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\Huge This article comes from Wikipedia
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\textbf{DVI} ("DeVice Independent") is the output file format of the TeX
typesetting program, designed by \textit{David R. Fuchs} in 1979. Unlike the TeX
markup files used to generate them, DVI files are not intended to be
human-readable; they consist of binary data describing the visual layout of a
document in a manner not reliant on any specific image format, display hardware
or printer (hence the DVI format's name). DVI files are typically used as input
to a second program (called a DVI driver) which translates DVI files to
graphical data. For example, most TeX software packages include a program for
previewing DVI files on a user's computer display; this program is a driver.
Drivers are also used to convert DVI files to popular document formats (e.g.
PostScript, PDF) and for printing. Wikipedia uses a PNG driver to generate
graphics for mathematical formulae in articles.

DVI is not a document encryption format, and TeX markup may be at least
partially reverse-engineered from DVI files, although this process is unlikely
to produce high-level constructs identical to those present in the original
markup, especially if the original markup used high-level \TeX extensions (e.g.
\LaTeX).

DVI differs from PostScript and PDF in that it does not support any form of
font embedding. (Both PostScript and PDF formats can either embed their fonts
inside the documents, or reference external ones.) For a DVI file to be printed
or even properly previewed, the fonts it references must be already installed.
Also, unlike PostScript, DVI is not a full, Turing-complete programming
language, though it does use a limited sort of machine language.  Contents


\section{Specification}

The DVI format was designed to be compact and easily machine-readable. Toward
this end, a DVI file is a sequence of commands which form "a machine-like
language", in Knuth's words. Each command begins with an eight-bit opcode,
followed by zero or more bytes of parameters. For example, an opcode from the
group \texttt{0x00} through \texttt{0x7F} (decimal 127), \verb|set_char_i|,
typesets a single character and moves the implicit cursor right by that
character's width. In contrast, opcode \texttt{0xF7} (decimal 247), \texttt{pre}
(the preamble, which must be the first opcode in the DVI file), takes at least
fourteen bytes of parameters, plus an optional comment of up to 255 bytes.

In a broader sense, a DVI file consists of a preamble, one or more pages, and a
postamble. Six state variables are maintained as a tuple of signed, 32-bit
integers: (h,v,w,x,y,z). h and v are the current horizontal and vertical
offsets from the upper-left corner (increasing v moves down the page), w and x
hold horizontal space values, y and z, vertical.

These variables can be pushed or popped from the stack. In addition, the
current font f is held as an integer value, but is not pushed and popped with
the rest of the state variables when the opcodes push or pop are encountered.
Font spacing information is loaded from TFM files. The fonts themselves are not
embedded in the DVI file, only referenced by an integer value defined in the
relevant \verb|fnt_defi| op. (This is done exactly twice for each loaded font: once
before it is referenced, and once in the postamble.) f contains an integer
value of up to four bytes in length, though in practice, TeX only ever outputs
font numbers in the range 0 through 63.

The DVI format also relies on the character encodings of the fonts it
references, not on those of the system processing it. This means, for instance,
that an EBCDIC-based system can process a DVI file that was generated by an
ASCII-based system, so long as it has the same fonts installed.

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