Page 194 - DCAP104_EXPOSURE_TO_COMPUTER_DISCPLINES

Unit 9: Software Development

Compilers are used to translate source code from a programming language Notes
into either object code or machine code. Object code needs further processing
to become machine code, and machine code is the central processing unit’s
native code, ready for execution.

Interpreted computer programs -in a batch or interactive session—are either
decoded and then immediately executed or are decoded into some efficient
intermediate representation for future execution. BASIC, Perl, and Python
are examples of immediately executed computer programs. Alternatively,
Java computer programs are compiled ahead of time and stored as a machine
independent code called bytecode. Bytecode is then executed on request by an
interpreter called a virtual machine.
The main disadvantage of interpreters is that computer programs run slower
than when compiled. Interpreting code is slower than running the compiled
version because the interpreter must decode each statement each time it is
loaded and then perform the desired action. However, software development
may be faster using an interpreter because testing is immediate when the
compiling step is omitted. Another disadvantage of interpreters is that at least
one must be present on the computer during computer program execution.
By contrast, compiled computer programs need no compiler present during
execution.

No properties of a programming language require it to be exclusively
compiled or exclusively interpreted. The categorization usually reflects
the most popular method of language execution. For example, BASIC is
thought of as an interpreted language and C a compiled language, despite
the existence of BASIC compilers and C interpreters. Some systems use
just-in-time compilation (JIT) whereby sections of the source are compiled
‘on the fly’ and stored for subsequent executions.
9.1.10 Self-Modifying Programs

A computer program in execution is normally treated as being different from the data the program
operates on. However, in some cases this distinction is blurred when a computer program modifies
itself. The modified computer program is subsequently executed as part of the same program.
Self-modifying code is possible for programs written in machine code, assembly language, Lisp,
C, COBOL, PL/1, Prolog and JavaScript (the eval feature) among others.
9.1.11 Execution and Storage

Typically, computer programs are stored in non-volatile memory until requested either
directly or indirectly to be executed by the computer user. Upon such a request, the program
is loaded into random access memory, by a computer program called an operating system,
where it can be accessed directly by the central processor. The central processor then executes
(“runs”) the program, instruction by instruction, until termination. A program in execution
is called a process. Termination is either by normal self-termination or by error, software
or hardware error.

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