Editing Fat binary (section)

===Crash-protected system files===
Under DOS, some files, by convention, have file extensions which do not reflect their actual file type.<ref group="nb" name="NB_NAMES"/> For example, [[COUNTRY.SYS]]<ref name="Paul_2001_COUNTRY"/> is not a DOS device driver,<ref group="nb" name="NB_KEYBOARD.SYS"/> but a binary [[National Language Support|NLS]] database file for use with the CONFIG.SYS [[COUNTRY (CONFIG.SYS directive)|COUNTRY directive]] and the [[NLSFUNC (DOS command)|NLSFUNC]] driver.<ref name="Paul_2001_COUNTRY"/> Likewise, the [[PC&nbsp;DOS]] and [[DR-DOS]] system files [[IBMBIO.COM]] and [[IBMDOS.COM]] are special binary images loaded by [[bootstrap loader]]s, not COM-style programs.<ref group="nb" name="NB_KEYBOARD.SYS"/> Trying to load COUNTRY.SYS with a DEVICE statement or executing IBMBIO.COM or IBMDOS.COM at the command prompt will cause unpredictable results.<ref group="nb" name="NB_NAMES"/><ref group="nb" name="NB_HIDDEN"/>

It is sometimes possible to avoid this by utilizing techniques similar to those described above. For example, [[DR-DOS&nbsp;7.02]] and higher incorporate a safety feature developed by Matthias R. Paul:<ref name="Paul_1997_NWDOSTIP"/> If these files are called inappropriately, tiny embedded stubs will just display some file version information and exit gracefully.<ref name="Paul_1997_OD-A3"/><ref name="Paul_1997_NWDOSTIP"/><ref name="Caldera_1998_NEW703"/><ref name="Paul_2001_COUNTRY"/> Additionally, the message is specifically crafted to follow certain [[magic number (programming)|"magic" pattern]]s recognized by the external [[NetWare]] & DR-DOS [[VERSION.EXE|VERSION]] file identification utility.<ref name="Paul_2001_COUNTRY"/><ref name="Paul_1997_NWDOSTIP"/><ref group="nb" name="NB_COUNTRY.SYS"/>

A similar protection feature was the 8080 instruction {{mono|C7h}} ("RST&nbsp;0") at the very start of Jay Sage's and Joe Wright's [[Z-System]] type-3 and type-4 "Z3ENV" programs<!-- introduced in 1988 --><ref name="Sage_1988_ZSystem"/><ref name="Sage_1992_ZSystem_1"/> as well as "Z3TXT" language overlay files,<ref name="Sage_1992_ZSystem_2"/> which would result in a [[warm boot]] (instead of a crash) under CP/M-80 if loaded inappropriately.<ref name="Sage_1988_ZSystem"/><ref name="Sage_1992_ZSystem_1"/><ref name="Sage_1992_ZSystem_2"/><ref group="nb" name="NB_RET"/>

In a distantly similar fashion, many (binary) [[list of file signatures|file format]]s by convention include a {{mono|1Ah}} byte ([[ASCII]] [[^Z]]) near the beginning of the file. This [[control character]] will be interpreted as "soft" [[end-of-file]] (EOF) marker when a file is opened in non-binary mode, and thus, under many operating systems (including the [[PDP-6]] monitor<ref name="DEC_1965_PDP-6"/> and [[RT-11]], [[OpenVMS|VMS]], [[TOPS-10]],<ref name="DEC_1969_PDP-10"/> CP/M,<ref name="DRI_1979_CPM20-IG"/><ref name="Hogan_1982_CP/M"/> DOS,<ref name="CHF_2010_DOS-1A"/> and Windows<ref name="Superuser_2011_TXT"/>), it prevents "binary garbage" from being displayed when a file is accidentally printed at the console.