PURE INTERPRETATION - IMPLEMENTATION METHODS

• Pure interpretation lies at the opposite end of implementation methods.
• The interpreter program acts as a software simulation of a machine whose fetch-execute cycle deals with high-level language program statements rather than machine instructions.
• This software simulation obviously provides a virtual machine for the language.
• Advantage: implementation of many source-level debugging operations, because all run-time error messages can refer to source-level units.
• For example, array index is found to be out of range, the error message can easily indicate the source line and the name of the array.
• Disadvantage: execution is 10 to 100 times slower than in compiled systems. The primary source of this slowness is the decoding of the high-level language statements, which are far more complex than machine language instructions.
• How many times a statement is executed, it must be decoded every time.
• Therefore, statement decoding, rather than the connection between the processor and memory, is the bottleneck of a pure interpreter.
• Another disadvantage: pure interpretation is that it often requires more space. In addition to the source program, the symbol table must be present during interpretation.
• Furthermore, the source program may be stored in a form designed for easy access and modification rather than one that provides for minimal size.
• Early languages of the 1960s (APL, SNOBOL, and LISP) were purely interpreted, by the 1980s, the approach was rarely used on high-level languages.
• pure interpretation has made a significant comeback with some Web scripting languages, such as JavaScript and PHP, which are now widely used.
• The process of pure interpretation is shown in below Figure.

Fig: Pure Interpretation