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        Load or execute (INT 21h, 4Bh - EXEC)

        Entry:
          AH = 4Bh
          AL = subfunction
                  0 = Load and execute program
                  1 = Load (Internal) but do not execute
                      Process ID set to new program's PSP; get with
                      function 62h
                  3 = Load overlay; do not create PSP
          DS:DX = filename
          ES:BX = parameter block
                     AL    Type
                     ----------------------------------------
                      0    Word      Segment of new environment (0=current)
                           DWord     Pointer to command line
                           DWord     Pointer to FCB 1
                           DWord     Pointer to FCB 2
                      1    Word      Segment of new environment (0=current)
                           DWord     Pointer to command line
                           DWord     Pointer to FCB 1
                           DWord     Pointer to FCB 2
                           DWord     Will hold SS:SP on return
                           DWord     Will hold entry point (CS:IP) on return
                      3    Word      Segment load address
                           Word      Segment relocation factor
        Return: CF = 1 if error
                        AX = Error Code