Bitwise Operations and Miscellaneous Topics

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Bitwise Operations andMiscellaneous Topics

• CS-2301, System Programming for Non-majors
• (Slides include materials from The C Programming
  Language, 2nd ed., by Kernighan and Ritchie and
  from C How to Program, 5th ed., by Deitel and
  Deitel)

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Topics for Today

• Bitwise Operations
• Discussion of Homework 4
• (A little bit of) insight into Operating Systems

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Bitwise Operations

• See 2.9 and 6.9 in Kernighan Ritchie
• Many situation, need to operate on the bits of a
  data word
• Register inputs or outputs
• Controlling attached devices
• Obtaining status

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Bitwise Operations
Corresponding bits of both operands are combined
by the usual logic operations.

• AND
• Result is 1 if both operand bits are 1
• OR
• Result is 1 if either operand bit is 1
• Exclusive OR
• Result is 1 if operand are different

• Complement
• Each bit is reversed
• ltlt Shift left
• Multiply by 2
• gtgt Shift right
• Divide by 2

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Examples

• unsigned int c, a, b
• c a b
• c a b
• c a b
• b a
• c a ltlt 2
• b a gtgt 3

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Example Printer Status Register
Clean
Low ink
Paper jam
Empty paper

• Traditional C definition of bit fields
• define EMPTY 01
• define JAM 02
• define LOW_INK 16
• define CLEAN 64

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Example Printer Status Register (cont.)
Clean
Low ink
Paper jam
Empty paper

• Traditional bit fields (continued)
• char status
• if (status (EMPTY JAM)) ...
• if (status EMPTY status JAM) ...
• while (! status LOW_INK) ...
• int flags CLEAN / turns on CLEAN bit /
• int flags JAM / turns off JAM bit /

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Traditional Bit Definitions

• Used very widely in C
• Including a lot of existing code
• No checking
• You are on your own to be sure the right bits are
  set
• Machine dependent
• Need to bit order in bytes, byte order in words
• Integer fields within a register
• Need to AND and shift to extract
• Need to shift and OR to insert

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Example Printer Status Register (cont.)
Clean
Low ink
Paper jam
Empty paper

• An integer field (traditional style)
• define COUNT (8163264128)
• int c (status COUNT) gtgt 3
• status (c ltlt 3) COUNT

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Alternative Bit-field Definitions
Clean
Low ink
Paper jam
Empty paper

• struct statusReg unsigned int emptyPaperTray
  1unsigned int paperJam 1
  2unsigned int lowInk
  1 1unsigned int
  needsCleaning 1
  1

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Example Printer Status Register (cont.)
Clean
Low ink
Paper jam
Empty paper

• struct statusReg unsigned int emptyPaperTray
  1unsigned int paperJam 1
  1unsigned int count
  5 1unsigned int
  lowInk 1
  1unsigned int needsCleaning 1
  1

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Alternative Bit-fields (continued)

• struct statusReg s
• if (s.empty s.jam) ...
• while(! s.lowInk) ...
• s.needsCleaning true
• s.Jam false
• int c s.count
• s.count - 1

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Warning

• Almost everything about bit fields is
  implementation dependent.
• Especially the order of fields in the struct!
• Consult your hardware and compiler implementation!

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Questions about Bit Fields?
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Revisit Homework 4

• A difficult learning exercise
• Messy algorithm
• Lots of states
• No apparent clean solution
• Not the kind of cut-and-dried problem assignment
  typical of this level of course
• Lessons are deep and subtle

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Ways to Approach a Programming Problem

• Top-down
• I.e., stepwise refinement
• Bottom-up
• I.e., work out the principle algorithm, and then
  build the system infrastructure around it
• Data-oriented
• I.e., define the shape and flow of the data,
  derive the algorithm from it

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Ways to Approach a Programming Problem

• Top-down
• I.e., stepwise refinement
• Bottom-up
• I.e., work out the principle algorithm, and then
  build the system infrastructure around it
• Data-oriented
• I.e., define the shape and flow of the data,
  derive the algorithm from it

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Ways to Approach a Programming Problem

• Top-down
• I.e., stepwise refinement
• Bottom-up
• I.e., work out the principle algorithm, and then
  build the system infrastructure around it
• Data-oriented
• I.e., define the shape and flow of the data,
  derive the algorithm from it

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Ways to Approach a Programming Problem

• Top-down
• I.e., stepwise refinement
• Bottom-up
• I.e., work out the principle algorithm, and then
  build the system infrastructure around it
• Data-oriented
• I.e., define the shape and flow of the data,
  derive the algorithm from it

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Top-down Approach

• Definition Step-wise refinement
• Partition global problem statement into a few
  macro steps
• For each step, refine it into a few sub-steps
• Continue (recursively) until you have the entire
  problem solved.
• Advocated by Edsger Dijkstra

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Application to Homework 4

• int main(int argc, char argv)
• for(i 1 i lt argc i) if (argvi '-')
  // Process Command line switches
• else
• // Open File // ReadAndPrint file with
  width tab // close file
• //for

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Application to Homework 4 (continued)

• int ReadAndPrint(FILE in, int width, int tab)
• while(/not end of file/) // read one
  paragraph (ends in \n or EOF) // Justify
  and print one paragraph

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Application to Homework 4 (continued)

• int ReadAndPrint(FILE in, int width, int tab)
• bool eof false
• while(!eof) while((c fgetc(in))! EOF c
  ! '\n') // append c to string //
  increase size of string if necessary // see
  code fragment from HW4 if (c
  EOF) eof true// Justify and print one
  paragraph

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Application to Homework 4 (continued)

• int JustifyPrint(char s, int width, int tab)
• while(not at end of string s) // scan to
  end of one line // expand tabs while scanning
  // figure out where word break is
• // print the line

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Application to Homework 4 (continued)

• int JustifyPrint(char s, int width, int tab)
• bool endOfPara false
• while(!endOfPara) // scan and copy to end
  of one line // expand tabs while copying
• if (s '\0') endOfPara true
• // print the line

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Application to Homework 4 (continued)

• int JustifyPrint(char s, int width, int tab)
• bool endOfPara false
• while(!endOfPara) // scan and copy to end
  of one line // expand tabs while copying
• if (s '\0') endOfPara true
• // print the line

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Homework 4 (continued)

• Issues and requirements for one line
• Copy each character of string s to line buffer
• I.e., a character array large enough to hold a
  line
• When copying '\t', fill in spaces to itab
• if (s '\t') do linei ' 'while (itab
  ! 0)
• Need to copy as many characters as fit in a line

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Homework 4 (continued)

• Issues and requirements (continued)
• However, if line ends in a middle of a word
• Remove characters back to end of previous word
• Remember them so they can be copied to next line
• Be sure to leading include spaces at beginning of
  paragraph
• But no leading spaces within a paragraph unless
  '\t'
• Special case a word with no spaces is too
  long to fit on one line
• Usually occurs with URLs
• Short lines at end of paragraph treated
  differently

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Stepwise Refinement for Homework 4

• Works pretty well
• until we get to nitty-gritty of the core
  algorithm.
• And then, it is not clear whether data structure
  or algorithm work out.

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Stepwise Refinement for Homework 4

• Works pretty well
• until we get to nitty-gritty of the core
  algorithm.
• And then, it is not clear whether data structure
  or algorithm work out.
• In fact, they didnt work out on first 2-3
  attempts

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What about Bottom-up Design

• Start with an algorithm to scan one line directly
  from file input
• Handle the special circumstances
• When reading from input, how to handle characters
  that dont fit at end of line
• And pass them to next line

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Bottom-up Design (continued)

• How do we deal with EOF and '\n', ?
• Need to communicate back up the function call
  stack
• Functions cannot return multiple values
• Need to pass information back by reference
• Very complex semantics, pre- and post-conditions

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Data-Oriented Design

• Scanning an input stream
• Need to un-scan when word does not fit at end
  of line
• Same problems as with Bottom-up Design

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Stepwise Refinement (again)

• First attempts at top-down approach were wrong!

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Application to Homework 4 (continued)

• int ReadAndPrint(FILE in, int width, int tab)
• while(/not end of file/) // read one
  paragraph (ends in \n or EOF) // Justify
  and print one paragraph

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Application to Homework 4 (continued)

• int ReadAndPrint(FILE in, int width, int tab)
• bool eof false
• while(!eof) while((c fgetc(in))! EOF c
  ! '\n') // append c to string //
  increase size of string if necessary // see
  code fragment from HW4 if (c
  EOF) eof true// Justify and print one
  paragraph

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Stepwise Refinement (again)

• First attempts at top-down approach were wrong!
• Needed to separate EOF from '\n' and add
  paragraph loop
• Not at all obvious on first attempt to develop
  the refinement
• Needed to bump into a brick wall in order to have
  enough information to do it right

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Stepwise Refinement (again)

• First attempts at top-down approach were wrong!
• Needed to separate EOF from '\n' and add
  paragraph loop
• Not at all obvious on first attempt to develop
  the refinement
• Needed to bump into a brick wall in order to have
  enough information to do it right
• Several times!

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Application to Homework 4 (again)

• int JustifyPrint(char s, int width, int tab)
• bool endOfPara false
• while(!endOfPara) // scan and copy to end
  of one line // expand tabs while copying
• if (s '\0') endOfPara true
• // print that line

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GetOneLine()

• Inputs
• Pointer to string s, line buffer line
• Starting character position n
• Tab width and line length maxLen
• Output stream FILE out
• Result
• Starting position of next line

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GetOneLine() (continued)

• int GetOneLine(char s, char line, int n, ...)
  int lp, sp nfor(lp 0 ssp!'\0'
  lpltmaxLen sp) if (ssp '\t') do
  linelp ' ' while (lptab ! 0)
  else linelp sspif
  (ssp'\0') linelp '\0' return
  spelse
• // scan backwards to end of last word// set
  linelp '\0'// scan forward to next
  non-blank, return sp

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GetOneLine() (continued)

• int GetOneLine(char s, char line, int n, ...)
  int lp, sp nfor(lp 0 ssp!'\0'
  lpltmaxLen sp) if (ssp '\t') do
  linelp ' ' while (lptab ! 0)
  else linelp sspif
  (ssp'\0') linelp '\0' return
  spelse
• // scan backwards to end of last word// set
  linelp '\0'// scan forward to next
  non-blank, return sp

Expand tabs!
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GetOneLine() (continued)

• int GetOneLine(char s, char line, int n, ...)
  int lp, sp nfor(lp 0 ssp!'\0'
  lpltmaxLen sp) if (ssp '\t') do
  linelp ' ' while (lptab ! 0)
  else linelp sspif
  (ssp'\0') linelp '\0' return
  spelse
• // scan backwards to end of last word// set
  linelp '\0'// scan forward to next
  non-blank, return sp

Copy character!
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GetOneLine() (continued)

• int GetOneLine(char s, char line, int n, ...)
  int lp, sp nfor(lp 0 ssp!'\0'
  lpltmaxLen sp) if (ssp '\t') do
  linelp ' ' while (lptab ! 0)
  else linelp sspif
  (ssp'\0') linelp '\0' return
  spelse
• // scan backwards to end of last word// set
  linelp '\0'// scan forward to next
  non-blank, return sp

Test for end of paragraph!
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GetOneLine() (continued)

• int GetOneLine(char s, char line, int n, ...)
  int lp, sp nfor(lp 0 ssp!'\0'
  lpltmaxLen sp) if (ssp '\t') do
  linelp ' ' while (lptab ! 0)
  else linelp sspif
  (ssp'\0') linelp '\0' return
  spelse
• // scan backwards to end of last word// set
  linelp '\0'// scan forward to next
  non-blank, return sp

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GetOneLine() (continued)

• There is still more to do
• Scan backward through line and string
• To find end of last word ( end of line)
• To find start of next word ( start of next line)
• Be sure not to get confused by expanded tabs

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Discussion or Questions?