The Program Abstraction Revealed as the Software/Hardware Reality: Data Storage and Memory

Tom Kelliher, CS18

Feb. 16, 1996

The Program Abstraction

Control Abstractions

The sequence structure
The selection structure
The repetition structure
Functions

Data Abstractions

C/C++ less abstract than BASIC

Constants

Decimal, octal, hexadecimal integers
Floating point numbers
Character constants (subset of integers)
Strings

Scalar Integer Types

char --- Yes, Virginia, it's a number
short
int
long

Plus unsigned

Scalar Float Types

float
double
long double

Structured Types

The array:

char string[] = "Hello, world.\n";
int data[10] = { 1, 2, 3, 4 };

cout << string;
string[5] = '\0';
cout << string << endl;
data[4] = 29;

The structure (record):

struct employeeRecord
{
   char name[NAMELEN];
   int age;
   int salary;   // unit is cents
};

employeeRecord tom;

strcpy(tom.name, "Thomas P. Kelliher";
tom.age = 34;
tom.salary = 500;

The class --- a further abstraction of the structure

The Software/Hardware Reality

Memory

Main memory --- RAM

Ordered sequence of bytes (value/location)
Byte = 8 bits ( binary digits)
Word size is CPU dependent
Grouped into words, half-words, long-words, etc.

Running program allocates memory for different purposes:

Code segment --- program instructions
Stack segment --- local variables
Heap segment --- dynamic variables
Data segment(s) --- static variables, ``large'' locals

Control Reality

Each instruction stored in a word of memory

The sequence structure

Consider the C code:

taxes = 1000;
netPay = grossPay - taxes;

The corresponding assembler code:

         li $16, 1000
         sw $16, taxes($0)
         lw $17, grossPay($0)
         sub $18, $17, $16
         sw $18, netPay($0)

The selection structure

C code:

if (num > max)
   max = num;

Assembler code:

         lw $16, num($0)
         lw $17, max($0)
         ble $16, $17, eif
         sw $16, max($0)
eif:     ...

The repetition structure

C code:

while (i < 10)
{
   sum += i;
   i++;
}

Assembler code:

         lw $16, i($0)
         lw $17, sum($0)
while:   bge $16, 10, ewhile
         add $17, $17, $16
         add $16, $16, 1
         j while
ewhile:  ...