The following paper (aka "The Monster") is available by FTP from ftp.dcs.glasgow.ac.uk: pub/glasgow-fp/papers/spineless-tagless-gmachine.dvi In the same directory you'll find various other papers from our group. Their titles are summarised in the file PAPER_LIST. Simon PJ, University of Glasgow. Implementing lazy functional languages on stock hardware: the Spineless Tagless G-machine Version 2.4 Simon L Peyton Jones, University of Glasgow The Spineless Tagless G-machine is an abstract machine designed to support non-strict higher-order functional languages. This presentation of the machine falls into three parts. Firstly, we give a general discussion of the design issues involved in implementing non-strict functional languages, which is intended to set the STG machine in a wider context. Next, we present the {\em STG language}, an austere but recognisably-functional language, which as well as a {\em denotational} meaning has a well-defined {\em operational} semantics. The STG language is the ``abstract machine code'' for the Spineless Tagless G-machine. Lastly, we discuss the mapping of the STG language onto stock hardware. The success of an abstract machine model depends largely on how efficient this mapping can be made, though this topic is often relegated to a short section. Instead, we give a detailed discussion of the design issues and the choices we have made. Our principal target is the C language, treating the C compiler as a portable assembler. This is a very long and detailed paper (82 pages), and it gives away all our trade secrets! It is to appear in the Journal of Functional Programing.