Editing Device independent file format (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 language|machine-like language]]", in [[Donald Knuth|Knuth]]'s words.<ref name="DVItype"/> Each command begins with an eight-bit [[opcode]], followed by zero or more bytes of parameters. For example, an opcode from the group <code>0x00</code> through <code>0x7F</code> (decimal 127), <code>set_char_''i''</code>, typesets a single character and moves the implicit cursor right by that character's width. In contrast, opcode <code>0xF7</code> (decimal 247), <code>pre</code> (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: <math>(h,v,w,x,y,z)</math>. ''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 to or popped from the [[stack (data structure)|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 <code>push</code> or <code>pop</code> are encountered. Font spacing information is loaded from [[TeX font metric|TFM]] files. The fonts themselves are not embedded in the DVI file, only referenced by an integer value defined in the relevant <code>fnt_def''i''</code> 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 255.

Similarly, the DVI format supports character codes up to four bytes in length, even though only the 0–255 range is commonly seen, as the [[TeX font metric|TFM]] format is limited to that range. Character codes in DVI files refer to the [[character encoding]] of the current font rather than that 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.