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4.6.7 Differences from AT&T 'ms'
--------------------------------

The 'groff' 'ms' macros are an independent reimplementation, using no
AT&T code.  Since they take advantage of the extended features of
'groff', they cannot be used with AT&T 'troff'.  'groff' 'ms' supports
features described above as Berkeley and Tenth Edition Research Unix
extensions, and adds several of its own.

   * The internals of 'groff' 'ms' differ from the internals of AT&T
     'ms'.  Documents that depend upon implementation details of AT&T
     'ms' may not format properly with 'groff' 'ms'.  Such details
     include macros whose function was not documented in the AT&T 'ms'
     manual.(1)  (*note Differences from AT&T ms-Footnote-1::)

   * The error-handling policy of 'groff' 'ms' is to detect and report
     errors, rather than to ignore them silently.

   * Tenth Edition Research Unix supported 'P1'/'P2' macros to bracket
     code examples; 'groff' 'ms' does not.

   * 'groff' 'ms' does not work in GNU 'troff''s AT&T compatibility
     mode.  If loaded when that mode is enabled, it aborts processing
     with a diagnostic message.

   * Multiple line spacing is not supported.  Use a larger vertical
     spacing instead.

   * 'groff' 'ms' uses the same header and footer defaults in both
     'nroff' and 'troff' modes as AT&T 'ms' does in 'troff' mode; AT&T's
     default in 'nroff' mode is to put the date, in U.S. traditional
     format (e.g., "January 1, 2021"), in the center footer (the 'CF'
     string).

   * Many 'groff' 'ms' macros, including those for paragraphs, headings,
     and displays, cause a reset of paragraph rendering parameters, and
     may change the indentation; they do so not by incrementing or
     decrementing it, but by setting it absolutely.  This can cause
     problems for documents that define additional macros of their own
     that try to manipulate indentation.  Use the 'ms' 'RS' and 'RE'
     macros instead of the 'in' request.

   * AT&T 'ms' interpreted the values of the registers 'PS' and 'VS' in
     points, and did not support the use of scaling units with them.
     'groff' 'ms' interprets values of the registers 'PS', 'VS', 'FPS',
     and 'FVS' equal to or larger than 1,000 (one thousand) as decimal
     fractions multiplied by 1,000.(2)  (*note Differences from AT&T
     ms-Footnote-2::) This threshold makes use of a scaling unit with
     these parameters practical for high-resolution devices while
     preserving backward compatibility.  It also permits expression of
     non-integral type sizes.  For example, 'groff -rPS=10.5p' at the
     shell prompt is equivalent to placing '.nr PS 10.5p' at the
     beginning of the document.

   * AT&T 'ms''s 'AU' macro supported arguments used with some document
     types; 'groff' 'ms' does not.

   * Right-aligned displays are available.  The AT&T 'ms' manual
     observes that "it is tempting to assume that '.DS R' will right
     adjust lines, but it doesn't work".  In 'groff' 'ms', it does.

   * To make 'groff' 'ms' use the default page offset (which also
     specifies the left margin), the 'PO' register must stay undefined
     until the first 'ms' macro is called.

     This implies that '\n[PO]' should not be used early in the
     document, unless it is changed also: accessing an undefined
     register automatically defines it.

   * 'groff' 'ms' supports the 'PN' register, but it is not necessary;
     you can access the page number via the usual '%' register and
     invoke the 'af' request to assign a different format to it if
     desired.(3)  (*note Differences from AT&T ms-Footnote-3::)

   * The AT&T 'ms' manual documents registers 'CW' and 'GW' as setting
     the default column width and "intercolumn gap", respectively, and
     which applied when 'MC' was called with fewer than two arguments.
     'groff' 'ms' instead treats 'MC' without arguments as synonymous
     with '2C'; there is thus no occasion for a default column width
     register.  Further, the 'MINGW' register and the second argument to
     'MC' specify a _minimum_ space between columns, not the fixed
     gutter width of AT&T 'ms'.

   * The AT&T 'ms' manual did not document the 'QI' register; Berkeley
     and 'groff' 'ms' do.

 -- Register: \n[GS]
     The register 'GS' is set to 1 by the 'groff' 'ms' macros, but is
     not used by the AT&T 'ms' package.  Documents that need to
     determine whether they are being formatted with 'groff' 'ms' or
     another implementation should test this register.

* Menu:

* Missing Unix Version 7 ms Macros::