pts-mips-emulator: MIPS I emulator in Perl 5 which can run Linux programs
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
pts-mips-emulator is a platform-independent, proof-of-concept Perl 5 script
which can run some Linux MIPS-I (R2000, R3000) ELF executables.

The code is a fork of the excellent
http://blog.schmorp.de/2015-07-04-emulating-linux-mips-in-perl-4.html . This
fork contains some bugfixes, usability improvements and memory utilization
improvements.

pts-mips-emulator is compatible with Perl 5 installations with both 32-bit
and 64-bit integer arithmetic. (It's faster on 64-bit, but it doesn't use
more memory.)

pts-mips-emulator has been tested and found working with:

* Perl 5.004_04 (i386, 32-bit integers, 1997-10-15)
* Perl 5.6.1 (i386, 32-bit integers, 2001-04-08)
* Perl 5.10.1 (i386, 32-bit integers, 2009-08-22)
* Perl 5.14.2 (i386, 64-bit integers, 2011-09-26)
* Perl 5.18.2 (amd64, 64-bit integers, 2014-01-06)
* Perl 5.24.1 (amd64, 64-bit integers, 2017-01-14)

using the following command lines:

  $ perl ./run dash.run -c 'echo $((6*7))'
  42
  $ perl ./run bash.run -c 'let ANSWER=6*7; echo $ANSWER'
  42
  $ perl ./run bash.run --version | head -1
  GNU bash, version 4.1.0(1)-release (mips-unknown-linux-uclibc)

The shells also work interactively:

  $ PS1='. ' perl ./run dash.run
  . exit
  $ PS1=', ' perl ./run bash.run --norc
  , exit

pts-mips-emulator implements a subset of the 32-bit MIPS-I instruction set
architecture (ISA), see
https://en.wikipedia.org/wiki/MIPS_architecture#MIPS_I introduced in 1985
with the R2000 processor, and also implemented by the R3000 processor
introduced in 1998. pts-mips-emulator doesn't implement floating point
instructions or the comprocessor (and some other instructions are also
missing). Some more details:

* MIPS-I is is big endian (MSB-first) and it's 32-bit.
* There are many architectures named MIPS, e.g. for 32-bit: MIPS-I, MIPS-II,
  MIPS-III, MIPS-IV, MIPS-V, MIPS32r1, ... MIPS32r6, microMIPS32r3 ...
  microMIPS32r6. For 64 bits: MIPS64r1 ... MIPS64r6, microMIPS64r3 ...
  microMIPS64r6. There are also application-specific extensions (ASE) for
  many of these, e.g. MIPS16e2. Some of these architectures are backwards
  compatible to each other, but most of them are not. Thus pts-mipus-emulator
  implements MIPS-I only, and nothing else.
* More details about MIPS architectures here:
  https://en.wikipedia.org/wiki/MIPS_architecture#MIPS_I .
* The corresponding Linux uname(2) machine type is mips, but that's confusing,
  because it includes MIPS-II, MIPS32r2, MIPS64r2 etc. There different Linux
  system call numbers assigned to 32-bit and 64-bit MIPS:
  https://fedora.juszkiewicz.com.pl/syscalls.html .
* Some precompiled busybox executables for MIPS-I: busybox-mips in:
  * https://busybox.net/downloads/binaries/1.16.1/
    Works, but some applets need floating point, which aborts the emulator.
  * https://busybox.net/downloads/binaries/1.17.2/
    Works, but some applets need floating point, which aborts the emulator.
  * https://busybox.net/downloads/binaries/1.21.1/
    It doesn't work.
  * https://busybox.net/downloads/binaries/1.24.0.git-defconfig-multiarch/
    It doesn't work.
  * https://busybox.net/downloads/binaries/1.26.2-defconfig-multiarch/
    It doesn't work.
  * https://busybox.net/downloads/binaries/1.28.1-defconfig-multiarch/
    It doesn't work.
* These precompiled executables have architecture MIPS32r2, so they don't work
  with pts-mips-emulator:
  * https://github.com/darkerego/mips-binaries

About emulation speed. For LZMA2 decompression, qemu-mips on amd64 is about
2.93 times slower than native amd64, and pts-mips-emulator is about 2755
times slower than native amd64. User time was compared. Detailed speed
measurements:

  $ wget -O busybox17 https://busybox.net/downloads/binaries/1.17.2/busybox-mips
  $ chmod +x busybox17  # Needed by qemu-mips.

  # pdftex.doc.tar.xz compressed_size=2404924 uncompressed_size=4218880
  $ time xzcat <pdftex.doc.tar.xz >tdx.tar
  0.15s user 0.01s system 99% cpu 0.157 total
  $ time qemu-mips busybox17 xz -cd <pdftex.doc.tar.xz >tdq.tar
  0.44s user 0.00s system 99% cpu 0.449 total
  $ cmp tdx.tar tdq.tar
  (empty, files are identical)
  $ time perl ./run busybox17 xz -cd <pdftex.doc.tar.xz >tdp.tar
  413.22s user 0.06s system 99% cpu 6:53.33 total
  $ cmp tdx.tar tdp.tar
  (empty, files are identical)

  # pdftex.i386-linux.tar.xz compressed_size=499796 decompressed_size=1372160
  $ time xzcat <pdftex.i386-linux.tar.xz >tix.tar
  0.03s user 0.00s system 96% cpu 0.039 total
  $ time qemu-mips busybox17 xz -cd <pdftex.i386-linux.tar.xz >tiq.tar
  0.12s user 0.00s system 99% cpu 0.121 total
  $ cmp tix.tar tiq.tar
  (empty, files are identical)
  $ time perl ./run busybox17 xz -cd <pdftex.i386-linux.tar.xz >tip.tar
  101.05s user 0.02s system 99% cpu 1:41.08 total
  $ cmp tix.tar tip.tar
  (empty, files are identical)

If you plan to use pts-mips-emulator for LZMA2 decompression (.xz, .lzma,
xzcat, xzdec, unlzma, lzmadec), then please take a look at muxzcat.pl (on
https://github.com/pts/muxzcat) instead, which is about 9.67 times faster
than pts-mips-emulator for this use case, and it runs on the same systems
(Perl >= 5.004_04, either 64-bit or 32-bit).

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