Substitute character

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A substitute character (␚) is a control character that is used in the place of a character that is recognized to be invalid or erroneous, or that cannot be represented 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 (1A hex). Standard keyboards transmit this code when the Ctrl and Z keys are pressed simultaneously (Ctrl+Z, by convention often described as ^Z).[1] Unicode encodes this character, 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[edit]

End of file[edit]

Historically, under PDP-6 monitor,[2] RT-11, VMS, TOPS-10[3] as well as 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 CP/M 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.[4][5][6][7] This extra space was filled with 1A16 (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,[8][9] so this was no longer a physical requirement but a mere 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 COPY CON: TYPEDTXT.TXT).

While no longer technically required to indicate the end of a file, 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[10]) still recommend a trailing EOF character to be appended as the last character in the file. However, typing Control+Z 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 if the ASCII calls it Substitute, and has other characters for this. Number 28 which is called "File Separator" has also been used for similar purposes.

Other uses[edit]

In Unix operating systems, this character is typically used to suspend the currently executing interactive process.[11] 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[12] recommends this character as a safe replacement for unmappable characters during character set conversion.

In many GUIs and applications Control+Z (⌘ Command+Z on Mac OS) can be used to undo the last action. In many applications earlier actions than the last one can also be undone by pressing Control+Z multiple times. Control+Z was one of a handful of keyboard sequences chosen by the program designers at Xerox PARC to control text editing. Presumably these particular keystrokes were chosen because of their location on a standard QWERTY keyboard, since the Z (undo), X (cut), C (copy), and V (paste) keys are located together at the left end of the bottom row of the standard QWERTY keyboard.

Representation[edit]

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[edit]

References[edit]

  1. ^ "Keyboard shortcuts for Windows". Microsoft Support. Microsoft. Retrieved 2012-06-02.
  2. ^ "PDP-6 Multiprogramming System Manual" (PDF). Digital Equipment Corporation (DEC). 1965. p. 43. Archived (PDF) from the original on 2014-07-14. Retrieved 2014-07-10.
  3. ^ "PDP-10 Reference Handbook, Book 3, Communicating with the Monitor" (PDF). Digital Equipment Corporation (DEC). 1969. p. 5-5. Archived (PDF) from the original on 2011-11-15. Retrieved 2014-07-10.
  4. ^ John C. Elliott (1998). CP/M 1.4 disc formats. ([1])
  5. ^ John C. Elliott (1998). CP/M 2.2 disc formats. ([2])
  6. ^ "2. Operating System Call Conventions". CP/M 2.0 Interface Guide (PDF) (1 ed.). Pacific Grove, California, USA: Digital Research. 1979. p. 5. Archived (PDF) from the original on 2020-02-28. Retrieved 2020-02-28. […] 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)
  7. ^ Hogan, Thom (1982). "3. CP/M Transient Commands". Osborne CP/M User Guide - For All CP/M Users (2 ed.). Berkeley, California, USA: A. Osborne/McGraw-Hill. p. 74. ISBN 0-931988-82-9. Retrieved 2020-02-28. […] 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. […] [3][4]
  8. ^ John C. Elliott (1998). CP/M 3.1 disc formats. ([5])
  9. ^ John C. Elliott (1998). CP/M 4.1 disc formats. ([6])
  10. ^ CSV-1203 format specification Archived 2016-05-16 at the Portuguese Web Archive
  11. ^ "Quick Reference: Unix Commands". IT Connect. University of Washington. Retrieved 2012-06-02.
  12. ^ Unicode Security Considerations report

Further reading[edit]