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\title{Simplifying modern computing by embracing the language}
\subtitle{A case study using Scheme}

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\author{Ekaitz Zárraga Río}
%% \authornote{Both authors contributed equally to this research.}
\email{ekaitz@elenq.tech}
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\begin{abstract}
  Unix based Operating Systems have dominated the computer market since its
  inception and its concepts and needs have impregnated almost every aspect of
  modern computing.
  In this paper a novel approach for optimization is proposed, one that is
  rooted on the simplification of personal computers by embracing the
  programming language \textit{Scheme}, and describes how this strategy
  provides a high level of abstraction and opens the door to many research
  topics that directly challenge the current computing ecosystem, which relies
  in the continuous delivery of heavily specialized hardware to support the
  ever growing software demands, mainly coming from Unix's heritage.
\end{abstract}

%% Document outline:
%% - Motivation: Simplification as a mean for more efficient computing and
%%   a more accessible manufacture. Modern computing systems are so complex
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%% - Kernels, Unix
%% - Interpreters
%%    - Shells
%%    - Are interpreters a kernel?
%% - Removing layers instead of adding layers
%%    - Unikernels: Good idea but miss the target.

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\keywords{Operating Systems, Programming Languages, Interpreters, CPU}
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%% Dates for the article
%% \received{20 February 2007}
%% \received[revised]{12 March 2009}
%% \received[accepted]{5 June 2009}


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\section{Context}

%% We need to understand context ...
  The computing industry, arguably due to its rapid emergence, has been driven
  by \textit{backwards compatibility}. This section summarizes the most
  influential milestones in computing history and their impact in modern
  computer hardware.

\subsection{Von Neumann model}
  The von Neumann model introduced in 1945 proposes a general purpose device
  consisting of a \textit{Central Processing Unit} (CPU) and a \textit{Store}.
  The \textit{Store} is often implemented as a \textit{Random Access Memory}
  (RAM or, simply, \textit{memory}).

  In the von Neumann model the data and the program are both written to and
  read from the \textit{Store}. This fact imposes limitations in any design
  that aims to have direct access to memory and multitasking, as any running
  program could read and write other running program's memory, and even
  manipulate its behavior, overwriting the program itself.

\subsection{Unix's heritage}
\epigraph{
  Applicants must also have extensive knowledge of Unix, although they should
  have sufficiently good programming taste to not consider this an achievement.
}{\textit{-- Hal Abelson}}

  Since its inception, Unix was a huge innovation in Operating Systems market.
  Its main features include \textit{multitasking} and \textit{multi-user}
  support, a programming interface, \textit{files as abstractions} for devices
  and other objects and a powerful \textit{shell} that facilitates program
  composition.

  In the Unix model, the Kernel, the core of the Operating System, is
  responsible for managing the hardware resources. For that job, it uses
  several concepts that systems designers and programmers are familiarized with
  and are discouraged to change. Those include \textit{virtual memory},
  \textit{processes}, \textit{shared-memory threads}, \textit{hierarchical
  filesystems} and \textit{system calls}.

  The \textit{shell} is run as a userspace program that has the ability to
  launch other programs using an outdated fork+exec mechanism that encourages
  memory overshoot\cite{fork:Baumann}. The shell in Unix systems is optimized
  for text processing as, in McIlroy's words, \textit{"text streams [are] the
  universal interface"} \cite{QuarterCenturyUnix:Salus}.

  Userspace programs are loaded in and given access to \textit{virtual memory},
  and they can only run the \textit{unprivileged} subset of the CPU
  instructions. For restricted operations, programs need to call the
  \textit{Kernel} using a \textit{system-call} that can be accepted or rejected
  by the latter, according to \textit{permissions} or resource availability. In
  order to achieve multitasking, many programs can be loaded in memory
  (\textit{processes}) simultaneously and the \textit{Kernel}
  \textit{schedules} which of the them will run at a given moment in time.


%  \paragraph{Concurrency}
%  If programs need to operate concurrently they can create many
%  \textit{processes} or use \textit{threads}. A \textit{thread} is a
%  lightweight version of a \textit{process} that shares the memory with the
%  \textit{process} that created it. Resource sharing in a concurrent system has
%  many security and reliability implications, and has proven to be a difficult
%  subject for computer programmers over the years
%  \cite{Threads:Lee}.
%
%
%\subsubsection{Interpreters}
%  Interpreters, like one in the \textit{shell}, are a fundamental part of
%  modern day programming. Interpreters are run as userspace programs, acting as
%  a \textit{host} for the program they interpret. The interpreter effectively
%  hides the details of the Operating System, often even implementing a virtual
%  machine for that job, in order to provide \textit{portability} and
%  \textit{usability} to the programmers. That is why the most used and demanded
%  programming languages nowadays are interpreted \cite{PLCommunity:Tambad}.

%  Unix was marketed as a system for multiple languages / supports many
%  languages via interpreters that ease the development experience. Describe how
%  they work and why they are useful
%  Interpreters are programs that run in userspace

\subsection{Computer hardware}
  Computer processors, often marketed as "\textit{general purpose}", are based
  on the von Neumann model\cite{LiberateFromVonNeumann:Backus}, a CPU and a
  Store that is, and designed for running an Operating System on them. They
  clearly separate \textit{privileged}, reserved for the kernel, and
  \textit{unprivileged} instructions, that any userspace program can use, in
  order to facilitate \textit{system-calls} and \textit{interrupt} and
  \textit{virtual memory} control.

  Contrary to what one could expect, improvements in the processor
  architectures come from specialization, instead of generalization, making
  processors heavily optimized machines for Operating Systems that follow the
  Unix model (including MS Windows), and a memory layout that resembles that of
  a \textit{C-like} program, which also comes from the days of Unix
  \cite{GeneralPurposeProcessor:Chisnall}, reducing the chance for other
  paradigms to succeed.


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\section{Simplification enables optimization}

  Choosing a language and committing to it enables aggressive simplification of
  the computing systems, taking advantage of that, many research areas open.
  Most of them confront Unix's heritage and modern computing but could provide
  a powerful but simple set of constructs that allow to create simple computers
  that can be maintained by an individual and reduce the economical and
  environmental costs of infrastructure of the modern CPU fabrication.

\subsection{Embracing the language: Scheme}

  \textit{Scheme} is a simple language, with a minimal standard, that enables a
  huge level of abstraction thanks to its minimal but powerful core concepts
  which are also present in mainstream programming languages today (Python,
  JavaScript).

  The \textit{Lisp} family of languages have proven to be flexible and powerful
  for system design \cite{LispMachine:Greenblatt} and particularly
  \textit{Scheme} has a long history of research in language and CPU design
  \cite{Lambda:Steele}.

  The nature of the Lisp family of languages also makes them suitable as file
  formats\cite{SXML:Kiselyov} for storage and configuration files, for
  extending the language or writing DSLs \cite{LanguageOriented:Culpepper},
  eliminating the need to rely on unstructured text.

\subsection{Operating System}

  An Operating System that embraces the language becomes a bare metal
  interpreter, eliminating the duplication of tasks present in Unix operating
  systems \cite{MIMOSA:Yvon}. The Operating System is accessible in the runtime
  environment of the programs. \textit{System calls} become \textit{procedure
  calls}. The \textit{Shell} is a system \textit{REPL}, an interactive
  environment for programming that exposes Operating System's facilities.

\subsubsection{Capability based security "lambda-style"}
  Most modern computers do not have more than a single user, but they preserve
  an antique user management system, inherited from Unix, recycled for program
  permissions.
  3L \cite{3L:Hintz} explored an OS where permission control is reduced to
  \textit{environments} with access to a limited set of bindings
  \cite{securityKernelLambda:Rees}. A program can only access the bindings
  provided by the system, which also facilitates fine grained control letting
  the user replace system bindings by ad-hoc versions when needed.

\subsubsection{Managed memory}
  Virtual memory is an attempt to isolate programs from each other and provide
  a permission system on top of a flawed von Neumann model, but it is a
  leaky abstraction that can be exploited\cite{SpectreMeltdown:HillMasters}.
  Preventing direct access to memory, replacing it with managed memory,
  obviates the need of virtual memory and decouples underlying implementations
  from user programs.

\subsubsection{Concurrency}
  Unix-style concurrency, reinforced by modern \textit{multi-core} CPU design
  and threads, is hard to reason about\cite{Threads:Lee}. A multitasking system
  can be approached in terms of \textit{Scheme}'s
  \textit{continuations}\cite{ContinuationsConcurrency:Hieb}, embracing its
  first-class constructs.



\subsection{Computer hardware}

  Attempts have been done to run \textit{Scheme} in a bare-metal environment
  using commodity hardware\cite{MIMOSA:Yvon}\cite{Loko:Weinholt}, but they do
  not take advantage of hardware optimized for \textit{Scheme}, which would
  provide huge performance benefits and simplify the interpreter and
  potentially the CPU.

\subsubsection{Optimization for tree structures}
  \textit{Scheme} is profoundly based on the \textit{cons cell}, similar to a
  \textit{linked-list} node, and the data structures that can be created from
  it (\textit{lists} and \textit{trees}). Optimizing the CPU for that case,
  with fast lookups and \code{car} and \code{cdr} operations would impact its
  performance.

\subsubsection{CPU as a reducer}
  Like other functional programming languages, \textit{Scheme} is based on
  expression reduction, in opposition to imperative languages, where the basis
  is the \textit{statement}, that matches current processor architectures
  better. This research area, was mainly abandoned after the modern CPU
  architectures flooded the market, but has a long
  history\cite{LiberateFromVonNeumann:Backus} and has been revived with the
  introduction of Field Programmable Gate Arrays,
  \textit{FPGA}\cite{Heron:Ramsay}.

\subsubsection{Hardware garbage collection}
  When the whole system uses managed memory, the Garbage Collection,
  \textit{GC}, can be pushed down in the stack. Both Oberon\cite{Oberon:Wirth}
  and MIMOSA \cite{MIMOSA:Yvon} propose Garbage Collector as part of the
  Operating System. A computing device that embraces \textit{Scheme} in its
  core may take a more radical approach and involve the hardware in the Garbage
  Collection process. Hardware assisted GC has been proven to improve the
  performance of managed memory in von Neumann style processors
  \cite{HWAssistedGC:Maas} and Lisp Machines\cite{LispMachine:Greenblatt}.

\subsubsection{Exploration via reconfigurable Hardware}
  A design based on a FPGA would facilitate incremental testing and evaluation
  of the impact of the proposed optimizations. If a Hardware Description
  Language, \textit{HDL}, and support for Partial and Dynamic
  Reconfiguration\cite{FPGAReconf:Vipin} are provided within the system, it
  could be updated with no distinction between hardware and software, while
  making the system more resistant to trusting-trust and supply-chain
  attacks\cite{riscvSelfHostingComputer:Somlo}. A \textit{rollback}
  system\cite{Guix:Courtes} could always recover the state of both hardware and
  software as a whole.

\subsection{Environmental implications}
  A computing system designed around its reconfigurability is less likely to
  become obsolete in the short term and encourages its repurpose once it
  happens. This attenuates the \textit{embodied} environmental impact of the
  device, which nowadays accounts for the majority of computing systems'
  \(CO_{2}\) emissions \cite{Carbon:Gupta}.

  While FPGAs are not as efficient as Application Specific Integrated Circuits,
  \textit{ASIC}, which would increase the \textit{operational} impact of the
  system, an aggressive simplification has been proven to have acceptable
  performance and energy usage in minimalistic processors running in a baseline
  FPGA\cite{Oberon:Wirth}, even without the optimization level proposed in this
  document. Additionally, the low-power nature of personal
  computing facilitates the decarbonization of its energy sources, reducing the
  environmental impact of the device operation to negligible levels.

\section{Conclusion}

  This paper introduces a computing system that combines a reconfigurable CPU
  and a simplified approach to Operating System design that embraces the
  programming language \textit{Scheme} and proposes several optimizations and
  research areas that its design would enable.

  The cited literature demonstrates systems with a level of simplification
  analogous to the one proposed in this document can be implemented and
  maintained by a single individual, down to the hardware, while keeping it
  secure, accessible and bootstrappable if carefully designed.

  The reconfigurability of the system directly challenges the current computer
  processor supply chain that relies on very specialized \textit{ASIC}
  manufacture. The proposed generalization of the hardware attenuates its
  embodied environmental impact pushing the specialization to the software
  side, the malleable part of the system, where a language with high level of
  abstraction can provide a solid foundation for a more human-centered design
  that finally obsoletes the antiquities from the times of Unix.


\clearpage

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