159.331Prog Lang & Alg1 159.331 Programming Languages & Algorithms Lecture 02 - Aspects of...

159.331 Prog Lang & Alg 1

159.331 Programming Languages & Algorithms

Lecture 02 - Aspects of Programming Languages - Part 1

(Figures from Bal & Grune)


What is a Programming Language?

• A vehicle for the user to tell the computer what to do to solve the user’s problem

• Enables the user to bridge the gap between problem domain and machine domain– Problem domain is all about the application– Machine domain is about memory and cpu,

peripherals and the computer’s capabilities


Which Programming Language?

• There are lots of programming languages (estimates vary but probably over 4000 in existence!)

• Not all languages equally important• Are grouped in families based around key ideas• So we don’t need to know them all (luckily!)• Some support some ideas and algorithms better than

others (“better” might mean in terms of speed or cost)• Its important to know which is best for a particular

sort of problem


Who is the Language For?

• The human - needs to be readable and in language that is memorable and understandable to the human who wrote it and the (possibly different) humans who have to read it

• The computer - just needs a compiler that understands the particular language– Sometimes it does not even need a compiler!


Machine Code• Machine code is about as low a level language as we can

get - maybe not even a “language” at all - just a list of numbers

• Specifies hardware machine instructions like:– Move the number in register X to memory location Y– Add two registers together leaving answer in another– Branch two instructions forward if register X is zero

• Often in hexadecimal - base 16 - translates into values that control the hardware - the voltages on the wires!

• (hex 0123456789ABCDEF)• Rare that we need to work at this level - tools to help us


A Reminder - What is a Computer?• Generally (although not necessarily) a von Neumann

machine (after John von Neumann)• Machine has a state (of the registers etc)• An instruction pointer (IP) telling it which to do

next• Flow of control is to march through its program code• Program acts on memory (and devices which might be

memory-mapped)• Can modify its program data and maybe even its

program code too


Assembly Language

• A set of textual macros and mnemonics for the numbers used in machine codes

• Often manufacturer supplies quite elaborate tools for developing in assembler for their CPU

• Still hard for humans to work with• Often only needed at lowest bootstrap level when

developing a new computer• Maybe needed to optimise speed of some very

important code eg for a device driver


Example is for Motorola 68000 Processor


Assembly language• A step forward from machine code• Macros can do some of the tedious address

calculations• Still machine (CPU product) specific• Only very primitive concepts supported:

– Arithmetic and Logic– Conditionals– Loops– Branches– Memory addressing


Higher-Level Languages• All programming languages are attempts to map a

problem to the machine• To link the user’s problem specific language to the nuts

and bolts of the machine.

• But we can do better than assembler!• Some early “High” level languages - Fortran and Cobol

and C use ideas (and syntax) that still exist in modern languages such as Ada and build on assembler ideas


More readable for Humans than Assembler - and theCompiler can do the loop address calculations for us!

Modern Fortran allows us to Use END DO instead of theNumbered continue statement


Loop that does nothing except change the loop counter

There might be a NOP no operation(a “do-nothing” instruction code)


Reminder - What is a Compiler?

• Compiler is the tool (program) that we run to convert our high level language into machine code

• Turns our text (that we understand) into numbers (that the computer can use)

• So who is our programming language text for?• Compilers ignore comments - they are for us!


Some History

• Remember Computer Science is still quite a young discipline

• The oldest material we are discussing is only late 1940s

• Only around 60 years and much in the last 40

• We can look at the decades as an approximate guide:


1940s- Pre-lingual Stage

• Programming early computers in machine code - toggle switches and punch cards

• Programs were necessarily very small

• Still useful to make machine do very tedious tasks

• Before this your “computer” was a human


1950s - Exploiting Machine Power

• Assembler Language used

• Lots of numerical analysis and number crunching applications

• Scientific (weather and bomb simulations)

• IBM Mathematical FORmula TRANslating System - function library for maths


1960s - Increasing Expressive Power

• FORTRAN developed further - Fortran 66• FORTRAN became a non-vendor specific

as other companies started competing with IBM

• COBOL, Lisp, algol 60, BASIC, PL/I…• “Serious” programming still done in

assembler - machine cycles still (much) more expensive than programmer time!


1970s Portability & Correctness

• Beginnings of the “software crisis”• “Structured Programming” and other

tools/approaches• Pascal, Algol 68, C, Fortran 77• Still lots of assembler• Continuation of the night hacker culture - use the

machine at unfriendly times when cycles are cheaper


1980s Reducing Complexity

• Start of a continuing problem

• Large complex codes

• Ada, Modula-2

• Smalltalk (Object oriented)

• Miranda (Functional)

• Lots of tools (Make) and Environments

• NASA and other still use assembler


1990s - Parallel and Distributed• Massive Parallelism - Special purpose languages for

expensive new computers• Data-Parallel (High Performance Fortran)• Message-Passing• Support in languages: Hermes, Linda, Orca, SR, Ada

9X• Support in special libraries for eg C++• Reduced Instruction Set Chip architectures (RISC) -

too hard for humans to write good assembler


2000’s - Virtual Machines

• Already happening in late 1990s

• Java Virtual Machine and Verification technologies

• Copied elsewhere

• Problem Solving - Very High level languages like Matlab, Mathematica are effectively virtual machines


Maybe the 2000’s were the era of “the virtual machine”eg Java, Matlab, Mathematica,…


Reminder - What is a VM?

• A compiler makes code from program text - eg C or C++ - and stores it in a file (of code)

• An interpreter reads the program text line by line and executes as it goes (usually very slow) - eg Perl or Python interpreters

• A Virtual Machine is a software machine that executes instructions in some language eg Java Bytecode, or Matlab Language– Combines compiling and interpreter ideas


Summary• Aspects of Programming Languages - Part 1

– What is a Programming language?

– What/Who is it For?

– Machines, Code, Assembler

– Higher Level languages

– A Brief history

• Bal & Grune Chapter 1; Sebesta Chapters 1&2• See Mashey’s article in ACM Queue (Jan 05)• Next - Programming Paradigms

159.331Prog Lang & Alg1 159.331 Programming Languages & Algorithms Lecture 02 - Aspects of...

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