Chapter 11-12 , Appendix D C Programs

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Chapter 11-12 , Appendix D C Programs

• Higher Level languages
• Compilers
• C programming
• Converting C to Machine Code
• C Compiler for LC-3

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Higher Level Languages

• High Level Languages give us
• Symbolic Names
• Expressions
• Libraries of functions/subroutines
• Abstraction of underlying hardware
• Readability
• Structure help keep bugs out

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Translating Higher Level Program

• Interpreters
• Compilers

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Compiling C
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Simple Example C Program

• /
• Program Name countdown, our first C program
  
• 
  
• Description This program prompts the user
  to type in
• a positive number and counts down from that
  number to 0,
• displaying each number along the way.
  
• /
• / The next two lines are preprocessor directives
  /
• include ltstdio.hgt
• define STOP 0
• / Function main
  /
• / Description prompt for input, then display
  countdown /
• int main()
• / Variable declarations /

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Terms, Etc.

• Pre processor directives
• define
• include
• Header Files
• ltstdio.hgt
• Data Types
• int
• char
• double
• Scope
• Local
• Global

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Scope

• include ltstdio.hgt
• int globalVar 2 / This variable is
  global /
• int main()
• int localVar 3 / This variable is local
  to main /
• printf("Global d Local d\n", globalVar,
  localVar)
• int localVar 4 / Local to this
  sub-block /
• printf("Global d Local d\n", globalVar,
  localVar)
• printf("Global d Local d\n", globalVar,
  localVar)
• return 0

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Pointers - IMPORTANT

• A pointer is a variable which contains the
  address in memory of another variable.
• We can have a pointer to any variable type.
• The unary or monadic operator gives the
  address of a variable''.
• The indirection or dereference operator gives
  the contents of an object pointed
• to by a pointer''.
• To declare a pointer to a variable do
• int pointer

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Precedence Order of Operations

• See Table 12.5
• Page 323

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Arithmetic Operators

• Symbol Operation Usage Precedence Assoc
• multiply x y 6 l-to-r
• / divide x / y 6 l-to-r
• modulo x y 6 l-to-r
• addition x y 7 l-to-r
• - subtraction x - y 7 l-to-r
• Evaluate expressions left to right.
• / have higher precedence than -.

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Bitwise Operators

• Symbol Operation Usage Precedence Assoc
• bitwise NOT x 4 r-to-l
• ltlt left shift x ltlt y 8 l-to-r
• gtgt right shift x gtgt y 8 l-to-r
• bitwise AND x y 11 l-to-r
• bitwise XOR x y 12 l-to-r
• bitwise OR x y 13 l-to-r
• Operate on variables bit-by-bit.
• Like LC-3 AND and NOT instructions.
• Shift operations are logical (not arithmetic).
  Operate on values -- neither operand is
  changed.

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Logical Operators

• Symbol Operation Usage Precedence Assoc
• ! logical NOT !x 4 r-to-l
• logical AND x y 14 l-to-r
• logical OR x y 15 l-to-r
• Treats entire variable (or value) as TRUE
  (non-zero) or FALSE (zero).
• Result is 1 (TRUE) or 0 (FALSE).

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Relational Operators

• Symbol Operation Usage Precedence Assoc
• gt greater than x gt y 9 l-to-r
• gt greater than or equal x gt y 9 l-to-r
• lt less than x lt y 9 l-to-r
• lt less than or equal x lt y 9 l-to-r
• equal x y 10 l-to-r
• ! not equal x ! y 10 l-to-r
• Result is 1 (TRUE) or 0 (FALSE).
• Note Don't confuse equality () with
  assignment ().

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Special Operators and --

• Changes value of variable before (or after) its
  value is used in an expression.
• Symbol Operation Usage Precedence Assoc
• postincrement x 2 r-to-l
• -- postdecrement x-- 2 r-to-l
• preincrement x 3 r-to-l
• - - predecrement --x 3 r-to-l
• Pre Increment/decrement variable before using
  its value.
• Post Increment/decrement variable after using
  its value.

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Special Operators , , etc.

• Arithmetic and bitwise operators can be
  combinedwith assignment operator.
• Statement Equivalent assignment
• x y x x y
• x - y x x - y
• x y x x y
• x / y x x / y
• x y x x y
• x y x x y
• x y x x y
• x y x x y
• x ltlt y x x ltlt y
• x gtgt y x x gtgt y

All have sameprecedence and associativity as
and associate right-to-left.
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Special Operator Conditional

• Symbol Operation Usage Precedence Assoc
• ? conditional x?yz 16 l-to-r
• If x is TRUE (non-zero), result is y else,
  result is z.
• Like a MUX, with x as the select signal.

y
z
1
0
x
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Example program

• include ltstdio.hgt
• int main()
• int amount / The number of bytes to be
  transferred /
• int rate / The average network transfer
  rate /
• int time / The time, in seconds, for the
  transfer /
• int hours / The number of hours for the
  transfer /
• int minutes / The number of mins for the
  transfer /
• int seconds / The number of secs for the
  transfer /
• / Get input number of bytes and network
  transfer rate /
• printf("How many bytes of data to be
  transferred? ")
• scanf("d", amount)
• printf("What is the transfer rate (in
  bytes/sec)? ")
• scanf("d", rate)

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Symbol Table

• Like assembler, compiler needs to know
  information associated with identifier
• Compiler keeps more information
• Name (identifier)
• Type
• Location in memory
• Scope

Name
Type
Offset
Scope
amount hours minutes rate seconds time
int int int int int int
0 -3 -4 -1 -5 -2
main main main main main main
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Local Variable Storage

• Local variables are stored in an activation
  record, also known as a stack frame.
• Symbol table offset gives thedistance from the
  base of the frame.
• R5 is the frame pointer holds addressof the
  base of the current frame.
• A new frame is pushed on therun-time stack each
  time a block is entered.
• Because stack grows downward,base is the highest
  address of the frame,and variable offsets are lt
  0.

seconds minutes hours time rate amount
R5
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Allocating Space for Variables

• Global data section
• All global variables stored here(actually all
  static variables)
• R4 points to beginning
• Run-time stack
• Used for local variables
• R6 points to top of stack
• R5 points to top frame on stack
• New frame for each block(goes away when block
  exited)
• Offset distance from beginningof storage area
• Global LDR R1, R4, 4
• Local LDR R2, R5, -3

0x0000
instructions
PC
R4
global data
R6
run-time stack
R5
0xFFFF
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Another Example

• / Include the standard I/O header file /
• include ltstdio.hgt
• int inGlobal / inGlobal is a global variable
  because /
• / it is declared outside of all
  blocks /
• int main()
• int inLocal / inLocal, outLocalA,
  outLocalB are all /
• int outLocalA / local to main
  /
• int outLocalB
• / Initialize /
• inLocal 5
• inGlobal 3
• / Perform calculations /
• outLocalA inLocal inGlobal
• outLocalB (inLocal inGlobal) - (inLocal -
  inGlobal)

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Example Symbol Table
Name Type Offset Scope
inGlobal int 0 global
inLocal int 0 main
outLocalA int -1 main
outLocalB int -2 main
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Example Code Generation

• main
• initialize variables
• AND R0, R0, 0 ADD R0, R0, 5
  inLocal 5 STR R0, R5, 0 (offset
  0) AND R0, R0, 0 ADD R0, R0, 3
  inGlobal 3 STR R0, R4, 0 (offset 0)

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Example (continued)

• first statement
• outLocalA inLocal inGlobal
• LDR R0, R5, 0 get inLocal ADD
  R1, R0, 1 increment STR R1, R5, 0
  store LDR R1, R4, 0 get inGlobal
  NOT R1, R1 inGlobal AND R2, R0, R1
  inLocal inGlobal STR R2, R5, -1
  store in outLocalA
  (offset -1)

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Example (continued)

• next statement
• outLocalB (inLocal inGlobal)
  - (inLocal - inGlobal)
• LDR R0, R5, 0 inLocal LDR R1, R4,
  0 inGlobal ADD R0, R0, R1 R0 is sum
  LDR R2, R5, 0 inLocal LDR R3, R4,
  0 inGlobal NOT R3, R3 ADD R3, R3,
  1 ADD R2, R2, R3 R2 is difference
  NOT R2, R2 negate ADD R2, R2, 1
  ADD R0, R0, R2 R0 R0 - R2 STR R0,
  R5, -2 outLocalB (offset -2)

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Another program

• include ltstdio.hgt
• define RADIUS 15.0 / This value is in
  centimeters /
• int main()
• const double pi 3.14159
• double area
• double circumference
• / Calculations /
• area pi RADIUS RADIUS / area
  pir2 /
• circumference 2 pi RADIUS /
  circumference /
• /
  2pir /
• printf("Area of a circle with radius f cm is
  f cm2\n",
• RADIUS, area)
• printf("Circumference of the circle is f
  cm\n",