Computer Software

Evolution of Programming Languages

• Machine Languages

• Assembly Languages

• High-Level Languages

• Fourth-Generation Languages

Machine Languages

• The native language of the hardware

• Patterns of binary bits

• Machine specific, is non-portable– is essentially the microcode of the processor

Assembly Languages

• Replace the pattern of binary bits with – mnemonics for the commands– variable names for memory locations

• Essentially a one-to-one mapping of the machine language

• Still machine specific, is non-portable

High-Level Languages

• Represents multiple machine-level statements with single statements

• Easier to learn and use

• Not machine specific, is portable

An ExampleMachine Code Assembly Code High-Level Code in Hex 27BB0001 ldah gp, main main() 23BD8050 lda gp, main {23DEFFF0 lda sp, -16(sp) int a, b, c; A61D8018 ldq r16, 8(gp) a = 3; A77D8010 ldq r27, printf b = 4;47E0F411 mov 7, r17 c = a + b;B75E0000 stq r26, (sp) printf(“\n%d\n”, c);6B5B4000 jsr r26, printf }27BA0001 ldah gp, main A75E0000 ldq r26, (sp) 23BD8050 lda gp, main 47FF0400 clr r0 23DE0010 lda sp, 16(sp) 6BFA8001 ret r26

Move from Language to Language

• Computers only understand machine language.

• Use an ASSEMBLER to convert a program written in assembly language to machine language.

• Use a COMPILER to convert a program written in a high-level language to machine language.

Executable Code

• Program code which is in machine language and can be run (executed) by the computer

• Is hardware dependent you can’t run it on just any machine

• On PCs PC usually have an EXE or a COM extension

Object Code

• Program code which is in machine language but which is not ready to be run (i.e. it is a partial program).

• Is hardware dependent.

• On PCs usually has an OBJ extension.

• Not commonly used by end-users.

Source Code• Program code which is written in a high-level

language.

• Cannot be run by the computer.

• Is usually a text file.

• File name extension usually reflects the high-level language– BASIC - BAS extension– Pascal - PAS extension

Compiler & Files

Object Code

Executable Code

Source

CodeCompiler

LinkerObject Code

Library

Error Messages

Program Listing

Dynamic Link Libraries

• Some Window programs use “run-time” linking and and require the presence of dynamic link library files which have a DLL extension.

• DLL files ideally contain code which would be used my a number of programs.

Object Libraries & DLLs

• Both contain “commonly” used code.

• With object libraries all the code is included in the executable file at compile time so:– programming effort is saved– no space saving is realized, executables which

use common code have their own copy of the library code

Object Libraries & DLLs

• With DLLs the code is “fetched” from the DLL file at “run time” so:– programming effort is saved– space savings “may” be realized– the more executables that use the library code

the greater the savings– typically only a small part of the library code is

used

Common HLLs

• FORTRAN

• COBOL

• BASIC

• Pascal

• C

• C++

• PL/1

• Ada

• JAVA

• LISP

• Prolog

Fourth Generation Languages

• First three generations focused on describing “HOW TO DO” a task.

• 4GLs focus on describing “WHAT TO ACCOMPLISH”

• The fourth generation language compiler must be able to translate the “what to accomplish” description into a machine level “how to” set of instructions.

Categories of 4GLs

• Query languages

• Report generators

• Graphics languages

• Application generators

• Very high-level programming languages

• Application software packages

• Microcomputer tools

Categories of Software

• System Software– Operating Systems (OS)– Language Translators– Utility Programs

• Application Software

Operating Systems

• It is the job of the operating system to manage the computers resources and control the execution of programs.

• Computer resources include– memory– device drivers for peripheral devices

• The OS is the interface between the hardware and the user/application program.

Layers of interaction

Hardware

System Software

Application Software

Users

Operating Systems• OSs run the full spectrum from

– single program-single user, to– multitasking, multi-user, with virtual memory

• The OS decides which program runs and for how long.

• The OS improves throughput by skipping over processes that are waiting for I/O.

Common Operating Systems

• CP/M

• DOS

• Windows 3.1

• Windows 95

• Windows NT

• OS/2

• System 7

• Mac OSX

• Linux

• UNIX

• VMS

• VM/CMS

More RAM needed!

• Newer OSs extend the capabilities of desktop computers but require more RAM

• Windows 3.0 - 1MB (std),2MB (real)

• Windows 95 - 8MB recommended

• Windows NT 4.2 - 16MB (32 rec.)

• Windows 2000 - 64MB min.

• Windows XP - 128MB rec.(64MB min)

• Windows Vista - 1GB rec. (512MB min)