Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...
, register transfer language (RTL) is a term used to describe a kind of intermediate representation (IR) that is very close to assembly language
Assembly language
An assembly language is a low-level programming language for computers, microprocessors, microcontrollers, and other programmable devices. It implements a symbolic representation of the machine codes and other constants needed to program a given CPU architecture...
A compiler is a computer program that transforms source code written in a programming language into another computer language...
. Academic papers and textbooks also often use a form of RTL as an architecture-neutral assembly language. RTL is also the name of a specific IR used in the GNU Compiler Collection
GNU Compiler Collection
The GNU Compiler Collection is a compiler system produced by the GNU Project supporting various programming languages. GCC is a key component of the GNU toolchain...
, and several other compilers, such as Zephyr.
In GCC
In GCC, RTL is generated from the GIMPLE representation, transformed by various passes in the GCC 'middle-end', and then converted to assembly language.
GCC's RTL is usually written in a form which looks like a Lisp
Lisp programming language
Lisp is a family of computer programming languages with a long history and a distinctive, fully parenthesized syntax. Originally specified in 1958, Lisp is the second-oldest high-level programming language in widespread use today; only Fortran is older...
S-expressions or sexps are list-based data structures that represent semi-structured data. An S-expression may be a nested list of smaller S-expressions. S-expressions are probably best known for their use in the Lisp family of programming languages...
This "side-effect expression" says "add the contents of register 138 to the contents of register 139 and store the result in register 140". The SI specifies the access mode for each registers. In the example it is "SImode", i.e. "access the register as 32-bit integer".
The sequence of RTL generated has some dependency on the characteristics of the processor for which GCC is generating code. However, the meaning of the RTL is more-or-less independent of the target: it would usually be possible to read and understand a piece of RTL without knowing what processor it was generated for. Similarly, the meaning of the RTL doesn't usually depend on the original high-level language of the program.
A register transfer language is a system for expressing in symbolic form the microoperation sequences among the registers of a digital module. It is a convenient tool for describing the internal organization of digital computers in concise and precise manner. It can also be used to facilitate the design process of digital systems.
