environment is a copy of the original command processor's  
	environment. Changes to the EXECed environment are not passed back to 
	the original. The environment is followed by a copy of the DS:DX  
	filename passed to the child process. A zero value will cause the  
	child process to inherit the environment of the calling process. The 
	segment address of the environment is placed at offset 2Ch of the 
	PSP of the program being invoked. 
     5) This function uses the same resident part of COMMAND.COM, but makes a 
	duplicate of the transient part.  
     6) How EXEC knows where to return to: Basically the vector for int\22
	holds the terminate address for the current process. When a process  
	gets started, the previous contents of int\22 get tucked away in the
	PSP for that process, then int\22 gets modified. So if Process A
	EXECs process B, while Process B is running, the vector for int\22
	holds the address to return to in Process A, while the save location in 
	Process B's PSP holds the address that process A will return to when 
	*it* terminates. When Process B terminates by one of the usual legal  
	means, the contents of int\22 are (surmising) shoved onto the stack,
	the old terminate vector contents are copied back to int\22 vector from
	Process B's PSP, then a RETF or equivalent is executed to return control 
	to process A. 
     7) To load an overlay file with 4B: first, don't de-allocate the memory 
	that the overlay will load into. With the other 4Bh functions, the 
	opposite is true--you have to free the memory first, with function 4Ah. 
	Second, the "segment address where the file will be loaded" (first item 
	in the parameter block for sub-function 03) should be a paragraph  
	boundary within your currently-allocated memory. Third, if the 
	procedures within the overlay are FAR procs (while they execute, CS will 
	be equal to the segment address of the overlay area), the relocation 
	factor should be set to zero. On the other hand, if the CS register 
	will be different from the overlay area's segment address, the 
	relocation factor should be set to represent the difference. You 
	determine where in memory the overlay file will load by using the 
	segment address mentioned above. Overlay files are .EXEs (containing 
	header, relocation table, and memory image). 
     8) When function 00h returns, all registers are changed, including the 
	stack. You must resore SS, SP, and any other required registers. 

 

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