High-level programming (Ch 11-14)

Transcription

1 High-level programming (Ch 11-14) hardware software H/w s/w interface Problems Algorithms Prog. Lang & Interfaces Instruction Set Architecture Microarchitecture (Organization) Circuits Devices (Transistors) Bits 2009 Vijaykumar ECE495K Lecture Notes: Chapter 11 1

2 Programming in C We will cover fundamental concepts in C and how those concepts are reduced to the lowlevel stuff we ve seen so far 2009 Vijaykumar ECE495K Lecture Notes: Chapter 11 2

3 Origins of high-level programming First common high-level language was FORTRAN FORmula TRANslator Originally developed to enable scientists to write programs To get an idea of high-level programming here is an example in C: dot product of n-dimensional vectors u and v Dot product = u[1]*v[1] + u[2]*v[2]+...+ u[n]*v[n] sum = 0 for (i = 1; i<= n; i++) { } sum = sum + u[i]*v[i]; printf ( The dot product is %d\n, sum); 2009 Vijaykumar ECE495K Lecture Notes: Chapter 11 3

4 Quick connection to assembly Like assembly, high-level programs get converted to 0's & 1's The converter is called the compiler (a software program) Compiler converts from high-level to assembly and then the assembler converts from assembly to machine instructions More on this a little later Vijaykumar ECE495K Lecture Notes: Chapter 11 4

5 Goals of high-level programming (1 of 4) Gives symbolic names to data Like labels in assembly but more powerful because programmer need not know where data actually stored E.g., sum, n, u, i, and v in dot product example Data objects (variables) may be in registers or memory Behavior same whether variable in register or memory Whether a name is in register or memory depends on the compiler Also frees programmer from burden of mundane tasks E.g., moving data between memory and registers 2009 Vijaykumar ECE495K Lecture Notes: Chapter 11 5

6 Goals of high-level programming (2 of 4) Gives abstract syntax for operations Programmer need not know details of computation E.g., + and * in dot product example (also for) LC3 doesn t have multiply, but this code does! When compiling this code, the compiler can replace the multiply operation by your homework #6 code One high-level language operation may correspond to lots of machine-level instructions (unlike assembly, which is 1-to-1) 2009 Vijaykumar ECE495K Lecture Notes: Chapter 11 6

7 Goals of high-level programming (3 of 4) Gives abstract syntax for control-flow Programmer need not know details of condition codes, branches, jumps, calls, returns, traps E.g., for, {}, printf in dot product example Compiler can insert register saves and restores in functions/subroutines programmer need not deal with that crud Abstracting data/operations/control-flow makes this independent of underlying machine instructions: portable Same high-level code works on any machine by compiling to that specific machine's instructions 2009 Vijaykumar ECE495K Lecture Notes: Chapter 11 7

8 Goals of high-level programming (4 of 4) Makes code more readable Reduces level of effort needed to maintain code Our example is mathematically simple, so is the code Imagine the effort needed to do this in assembly! Also provides means to protect against bugs! Enforce properties that the code checks E.g., stack overflows/underflows 2009 Vijaykumar ECE495K Lecture Notes: Chapter 11 8

9 Benefits of high-level programming Computer Science/Engineering community has built sophisticated tools to translate high-level programs to lowlevel machine code so that high-level programs run almost as fast as assembly programming Without these sophisticated tools programmers would have to choose between ease of programming and performance, but because of the tools, this bad choice does not arise 2009 Vijaykumar ECE495K Lecture Notes: Chapter 11 9

10 We know computers don t execute highlevel languages! They execute machine code. So, let s convert our high-level language to machine code Vijaykumar ECE495K Lecture Notes: Chapter 11 10

11 Ultimately, everything is 1s and 0s Even assembly language is just a veneer over the 1s and 0s of the instruction set; so is a high-level language High-level language must be converted to machine operations Option 1: Compiler Program that analyzes a high-level program and converts it into another program (usually to machinelevel) This is common for C, Fortran, C++, Java, etc. Assembler is a special case which takes assembly instead of high-level program! 2009 Vijaykumar ECE495K Lecture Notes: Chapter 11 11

12 Ultimately, everything is 1s and 0s Even assembly language is just a veneer over the 1s and 0s of the instruction set; so is a high-level language High-level language must be converted to machine operations Option 2: Interpreter Program that itself performs actions indicated by highlevel program instead of converting high-level code into machine instructions (e.g., LC-3 simulator, shell for shell scripts, Perl, Java, etc.) 2009 Vijaykumar ECE495K Lecture Notes: Chapter 11 12

13 Interpreter: Code development easier Pros and Cons Slow execution because interpreter, the middle man, has to do the work Compiler: Fast execution because converts to machine code which runs directly (no middle man) and compiler can optimize the code for higher speed and less memory (ECE 436) e.g., x = y+y z = x y w = x z Print w Modern optimizing compiler will replace above with Print y 2009 Vijaykumar ECE495K Lecture Notes: Chapter 11 13

14 A simple example /* Program countdown -- counts down to 0 starting from user input number */ #include <stdio.h> #define STOP 0 int main () { int counter; /* intermediate count values */ int startpoint; /* user input starting point */ /* Prompt user for input */ printf ( *** Countdown program **** \n ); printf ( Enter a Positive number: ); 2009 Vijaykumar ECE495K Lecture Notes: Chapter 11 14

15 scanf ( %d, &startpoint); A simple example } /* Count down and print */ for (counter = startpoint; counter >= STOP; counter--) { } printf ( %d\n, counter); printf is a call to a routine that prints a string to display (the routine is similar to our display routine in assembly) scanf is a call to a routine that reads input from keyboard (the routine is similar to our keyboard routine in assembly) 2009 Vijaykumar ECE495K Lecture Notes: Chapter 11 15

16 C compilation process Fig Vijaykumar ECE495K Lecture Notes: Chapter 11 16

17 Preprocessor C compilation process Like assembler directives there are preprocessor directives #define STOP 0 Handy for defining constants such as STOP Easy to change stopping value to something else later Preprocessor replaces STOP with 0 #include <stdio.h> Literally insert the file stdio.h in place of #include <stdio.h> stdio is a library of ready-made I/O routines (printf and scanf) to help progammer so she does not have to write I/O routines More on this later 2009 Vijaykumar ECE495K Lecture Notes: Chapter 11 17

18 C compilation process Compiler Translates high-level program to assembly instructions One high-level statement can and usually do map to many assembly instructions Linker So compilers take other programs as inputs and output the assembly programs (just like assembler takes assembly programs as input and outputs machine code) Each high-level code file coverts to one assembly file which converts to one machine code object file links multiple object files together to produce a executable image 2009 Vijaykumar ECE495K Lecture Notes: Chapter 11 18

19 Formatting Formatting & comments Amount of white space and empty lines between words and lines don't matter but capitals and small matter Make statements line up (indentation) so it is easy to read Put empty lines between sections of code Use meaningful names for variables Entirely for human consumption so it is easy to read Comments Enclosed in between /* and */ -- can be multiple lines Put comments so it is easy for you to understand later when you come back to the code, or for someone else to read and modify it 2009 Vijaykumar ECE495K Lecture Notes: Chapter 11 19

20 Output (printf) and input (scanf) Printf has two parts formatting and values printf ( 43 is a prime ) printf ( %d is a prime, 43) %d is placeholder for an integer printf ( 4 +5 is %d, 4+5) printf ( value of x is %d and y is %d, x, y); %ds take x and y values in order scanf( %d, &count); & is the address of variable count Scanf is input and you want input function to put the value in the memory location of variable count If you forget &, scanf will BLINDLY treat value as address 2009 Vijaykumar ECE495K Lecture Notes: Chapter 11 20