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 Multi-platform support of Isabelle
 ==================================
 
 Preamble
 --------
 
 The general programming model is that of a stylized ML + Scala + POSIX
 environment, with a minimum of system-specific code in user-space
 tools.
 
 The Isabelle system infrastructure provides some facilities to make
 this work, e.g. see the ML and Scala modules File and Path, or
 functions like Isabelle_System.bash.  The settings environment also
 provides some means for portability, e.g. the bash function
 "platform_path" to keep the impression that Windows/Cygwin adheres to
 Isabelle/POSIX standards, although Poly/ML and the JVM are native on
 Windows.
 
 When producing add-on tools, it is important to stay within this clean
 room of Isabelle, and refrain from non-portable access to operating
 system functions. The Isabelle environment uses peculiar scripts for
 GNU bash and perl as system glue: this style should be observed as far
 as possible.
 
 
 Supported platforms
 -------------------
 
 The following hardware and operating system platforms are officially
 supported by the Isabelle distribution (and bundled tools), with the
 following base-line versions (which have been selected to be neither
 too old nor too new):
 
   x86_64-linux      Ubuntu 14.04 LTS
 
   x86_64-darwin     Mac OS X 10.10 Yosemite (macbroy31 MacBookPro6,2)
                     Mac OS X 10.11 El Capitan (macbroy2 MacPro4,1)
                     macOS 10.12 Sierra (macbroy30 MacBookPro6,2)
                     macOS 10.13 High Sierra (lapbroy68 MacBookPro11,2)
                     macOS 10.14 Mojave (lapnipkow3 MacBookPro9,2)
                     macOS 10.15 Catalina (laramac01 Macmini8,1)
 
   x86_64-windows    Windows 7
-  x86_64-cygwin     Cygwin 2.10 https://isabelle.sketis.net/cygwin_2018 (x86_64/release)
+  x86_64-cygwin     Cygwin 2.10 https://isabelle.sketis.net/cygwin_2021 (x86_64/release)
 
 All of the above platforms are 100% supported by Isabelle -- end-users
 should not have to care about the differences (at least in theory).
 
-Exotic platforms like BSD, Solaris, NixOS are not supported.
+Partial support for ARM (e.g. Raspberry Pi) is available:
+
+  arm64-linux       Ubuntu 20.04 LTS
+
+Exotic operating systems like BSD, Solaris, NixOS are unsupported.
 
 
 64 bit vs. 32 bit platform personality
 --------------------------------------
 
 Isabelle requires 64 bit hardware running a 64 bit operating
 system. Windows and Mac OS X allow x86 executables as well, but for
 Linux this requires separate installation of 32 bit shared
 libraries. The POSIX emulation on Windows via Cygwin64 works
 exclusively for x86_64.
 
 Poly/ML supports both for x86_64 and x86, and the latter is preferred
 for space and performance reasons. Java is always the x86_64 version
 on all platforms.
 
 Add-on executables are expected to work without manual user
 configuration. Each component settings script needs to determine the
 platform details appropriately.
 
 
 The Isabelle settings environment provides the following important
 variables to help configuring platform-dependent tools:
 
   ISABELLE_PLATFORM64  (potentially empty)
   ISABELLE_PLATFORM32  (potentially empty)
 
 Each can be empty, but not both at the same time. Using GNU bash
 notation, tools may express their platform preference, e.g. first 64
 bit and second 32 bit, or the opposite:
 
   "${ISABELLE_PLATFORM64:-$ISABELLE_PLATFORM32}"
   "${ISABELLE_PLATFORM32:-$ISABELLE_PLATFORM64}"
 
 
 There is a another set of settings for native Windows (instead of the
 POSIX emulation of Cygwin used before):
 
   ISABELLE_WINDOWS_PLATFORM64
   ISABELLE_WINDOWS_PLATFORM32
 
 These are always empty on Linux and Mac OS X, and non-empty on
 Windows. They can be used like this to prefer native Windows and then
 Unix (first 64 bit second 32 bit):
 
   "${ISABELLE_WINDOWS_PLATFORM64:-${ISABELLE_WINDOWS_PLATFORM32:-${ISABELLE_PLATFORM64:-$ISABELLE_PLATFORM32}}}"
 
 
 Dependable system tools
 -----------------------
 
 The following portable system tools can be taken for granted:
 
 * Scala on top of Java.  Isabelle/Scala irons out many oddities and
   portability issues of the Java platform.
 
 * GNU bash as uniform shell on all platforms. The POSIX "standard"
   shell /bin/sh does *not* work portably -- there are too many
   non-standard implementations of it. On Debian and Ubuntu /bin/sh is
   actually /bin/dash and introduces many oddities.
 
 * Perl as largely portable system programming language, with its
   fairly robust support for processes, signals, sockets etc.
 
 
 Known problems
 --------------
 
 * Mac OS X: If MacPorts is installed there is some danger that
   accidental references to its shared libraries are created
   (e.g. libgmp).  Use otool -L to check if compiled binaries also work
   without MacPorts.
 
 * Mac OS X: If MacPorts is installed and its version of Perl takes
   precedence over /usr/bin/perl in the PATH, then the end-user needs
   to take care of installing extra modules, e.g. for HTTP support.
   Such add-ons are usually included in Apple's /usr/bin/perl by
   default.
 
 * Common Unix tools like /bin/sh, /bin/kill, sed, ulimit are
   notoriously non-portable an should be avoided.
 
 * The traditional "uname" Unix tool only tells about its own executable
   format, not the underlying platform!