SUMMARY

To give the user quick response and a full set of MS-DOS Shell features, two files are used, DOSSHELL.COM and DOSSHELL.EXE.

When MS-DOS finds an .EXE and .COM file with the same name, the .COM file takes precedence. The .COM file is a subset of the .EXE file. A .COM file contains only a single segment for its code and data, so it can be quickly loaded. The .EXE file, however, may contain many code and data segments, so it takes more time to load. The DOSSHELL.EXE program will not fit into a COM file's single 64K segment.

The DOSSHELL.COM file is the resident part of MS-DOS Shell. It is not absolutely necessary to use the DOSSHELL.COM, because the same code is contained in DOSSHELL.EXE, but using DOSSHELL.COM speeds up the load time. When you execute the DOSSHELL command at the MS-DOS prompt, the DOSSHELL.COM file gets executed first and then the DOSSHELL.COM file calls the DOSSHELL.EXE file. If the DOSSHELL.COM file does not exist, DOSSHELL.EXE is executed. Finally, when you execute the DOSSHELL.COM file, you can save about 16 bytes of memory.