The Jargon File (version 4.4.7, 29 Dec 2003):

bare metal
 n.

    1. [common] 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 machine. Real
    bare-metal programming involves things like building boot proms and BIOS
    chips, implementing basic monitors used to test device drivers, and writing
    the assemblers that will be used to write the compiler back ends that will
    give the new machine 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, esp. tricks for speed and space optimization that rely on
    crocks such as overlapping instructions (or, as in the famous case
    described in The Story of Mel' (in Appendix A), interleaving of opcodes on
    a magnetic drum to minimize fetch delays due to the device's rotational
    latency). This sort of thing has become rare as the relative costs of
    programming time and machine resources have changed, but is still found in
    heavily constrained environments such as industrial embedded systems. See 
    Real Programmer.


The Free On-line Dictionary of Computing (30 December 2018):

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
   PROMs and BIOS chips, implementing basic monitors used to
   test device drivers, 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]