Source : Free On-Line Dictionary of Computing

bare metal
     
        1. New computer hardware, unadorned with such snares and
        delusions as an {operating system}, an {HLL}, or even
        {assembler}.  Commonly used in the phrase "programming on the
        bare metal", which refers to the arduous work of {bit bashing}
        needed to create these basic tools for a new computer.  Real
        bare-metal programming involves things like building {boot
        PROM}s and {BIOS} chips, implementing basic {monitor}s used to
        test {device driver}s, and writing the assemblers that will be
        used to write the compiler back ends that will give the new
        computer a real development environment.
     
        2. "Programming on the bare metal" is also used to describe a
        style of {hand-hacking} that relies on bit-level peculiarities
        of a particular hardware design, especially tricks for speed
        and space optimisation that rely on crocks such as overlapping
        instructions (or, as in the famous case described in {The
        Story of Mel}, interleaving of opcodes on a magnetic drum to
        minimise fetch delays due to the device's rotational latency).
        This sort of thing has become less common as the relative
        costs of programming time and computer resources have changed,
        but is still found in heavily constrained environments such as
        industrial embedded systems, and in the code of hackers who
        just can't let go of that low-level control.  See {Real
        Programmer}.
     
        In the world of personal computing, bare metal programming is
        often considered a {Good Thing}, or at least a necessary evil
        (because these computers have often been sufficiently slow and
        poorly designed to make it necessary; see {ill-behaved}).
        There, the term usually refers to bypassing the BIOS or OS
        interface and writing the application to directly access
        device registers and computer addresses.  "To get 19.2
        kilobaud on the serial port, you need to get down to the bare
        metal."  People who can do this sort of thing well are held in
        high regard.
     
        [{Jargon File}]