To compile the new C-language source files separately, you must refer to each source file in the makefile. Since this application is a medium-model application, use the /AM option when compiling. For clarity, you should also name each segment by using the /NT option.
You will also need to change the link command line so that it refers to the medium-model library MLIBCEW.LIB rather than the small-model library SLIBCEW.LIB.
The makefile for the Memory application should look like this:
memory.res: memory.rc memory.h
rc /r memory.rc
memory1.obj: memory1.c memory.h
cl /c /AM /Gsw /Zp /NT MEMORY_MAIN memory1.c
memory2.obj: memory2.c memory.h
cl /c /AM /Gsw /Zp /NT MEMORY_INIT memory2.c
memory3.obj: memory3.c memory.h
cl /c /AM /Gsw /Zp /NT MEMORY_WNDPROC memory3.c
memory4.obj: memory4.c memory.h
cl /c /AM /Gsw /Zp /NT MEMORY_ABOUT memory4.c
memory.exe: memory1.obj memory2.obj memory3.obj memory4.obj \
memory.def
link memory1 memory2 memory3 memory4,memory.exe,,mlibcew \
libw,memory.def
rc memory.res
memory.exe: memory.res
rc memory.res