Stage 11

ExPL Language

Primitive Data Types

int a, b, c;
str mystring;

int a[10];
str stringlist[10];

User Defined Types

• These types must be allocated dynamically.

mytype {
    int a;
    str b;
}

mytype var1, var2;

var = alloc();

var.a = 10;

retval = free(val);

General Program Structure

• Type Definitions - (For user defined)
• Global Declarations
• Functional Definitions

Type Definitions

type
linkedlist {
int data;
linkedlist next;
}

marklist{
str name;
linkedlist marks;
}

endtype

// global declarations
// functions

Global Declarations

• Begins with decl keyword and ends with keyword enddecl.
• string, integer or user defined type.
• Only single dimensional arrays are allowed.
• Values cannot be assigned during this phase.
• Every function except main must be declared before it is used. This is known as Forward Declaration (So that the compiler gets to know about the signature beforehand)
• Parameter Passing - Copy-by-Value and Copy-by-Reference
• Arrays cannot be passed as the parameters (must be globally)
• ExPL allows you to group different kinds of entities of same type (variables, arrays and functions)

str t, q[10], f3(str x)

Function Definitions and the Main function

int fun(int a, int b)
{
    decl
        int c, d;
    enddecl
    begin
        c = a + b;
        d = a - b;
        write(c);
        write(d);
        return c;
    end
}

1. Assignment Statement
2. Conditional Statement
3. Iterative statement
4. Return statement
5. Input/Output statements
6. Break statement
7. Continue statement

Statements and Expressions

• integer or quoted string
• NULL is allowed

Arithmetic and String Expressions

• +,-,*,/,%
• Eg. 5, a[a[5+x]]+x , (f2() + b[x] + 5), sum + listObject.data , a[listObject.data] + f2(listObject) * 8

Conditional Statement

if < Logical Expression > then
    Statements
else 
    Statements
endif;

Iterative Statement

while < Logical Expression > do
    Statements
endwhile;

Breakpoint Statement

breakpoint;

Dynamic memory Allocation

intialize();
t = alloc();
retval = free(t); -> should return NULL

Parameter Passing

type
node {
int x;
}
endtype

decl
node fun(node t);
enddecl

node fun(node t) {
begin
t.x = 3;
t = null;
return t;
end
}

. Visualizing the Stack vs Heap

Imagine r is a piece of paper in main's pocket that has the address "123 Heap Street" written on it.

1. When you call fun(r), you give fun a copy of that paper. Now fun has a piece of paper (t) that also says "123 Heap Street".

2. fun goes to "123 Heap Street" and paints the door red (t.x = 3).

3. Then, fun takes its copy of the paper and scribbles it out (t = null).

4. When you go back to main, main still has its original piece of paper (r) that says "123 Heap Street", and the door at that house is still red.

High Level Library Interface

t = exposcall(fun_code, arg1, arg2, arg3)

(https://exposnitc.github.io/os_spec-files/dynamicmemoryroutines.html)

The libary.lib occupies two pages of disk (13,14) and resides in (63,64)

Important

For any library implementation, remember to save to context of the used registers before call the interrupt

CALL is used for subroutines, but INT is used for interrupt handlers, for library - CALL 0