The following changes were made to the DLXOS before Project #4.  In
addition, you'll need the latest version of dlxos.tar.gz to get
diskio.c and diskio.h.  Also, note that proj2.s from Project #3 and
earlier will not work with the latest version of dlxos.tar.gz (Project
#4); however, the proj2.s supplied with the latest version of
dlxos.tar.gz DOES work with the rest of the most recent DLXOS.

----------

process.c:
  Add
	DiskModuleInit ();
  after ProcessModuleInit ();

----------
traps.c:
  Add
	case TRAP_DISK:
	{
	  DiskFinishIo ();
	  break;
	}
   to the big case statement after 'case TRAP_PAGEFAULT:'

----------
traps.h

Add
#define       TRAP_DISK               0x50    // disk trap
#define       DLX_DISK_REQ            0xfff00400
#define       DLX_DISK_STATUS         0xfff00408
somewhere in the file.
----------
filesys.c

Fix the following line:

return ((fd > 0) && (fd < FS_MAX_OPEN_FILES) && (openfiles[fd].flags != 0));

to

return ((fd >= 0) && (fd < FS_MAX_OPEN_FILES) && (openfiles[fd].flags != 0))

around line 34.