Editing Lua (section)

== Implementation ==
Lua programs are not [[Interpreter (computing)|interpreted]] directly from the textual Lua file, but are [[Compiler|compiled]] into bytecode, which is then run on the Lua [[virtual machine]] (VM). The compiling process is typically invisible to the user and is performed during [[Run time (program lifecycle phase)|run-time]], especially when a [[just-in-time compilation]] (JIT) compiler is used, but it can be done offline to increase loading performance or reduce the memory footprint of the host environment by leaving out the compiler. Lua bytecode can also be produced and executed from within Lua, using the <code>dump</code> function from the string library and the <code>load/loadstring/loadfile</code> functions. Lua version 5.3.4 is implemented in approximately 24,000 lines of C code.<ref name="luaspe"/><ref name="luaabout"/>

Like most CPUs, and unlike most virtual machines (which are [[Stack machine|stack-based]]), the Lua VM is [[Register machine|register-based]], and therefore more closely resembles most hardware design. The register architecture both avoids excessive copying of values, and reduces the total number of instructions per function. The virtual machine of Lua 5 is one of the first register-based pure VMs to have a wide use.<ref name=lua5impl>{{cite journal |last1=Ierusalimschy |first1=R. |last2=Figueiredo |first2=L. H. |last3=Celes |first3=W. |year=2005 |title=The implementation of Lua 5.0 |journal=J. Of Universal Comp. Sci. |volume=11 |issue=7 |pages=1159–1176 |url=http://www.jucs.org/jucs_11_7/the_implementation_of_lua/jucs_11_7_1159_1176_defigueiredo.html |doi=10.3217/jucs-011-07-1159 |doi-access=free}}</ref> [[Parrot virtual machine|Parrot]] and [[Android (operating system)|Android]]'s [[Dalvik (software)|Dalvik]] are two other well-known register-based VMs. PCScheme's VM was also register-based.<ref>{{cite book |last1=Texas Instruments |year=1990 |title=PC Scheme: Users Guide and Language Reference Manual, Trade Edition |publisher=MIP Press |isbn=0-262-70040-9}}</ref>

This example is the bytecode listing of the factorial function defined [[#Factorial example|above]] (as shown by the <code>luac</code> 5.1 compiler):<ref name=luavm51instr>{{cite web |last1=Man |first1=Kein-Hong |year=2006 |title=A No-Frills Introduction to Lua 5.1 VM Instructions |url=https://talk.pokitto.com/uploads/default/original/2X/7/716c67a0b5b1636cbc4dc1fec232ca2536cb74d1.pdf}}</ref><!-- See earlier revisions for now-dead original and archive.org snapshots of this link, in case it moves again.  It is CC-BY-NC-SA so it can be rehosted if needed. -->

 function <factorial.lua:1,7> (9 instructions, 36 bytes at 0x8063c60)
 1 param, 6 slots, 0 upvalues, 6 locals, 2 constants, 0 functions
 	1	[2]	LOADK    	1 -1	; 1
 	2	[3]	LOADK    	2 -2	; 2
 	3	[3]	MOVE     	3 0
 	4	[3]	LOADK    	4 -1	; 1
 	5	[3]	FORPREP  	2 1	; to 7
 	6	[4]	MUL      	1 1 5
 	7	[3]	FORLOOP  	2 -2	; to 6
 	8	[6]	RETURN   	1 2
 	9	[7]	RETURN   	0 1