Why Study Low-Level Programming - Lecture Slides | CMSC 212, Study notes of Computer Science

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Announcements

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Program #5– is due Today

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Final– Monday 4-6 in J. M. Patterson 3201

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Pickup old assignments in my office– Midterms (including re-grades)– Programming assignments

Review Lecture

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Final covers all material from the class

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Today's lecture is set of highlights– Also, things not in today's review will likely be on the final!

The “C” Programming Language

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Widely Used for System Programming– Writing Operating Systems and Compilers

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“Middle Aged” Language – created in late 1970’s– Reflects lessons learned from Fortran, COBOL

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Goals– Designed to allow low-level programming– Small language

• Syntax is relatively simple

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Major Differences from JAVA– Explicit Memory Allocation and Deallocation– Procedural rather than object oriented–

Compiles

directly to machine code (rather than byte codes)

Preprocessor

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Not everything is in one file– Definitions of routines are in separate files

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Function

Prototypes

• Define name, parameters, and return types– Sort of like how methods are defined in class definitions– Example: int myFunc(int a, int b); z^

#include– provides ability to include definitions from other files– Generally only include prototypes from external files

• Actual code is kept in a separate file• Files are generally compiled seperately

Stored Program Computer

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A Program:– Consists of a sequence of operations– Is loaded into a computer’s memory to execute

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Standardize set of allowed operations, called

Instructions

• each instruction performs a specific operation– Instructions can
  • read or change memory• compute a value• read or write output
    • What instruction to execute next:
      • normally it is the next instruction• provide control to select alternate next instruction

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Program and data are both stored in memory

Completeness of Instructions

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Need to ensure that the computer can execute anycomputable function.– more instructions may make the machine faster

• but not more powerful

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Arithmetic– add, complement (negate)– AND and NOT (provides NAND which is enough)

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Data movement instructions– Load from memory, Store to memory

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Control instructions– check values of memory or other locations– provide a way to change the next instruction to execute

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Input/Output Instructions

Make: A Tool to Compile Programs

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Problem: Compiling programs is tedious

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Solution: Automate it!

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Done in Unix by the make command–

Uses a file called Makefile

• A makefile is a file (script) containing :
  • Project structure (files, dependencies)– Instructions for files creation

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Make is not limited to C programs

Adapted from: www.cs.tau.ac.il/~dcor/Software1/Makefile.

ppt

Function Arguments

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Comma separated list of values

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All parameters are passed by value– called function can modify values– arrays are passed by the address of their 0th element

• modifying elements of array seen by calling function

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Can pass address of variable to change valuesint foo(int *a) {

*a = 3; } int x;foo(&x);

Bit Fields

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Part of the Power of C is control over data layout– If you need a field with exactly 13 bits, you can define it– Useful for:

• defining fields that interact with hardware– managing pre-defined file formats– saving every last bit of space
  • Syntax: type variable:size; z^

Examples:int foo:13;unsigned int foo:4;typedef struct {

/* from project #1 */

unsigned int opCode:4;unsigned int r1:4;unsigned int r2:4;unsigned int r3:4;unsigned int address:16;} instruction;

Bit Shift Operator

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C has operators to bit shift numbers– number of bits changed depends on size of variable

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Left Shift (number << xxx)– Move each bit of number to the left by xxx bit positions– Leftmost xxx bits discarded– Rightmost xxx bits gets 0

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Right Shift (number >> xxx)– Move each bit of number to the right by xxx bit positions– Rightmost xxx bits discarded– Leftmost xxx bits gets 0

or replicate sign bit

• for unsigned gets 0• for signed, its

implementation dependent

Precedence

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There is a set of rules about precedence of operator– Most important precedence rule:

• When in doubt, put in () to ensure correct order
  • Order (highest to lowest):
    • ()• Function call, subscript, postfix increment/decrement• rest of unary operators• type conversion• Arithmetic operators• Relational operators• Bit operators• Assignment operators• ,

Standard I/O Library

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#include <stdio.h>– includes prototypes for standard I/O routines

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Most I/O is stream based– buffered to allow efficient operations

• mostly to disks or networks
  • interactive I/O is normally flushed at useful points
    • printf("foo\n");

/* \n causes a flush */

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FILE type– used to describe an active I/O connection– three default ones supplied on entry to main:

• stdin - an input stream for default input (often keybaord)• stdout - default output stream (often terminal window)• stderr - default output stream for errors

Strings

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Zero or more characters followed by NUL– not counted as part of string– string.h defines prototypes for string routines

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Copying Strings– size_t strlen(char const *str);

• returns count of characters in str
  • char *strncpy(char *dst, char const *src, size_t len);
    • copy src to dst• copy until NUL in src or at most len characters• pad extra characters will NUL• Safety tip:

dst[len-1] = '\0'; to force termination

• char *strncat(char *dst, char const *src, size_t len);
  • append src onto the end of dst• always appends NUL ('\0') to end of string

Arrays of Pointers

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int *api[10];– An array of 10 pointers to integers;

char *keyword[] = {

"Load","Store","Negate",…