XPL is a programming language based on PL/I, a portable one-pass compiler written in its own language, and a parser generator tool for easily implementing similar compilers for other languages. XPL was designed in 1967 as a way to teach compiler design principles and as starting point for students to build compilers for their own languages. XPL was designed and implemented by William M. McKeeman, David B. Wortman, James J. Horning and others at Stanford University. XPL was first announced at the 1968 Fall Joint Computer Conference. The methods and compiler are described in detail in the 1971 textbook A Compiler Generator. They called the combined work a 'compiler generator'. But that implies little or no language- or target-specific programming is required to build a compiler for a new language or new target. A better label for XPL is a translator writing system. It helps to write a compiler with less new or changed programming code. Language The XPL language is a simple, small, efficient dialect of PL/I intended mainly for the task of writing compilers. The XPL language was also used for other purposes once it was available. XPL can be compiled easily to most modern machines by a simple compiler. Compiler internals can be written easily in XPL, and the code is easy to read. The PL/I language was designed by an IBM committee in 1964 as a comprehensive language replacing Fortran, COBOL, and ALGOL, and meeting all customer and internal needs. These ambitious goals made PL/I complex, hard to implement efficiently, and sometimes surprising when used. XPL is a small dialect of the full language. XPL has one added feature not found in PL/I: a STRING datatype with dynamic lengths. String values live in a separate text-only heap memory space with automatic garbage collection of stale values. Much of what a simple compiler does is manipulating input text and output byte streams, so this feature helps simplify XPL-based compilers. Components XCOM The XPL compiler, called XCOM, is a one-pass compiler using a table-driven parser and simple code generation techniques. Versions of XCOM exist for different machine architectures, using different hand-written code generation modules for those targets. The original target was IBM System/360, which is a proper subset of IBM System/370, IBM System/390 and IBM System z. XCOM compiles from XPL source code, but since XCOM itself is written in XPL it can compile itself – it is