Systems Software

Systems Software Translator Assembler, Cross-Assembler and Disassembler Compiler, Cross-Compiler and Decompiler Interpreters Pre-processors and Macro Processors Loaders and Linkers Device Drivers Operating System Various System Tools Editors Debuggers Integrated Development Environment (IDE) Profilers Project Managers

Need for Systems Software A computer understands only the language of binary 1s and 0s. To implement even a simple function, several thousand lines of binary code may be required, which is not practical for a computer programmer.   So, a semantic gap is said to exist between the application domain (suitable for a programmer) and execution domain (used by the computer).

This semantic gap is made manageable by introduction of a new domain called the Programming Language (PL) Domain. The software developer bridges the specification gap by specifying application programs in terms of programming language i.e. a software developer converts the specification of an application into a computer program.   The system programmer bridges the execution gap by designing system software to interface with the machine, in machine language itself.

Translator

Assembler : An Assembler is a translator denoted as follows: ATAA   Imagine an 8086 ALP program (the actual assembler) that reads an input some other 8086 ALP program and generates its machine code for 8086 machine.

Cross-Assembler: A Cross-Assembler is a translator denoted as follows: ATAB A Cross-Assembler runs on one machine, but assembles ALP of another machine and generates machine code for that machine

Disassembler it converts machine code of a particular machine back to its assembly language. (Recall: reverse engineering).   This idea of reverse engineering is used to recover some lost source code file using its object file.

A Compiler is a language translator that takes as input a source program in some HLL and converts it into a lower-level language (i.e. machine or assembly language).

Cross-Compiler   A Cross-Compiler runs on one machine, but generates machine or assembly code for another machine.

Decompiler Similar to Disassembler for Assembly Language, a Decompiler traces back the machine or assembly code and converts it into source HLL program.

Interpreters An Interpreter is similar to a compiler, but one big difference is that it executes each line of source code as soon as its equivalent machine code is g.enerated

A Pre-processor converts one HLL / ALP into another HLL/ALP . Pre-processors A Pre-processor converts one HLL / ALP into another HLL/ALP . A Macro Processor is an example of pre-processor

Macro Processor A Macro is defined as a single-line abbreviation of a small sequence of statements.   A Macro Processor expands the macro calls and removes macro definitions from an as input source code, which contains these macro definitions and calls.

Linker   A Linker (or a Linkage Editor) takes the object file, loads and compiles the external sub-routines from the library and resolves their external references in the main-program.

Loaders   Programmers usually define the program to be placed at some pre-defined location in the memory. But this loading address given by the programmer may never be used, as it has not been coordinated with the OS.   A Loader does the job of coordinating with the OS to get the initial loading address for the program, prepares the program for execution (i.e. generates an .exe file) and loads it at that address.

Device Drivers A Device Driver is a system software that allows the OS and other applications to communicate with a specific hardware device.  Every different hardware device can understand only its own low-level commands; A Device Driver translates high-level commands by OS or other applications into machine code instructions that are directly executable on the hardware device.

Operating System An operating system can be viewed as an integration of system programs that act as an interface between the user and his computer.   An OS manages CPU and different hardware devices connected to the computer; it also provides system calls for efficient execution of common services needed by the applications.

System Tools System Tools are not complete systems software; rather they assist in writing system or application software and perform various actions on it. Editors Debuggers IDE Profilers Project manager

Summary System programs are needed because it is impractical for a programmer to write, debug and manage huge programs in binary 0s and 1s. Translators convert code of one language to another. Assembler translates assembly language to machine code; Disassembler performs the reverse function. Compiler translates HLL code into machine code; Decompiler uses source HLL information in machine code to do reverse of compilation process. Interpreters compile programs “line-by-line”. So, they are traditionally slow. But a main advantage is that chain of errors can be avoided.

Pre-processors are not translators; input and output languages are the same, only input source is modified to eliminate elements like whitespaces and comments. Linker links object files of various subroutines with that of main program and generates .exe file. Loader manages allocation and relocation of this .exe file in main memory. Device Drivers convert high-level commands from OS (or other applications) into machine code instructions for direct execution on the device. Operating System is an integration of various system programs that act as an interface between a user and his computer.