In
software engineering, porting is the process of adapting
software for the purpose of achieving some form of execution in a
computing environment that is different from the one that a given program (meant for such execution) was originally designed for (e.g., different
CPU, operating system, or third party
library). The term is also used when software/hardware is changed to make them usable in different environments.
Software is
''portable'' when the cost of porting it to a new platform is significantly less than the cost of writing it from scratch. The lower the cost of porting software relative to its implementation cost, the more portable it is said to be.
Etymology
The term "port" is derived from the Latin ''
portāre'', meaning "to carry". When code is not compatible with a particular
operating system or
architecture, the code must be "carried" to the new system.
The term is not generally applied to the process of adapting software to run with less memory on the same CPU and operating system, nor is it applied to the rewriting of source code in a different
language (i.e. language conversion or translation).
Software developers often claim that the software they write is ''
portable'', meaning that little effort is needed to adapt it to a new environment. The amount of effort actually needed depends on several factors, including the extent to which the original environment (the ''source platform'') differs from the new environment (the ''target platform''), the experience of the original authors in knowing which
programming language constructs and third party library calls are unlikely to be portable, and the amount of effort invested by the original authors in only using portable constructs (platform specific constructs often provide a cheaper solution).
History
The number of significantly different CPUs and operating systems used on the desktop today is much smaller than in the past. The dominance of the
x86 architecture means that most desktop software is never ported to a different CPU. In that same market, the choice of operating systems has effectively been reduced to three:
Microsoft Windows,
macOS, and
Linux. However, in the
embedded systems and
mobile markets,
portability remains a significant issue, with the
ARM being a widely used alternative.
International standards, such as those promulgated by the
ISO, greatly facilitate porting by specifying details of the computing environment in a way that helps reduce differences between different standards-conforming
platforms. Writing software that stays within the bounds specified by these standards represents a practical although nontrivial effort. Porting such a program between two standards-compliant platforms (such as
POSIX.1) can be just a matter of loading the source code and
recompiling it on the new platform. However, practitioners often find that various minor corrections are required, due to subtle platform differences. Most standards suffer from "gray areas" where differences in interpretation of standards lead to small variations from platform to platform.
There also exists an ever-increasing number of tools to facilitate porting, such as the
GNU Compiler Collection, which provides consistent programming languages on different platforms, and
Autotools, which automates the detection of minor variations in the environment and adapts the software accordingly before compilation.
The compilers for some
high-level programming languages (e.g.
Eiffel,
Esterel) gain portability by outputting source code in another high level
intermediate language (such as
C) for which compilers for many platforms are generally available.
Two activities related to (but distinct from) porting are
emulating and
cross-compiling.
Porting compilers
Instead of translating directly into
machine code, modern
compilers translate to a machine independent
intermediate code in order to enhance portability of the compiler and minimize design efforts.
The intermediate language defines a ''
virtual machine'' that can execute all programs written in the
intermediate language (a machine is defined by its language and vice versa).
[ describes the terms and their relations.] The intermediate code instructions are translated into equivalent machine code sequences by a ''code generator'' to create
executable code. It is also possible to skip the generation of machine code by actually implementing an
interpreter or
JIT for the virtual machine.
The use of intermediate code enhances portability of the compiler, because only the machine dependent code (the interpreter or the code generator) of the compiler itself needs to be ported to the target machine. The remainder of the compiler can be imported as intermediate code and then further processed by the ported code generator or interpreter, thus producing the compiler software or directly executing the intermediate code on the interpreter.
The machine independent part can be developed and tested on another machine (the ''host machine''). This greatly reduces design efforts, because the machine independent part needs to be developed only once to create portable intermediate code.
An interpreter is less complex and therefore easier to port than a code generator, because it is not able to do code optimizations due to its limited view of the program code (it only sees one instruction at a time, and you need a sequence to do optimization). Some interpreters are extremely easy to port, because they only make minimal assumptions about the instruction set of the underlying hardware. As a result, the virtual machine is even simpler than the target CPU.
Writing the compiler sources entirely in the programming language the compiler is supposed to translate, makes the following approach, better known as ''
compiler bootstrapping'', feasible on the target machine:
# Port the interpreter. This needs to be coded in
assembly code, using an already present
assembler on the target.
# Adapt the source of the code generator to the new machine.
# Execute the adapted source using the interpreter with the code generator source as input. This will generate the machine code for the code generator.
The difficult part of coding the optimization routines is done using the high-level language instead of the assembly language of the target.
According to the designers of the
BCPL language, interpreted code (in the BCPL case) is more compact than machine code; typically by a factor of two to one. Interpreted code however runs about ten times slower than compiled code on the same machine.
The designers of the
Java programming language try to take advantage of the compactness of interpreted code, because a Java program may need to be transmitted over the Internet before execution can start on the target's
Java Virtual Machine.
Porting of video games
Porting is also the term used when a
video game designed to run on one platform, be it an
arcade,
video game console, or
personal computer, is converted to run on a different platform. From the beginning of video games through to the 1990s, "ports", at the time often known as "conversions", were often not true ports, but rather reworked versions of the games due to limitations of different systems. For example, the 1982 game ''
The Hobbit'', a text adventure augmented with graphic images, has significantly different graphic styles across the range of personal computers that its ports were developed for.
However, many 21st century video games are developed using software (often in
C++) that can output code for one or more consoles as well as for a PC without the need for actual porting (instead relying on the common porting of individual component
libraries).
Porting arcade games to home systems with inferior hardware was difficult. The
ported version of
Pac-man for the
Atari 2600 omitted many of the visual features of the original game to compensate for the lack of
ROM space and the hardware struggled when multiple ghosts appeared on the screen creating a flickering effect. The poor performance of the
ported Pac-Man is cited by some scholars as a cause of the
video game crash of 1983.
Many early ports suffered significant gameplay quality issues because computers greatly differed.
Richard Garriott stated in 1984 at
Origins Game Fair that
Origin Systems developed computer games for the
Apple II series first then ported them to
Commodore 64 and
Atari 8-bit, because the latter machines'
sprites and other sophisticated features made porting from them to Apple "far more difficult, perhaps even impossible".
Reviews complained of ports that suffered from "Apple conversionitis",
retaining the Apple's "lousy sound and black-white-green-purple graphics";
after Garriott's statement, when
Dan Bunten asked "Atari and Commodore people in the audience, are you happy with the Apple rewrites?" the audience shouted "No!" Garriott responded, "
therwisethe Apple version will never get done. From a publisher's point of view that's not money wise".
Others worked differently.
Ozark Softscape, for example, wrote ''
M.U.L.E.'' for the Atari first because it preferred to develop for the most advanced computers, removing or altering features as necessary during porting. Such a policy was not always feasible; Bunten stated that "M.U.L.E. can't be done for an Apple",
and that the non-Atari versions of ''
The Seven Cities of Gold'' were inferior.
''
Compute!'s Gazette'' wrote in 1986 that when porting from Atari to Commodore the original was usually superior. The latter's games' quality improved when developers began creating new software for it in late 1983, the magazine stated.
In porting
arcade games, the terms "arcade perfect" or "arcade accurate" were often used to describe how closely the gameplay, graphics, and other assets on the ported version matched the arcade version. Many arcade ports in the early 1980s were far from arcade perfect as home consoles and computers lacked the sophisticated hardware in arcade games, but games could still approximate the gameplay. Notably, ''
Space Invaders'' on the
Atari VCS became the console's
killer app despite its differences, while the later
''Pac-Man'' port was notorious for its deviations from the arcade version.
Arcade-accurate games became more prevalent starting in the 1990s, starting with the
Neo Geo system from
SNK, which offered both a home console and an arcade system with more advanced versions of the home console's main hardware. This allowed near-arcade perfect games to be played at home. Further consoles such as the
PlayStation and
Dreamcast, also based on arcade hardware, made arcade-perfect games a reality.
"(Console) port" is a game that was originally made for a console (such as
Wii or
Xbox 360) before an identical version is created which can be played on a
personal computer or any other console. This term has been widely used by the gaming community. The process of porting a game from a console to a PC is often regarded negatively due to the higher levels of performance that computers generally have being underutilized, partially due to console hardware being fixed throughout their run (with games being developed for console specs), while PCs become more powerful as hardware evolves, but also due to ported games sometimes being poorly optimized for PCs, or lazily ported. Whilst broadly similar, architectural differences may exist such as the use of
unified memory on a console.
See also
*
Software portability
*
Language binding
*
Console emulator
*
List of system quality attributes
*
Source port
*
Write once, compile anywhere
*
Poshlib
*
Cross-platform
*
Program transformation
*
Meaning of ''unported''
Notes
References
*
*{{cite book |author-link=Andrew S. Tanenbaum |first=Andrew S. |last=Tanenbaum |title=Structured computer organization |year=1984 |isbn=0-13-854605-3
Category:Interoperability
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