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  Description:  Execute Program (EXEC)
 
  Allows an application program to run another program, regaining control
  when it is finished. Can also be used to load overlays, although this
  use is uncommon.
 
  Input                                        Output
  ═════════════════════════════════            ════════════════════════════
  AH=4BH                                       If function successful
  AL=subfunction                               Carry flag=clear
     00H=Load and Execute Program              all registers except for CS:
     03H=Load Overlay                           IP may be destroyed
  ES:BX=segment:offset of parameter            [3.0+] registers are pre-
        block                                  served in the usual fashion
  DS:DX=segment:offset of program              If function unsuccessful
        specification                          Carry flag=set
                                               AX=error code
                                    -♦-