Substitute Character

In computer data, a substitute character (␚) is a control character that is used to pad transmitted data in order to send it in blocks of fixed size, or to stand in place of a character that is recognized to be invalid, erroneous or unrepresentable on a given device. It is also used as an escape sequence in some programming languages.

In the ASCII character set, this character is encoded by the number 26 ( hex). Standard keyboards transmit this code when the and keys are pressed simultaneously (, often documented by convention as '')''. Unicode inherits this character from ASCII, but recommends that the replacement character (�, U+FFFD) be used instead to represent un-decodable inputs, when the output encoding is compatible with it.

Uses

End of file

Historically, under PDP-6 monitor, RT-11, VMS, and TOPS-10, and in early PC CP/M 1 and 2 operating systems (and derivatives like MP/M) it was necessary to explicitly mark the end of a file (EOF) because the native filesystem could not record the exact file size by itself; files were allocated in extents (records) of a fixed size, typically leaving some allocated but unused space at the end of each file. This extra space was filled with ₁₆ (hex) characters under CP/M. The extended CP/M filesystems used by CP/M 3 and higher (and derivatives like Concurrent CP/M, Concurrent DOS, and DOS Plus) did support byte-granular files, so this was no longer a requirement, but it remained as a convention (especially for text files) in order to ensure backward compatibility.

In CP/M, 86-DOS, MS-DOS, PC DOS, DR-DOS, and their various derivatives, the SUB character was also used to indicate the end of a character stream, and thereby used to terminate user input in an interactive command line window (and as such, often used to finish console input redirection, e.g. as instigated by the command COPY CON: TYPEDTXT.TXT).

While no longer technically required to indicate the end of a file, as of 2017 many text editors and program languages still support this convention, or can be configured to insert this character at the end of a file when editing, or at least properly cope with them in text files. In such cases, it is often termed a "soft" EOF, as it does not necessarily represent the physical end of the file, but is more a marker indicating that "there is no useful data beyond this point". In reality, more data may exist beyond this character up to the actual end of the data in the file system, thus it can be used to hide file content when the file is entered at the console or opened in editors. Many file format standards (e.g. PNG or GIF) include the SUB character in their headers to perform precisely this function. Some modern text file formats (e.g. CSV-1203) still recommend a trailing EOF character to be appended as the last character in the file. However, typing does not embed an EOF character into a file in either DOS or Windows, nor do the APIs of those systems use the character to denote the actual end of a file.

Some programming languages (e.g. Visual Basic) will not read past a "soft" EOF when using the built-in text file reading primitives (INPUT, LINE INPUT etc.), and alternate methods must be adopted, e.g. opening the file in binary mode or using the File System Object to progress beyond it.

Character 26 was used to mark "End of file" even though ASCII calls this character Substitute, and has other characters to indicate "End of file". Number 28 which is called " File Separator" has also been used for similar purposes.

Other uses

In Unix-like operating systems, this character is typically used in shells as a way for the user to suspend the currently executing interactive process. The suspended process can then be resumed in ''foreground'' (interactive) mode, or be made to resume execution in '' background'' mode, or be terminated. When entered by a user at their computer terminal, the currently running foreground process is sent a "terminal stop" (SIGTSTP) signal, which generally causes the process to suspend its execution. The user can later continue the process execution by using the "foreground" command ( fg) or the " background" command ( bg).

The Unicode Security Considerations report recommends this character as a safe replacement for unmappable characters during character set conversion.

In many GUIs and applications, ( on macOS) can be used to undo the last action. In many applications, earlier actions than the last one can also be undone by pressing multiple times. was one of a handful of keyboard sequences chosen by the program designers at Xerox PARC to control text editing.

Representation

ASCII and Unicode representation of "substitute":
* Octal code: 32
* Decimal code: 26
* Hexadecimal code: 1A, U+001A
* Mnemonic symbol: SUB
* Binary value: 11010

See also

* C0 and C1 control codes ( ISO 646)
* U+FFFD (Unicode replacement character �)
* Access key
* Control-C
* Control-G
* Control-V
* Control-X
* Control-\
* Keyboard shortcut
* List of file signatures
* , a symbol (sometimes called by the slang term ''tofu'') used to represent a missing character
** Noto fonts, a Google project to eliminate missing characters

References

{{reflist, refs=
{{cite book , title=CP/M 2.0 Interface Guide , chapter=2. Operating System Call Conventions , date=1979 , edition=1 , publisher=Digital Research , location=Pacific Grove, California, USA , page=5 , url=http://bitsavers.org/pdf/digitalResearch/cpm/2.0/CPM_2_0_Interface_Guide_1979.pdf , access-date=2020-02-28 , url-status=live , archive-url=https://web.archive.org/web/20200228175812/http://bitsavers.org/pdf/digitalResearch/cpm/2.0/CPM_2_0_Interface_Guide_1979.pdf , archive-date=2020-02-28 , quote= ..The end of an ASCII file is denoted by a control-Z character (1AH) or a real end of file, returned by the CP/M read operation. Control-Z characters embedded within machine code files (e.g., COM files) are ignored, however, and the end of file condition returned by CP/M is used to terminate read operations. ..} (56 pages)
{{cite book , title=Osborne CP/M User Guide - For All CP/M Users , chapter=3. CP/M Transient Commands , author-first=Thom , author-last=Hogan , publisher= A. Osborne/McGraw-Hill , date=1982 , edition=2 , location=Berkeley, California, USA , isbn=0-931988-82-9 , pag
74
, url=https://archive.org/details/osborne-cpm-users-guide_2nd-ed , access-date=2020-02-28 , quote= .. CP/M marks the end of an ASCII file by placing a CONTROL-z character in the file after the last data character. If the file contains an exact multiple of 128 characters, in which case adding the CONTROL-Z would waste 127 characters, CP/M does not do so. Use of the CONTROL-Z character as the end-of-file marker is possible because CONTROL-z is seldom used as data in ASCII files. In a non-ASCII file, however, CONTROL-Z is just as likely to occur as any other character. Therefore, it cannot be used as the end-of-file marker. CP/M uses a different method to mark the end of a non-ASCII file. CP/M assumes it has reached the end of the file when it has read the last record (basic unit of disk space) allocated to the file. The disk directory entry for each file contains a list of the disk records allocated to that file. This method relies on the size of the file, rather than its content, to locate the end of the file. ..

https://archive.org/download/osborne-cpm-users-guide_2nd-ed/OsborneCpmUsersGuideSecondEdition.pdf]
{{cite book , title=PDP-6 Multiprogramming System Manual , chapter=Table of IO Device Characteristics - Console or Teletypewriters , id=DEC-6-0-EX-SYS-UM-IP-PRE00 , publisher=Digital Equipment Corporation (DEC) , publication-place=Maynard, Massachusetts, USA , date=1965 , page=43 , url=http://bitsavers.trailing-edge.com/pdf/dec/pdp6/DEC-6-0-EX-SYS-UM-IP-PRE00_Multiprogramming_System_Manual_1965.pdf , access-date=2014-07-10 , url-status=live , archive-url=https://web.archive.org/web/20140714140253/http://bitsavers.trailing-edge.com/pdf/dec/pdp6/DEC-6-0-EX-SYS-UM-IP-PRE00_Multiprogramming_System_Manual_1965.pdf , archive-date=2014-07-14 (1+84+10 pages)
{{cite book , title=PDP-10 Reference Handbook: Communicating with the Monitor - Time-Sharing Monitors , volume=3 , chapter=5.1.1.1. Device Dependent Functions - Data Modes - Full-Duplex Software A(ASCII) and AL(ASCII Line) , publisher=Digital Equipment Corporation (DEC) , date=1969 , pages=5-3 – 5-6 -5 (431), url=http://bitsavers.org/pdf/dec/pdp10/1970_PDP-10_Ref/1970PDP10Ref_Part3.pdf , access-date=2014-07-10 , url-status=live , archive-url=https://web.archive.org/web/20111115083418/http://www.bitsavers.org/pdf/dec/pdp10/1970_PDP-10_Ref/1970PDP10Ref_Part3.pdf , archive-date=2011-11-15 (207 pages)
{{cite web , title=Keyboard shortcuts for Windows , work=Microsoft Support , publisher=Microsoft , url=http://support.microsoft.com/kb/126449 , access-date=2012-06-02
{{cite web , author-first=John C. , author-last=Elliott , date=1998 , title=CP/M 1.4 disc formats , url=http://www.seasip.info/Cpm/format14.html , access-date=2021-11-18 , url-status=live , archive-url=https://web.archive.org/web/20201114231913/http://www.seasip.info/Cpm/format14.html , archive-date=2020-11-14
{{cite web , author-first=John C. , author-last=Elliott , date=1998 , title=CP/M 2.2 disc formats , url=http://www.seasip.info/Cpm/format22.html , access-date=2021-11-18 , url-status=live , archive-url=https://web.archive.org/web/20201105204828/http://www.seasip.info/Cpm/format22.html , archive-date=2020-11-05
{{cite web , author-first=John C. , author-last=Elliott , date=1998 , title=CP/M 3.1 disc formats , url=http://www.seasip.info/Cpm/format31.html , access-date=2021-11-18 , url-status=live , archive-url=https://web.archive.org/web/20211026154048/https://www.seasip.info/Cpm/format31.html , archive-date=2021-10-26
{{cite web , author-first=John C. , author-last=Elliott , date=1998 , title=CP/M 4.1 disc formats , url=http://www.seasip.info/Cpm/format41.html , access-date=2021-11-18 , url-status=live , archive-url=https://web.archive.org/web/20201105174304/http://www.seasip.info/Cpm/format41.html , archive-date=2020-11-05
{{cite web , title=Quick Reference: Unix Commands , work=IT Connect , publisher=University of Washington , url=http://www.washington.edu/computing/unix/unixqr.html , access-date=2012-06-02
CSV-1203 format specification
{{Webarchive, url=http://arquivo.pt/wayback/20160516100434/http://www.mastpoint.com/csv-1203 , date=2016-05-16
Unicode Security Considerations report
/ref>

Further reading

* Federal Standard 1037C

Control characters