How does XeLaTeX and LuaLaTeX extend the dvi format?

Question

The structure of the DVI file format is documented by Donald Knuth in TEX: the Program, Addison-Wesley, 1986.

Although PDF is the primary output format, LuaTeX can also output DVI files. Are these DVI files different from what is found in the original specification?

XeTeX can output XDV files, which are an extension of the DVI format. Is the XDV format documented anywhere?

Answer 1

I think luatex generated dvi is standard.

For xdv, xetex.web defines the extended format, the relevant parts being

\yskip\noindent Commands 250--255 are undefined in normal \.{DVI} files, but
the following commands are used in \.{XDV} files.

\yskip\hang\vbox{\halign{#&#\hfil\cr
|define_native_font| 252 & |k[4]| |s[4]| |flags[2]| |l[1]| |n[l]| |i[4]|\cr
& |if (flags and COLORED) then| |rgba[4]|\cr
& |if (flags and EXTEND) then| |extend[4]|\cr
& |if (flags and SLANT) then| |slant[4]|\cr
& |if (flags and EMBOLDEN) then| |embolden[4]|\cr
}}

\yskip\hang|set_glyphs| 253 |w[4]| |k[2]| |xy[8k]| |g[2k]|.

\yskip\noindent Commands 250 and 255 are undefined in normal \.{XDV} files.

@ @d set_char_0=0 {typeset character 0 and move right}
@d set1=128 {typeset a character and move right}
@d set_rule=132 {typeset a rule and move right}
@d put_rule=137 {typeset a rule}
@d nop=138 {no operation}
@d bop=139 {beginning of page}
@d eop=140 {ending of page}
@d push=141 {save the current positions}
@d pop=142 {restore previous positions}
@d right1=143 {move right}
@d w0=147 {move right by |w|}
@d w1=148 {move right and set |w|}
@d x0=152 {move right by |x|}
@d x1=153 {move right and set |x|}
@d down1=157 {move down}
@d y0=161 {move down by |y|}
@d y1=162 {move down and set |y|}
@d z0=166 {move down by |z|}
@d z1=167 {move down and set |z|}
@d fnt_num_0=171 {set current font to 0}
@d fnt1=235 {set current font}
@d xxx1=239 {extension to \.{DVI} primitives}
@d xxx4=242 {potentially long extension to \.{DVI} primitives}
@d fnt_def1=243 {define the meaning of a font number}
@d pre=247 {preamble}
@d post=248 {postamble beginning}
@d post_post=249 {postamble ending}

@d define_native_font=252 {define native font}
@d set_glyphs=253 {sequence of glyphs with individual x-y coordinates}