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Spec
The language specification given here is informal and gives a lot of flexibility for the designer to write the grammatical specifications to his/her own taste. The following features are the minimal requirements for the language.
Global Declarations Function Definitions Main Function Definition
The global declaration part of a SIL program begins with the keyword decl and ends with the keyword enddecl. Declarations should be made for global variables and functions defined in the SIL program.
Global variables may be of type Integer, String, Integer array or String array. The variables declared globally must be allocated statically. String variables can hold strings of maximun length 15. Global variables are visible throughout the program unless suppressed by a redeclaration within the scope of some function. Array type variables can be declared only globally. Only single dimensional arrays are allowed. Variables cannot be assigned values during the declaration phase.
For every function except the main function defined in a SIL program, there must be a declaration. A function declaration should specify the name of the function, the name and type of each of its arguments and the return type of the function. A function can have integer/boolean arguments. Parameters may be passed by value or reference. Arrays cannot be passed as arguments. If a global variable name appears as an argument, then within the scope of the function, the new declaration will be valid and global variable declaration is suppressed. Different functions may have arguments of the same name. However, the same name cannot be given to different arguments in a function. The return type of a function must be either integer or string. The general form of declarations is as follows: Type VarName/FunctionName [ArraySize]/(ParameterList); //Third part needed only for arrays/functions
Example:
decl integer x,y,a[10],b[20]; // x,y are integers, a,b are integer arrays
string u,v,w[10];//u,v are strings ,w is a string array
integer f1(integer a1,a2; boolean b1; integer &c1), f2(); // c1 is passed by reference, rest by value
boolean t, q[10], f3(integer x,string y); // variable, array and a functions declared together
integer swap(integer &x, &y); // x, y are passed by reference
enddecl // Please note the use of "," and ";"
Declaring functions at the beginning avoids the "forward reference" problem and facilitates simpler single pass compilation. Note that the declaration syntax of functions is structurally same as that for variables. Finally, inside swap, the global variables x and y are no more visible because of the re-declaration and global declaration for x is suppressed in f3. If a variable/function is declared twice at the same point, a compilation error should result.
All globally declared variables are visible inside a function, unless suppressed by a re-declaration. Variables declared inside a function are invisible outside. The general form of a function definition is given below:
FunctionName(ArgumentList) {
Local Declarations
Function Body
}
The arguments and return type of each function definition should match exactly with the corresponding declaration. Argument names must be type checked for name equivalence against the declaration.Every declared function must have a definition. The compiler should report error otherwise. The syntax of local declarations and definitions are similar to those of global declarations except that arrays and functions cannot be declared inside a function. Local variables are visible only within the scope of the function where they are declared. Scope rules for parameters are identical to those for variables. The main() function, by specification, must be a zero argument function of type integer. Program execution begins from the body of the main function. The main function need not be declared. The definition part of main should be given in the same format as any other function. The Body of a function is a collection of statements embedded within the keywords begin and end. Example: The definition of swap declared above may look like the following: integer swap (integer &x, &y) { decl integer q // q is re-declared causing suppression of global declaration enddecl begin q = x; x = y; y = q; // Note the syntax for using variables passed by reference. return 1; // swap must return an integer. end // Note that SIL doesn’t support void functions. } Local Variables and parameters should be allocated space in the run-time stack. The language supports recursion and static scope rules apply. 5. Main and Function Body A Body is a collection of statements embedded within the keywords begin and end. Each statement should end with a ‘;’ which is called the terminator. There are five types of statements in SIL. They are: a. Assignment Statement b. Conditional Statement c. Iterative statement d. Return statement e. Input/Output Before taking up statements, we should look at the different kinds of expressions supported by SIL. 6. Expressions SIL has two kinds of expressions, a) Arithmetic and b) Logical 6.1 Arithmetic Expressions Any constant, integer variable or an indexed array variable is a SIL expression provided the expression is within the scope of the concerned variable declarations. SIL treats a function as an expression and the value of a function is its return value. SIL supports recursion. SIL provides five arithmetic operators, viz., +, -, , / (Integer Division) and % (Modulo operator) through which arithmetic expressions may be combined. Expression syntax and semantics are similar to standard practice in programming languages and normal rules of precedence, associativity and paranthesization hold. SIL is strongly typed and any type mismatch must be reported at compile time. Examples: 5, a[a[5+x]]+x , (f2() + b[x] + 5) etc. are arithmetic expressions. 6.2 Logical Expressions Logical expressions can take values TRUE or FALSE. Logical expressions may be formed by combining arithmetic expressions/strings using relational operators. The relational operators supported by SIL are <, >, <=, >=, ==, and !=. The only logical operation supported by strings is == (equality check). Again standard syntax and semantics conventions apply. TRUE and FALSE are constant logical expressions. Every boolean variable is a logical expression and its value is the value stored in its location. Logical expressions themselves may be combined using logical operators AND, OR and NOT. Logical expressions may be assigned to boolean variables only. Note that a relational operator can compare only two arithmetic expressions and not two logical expressions. Similarly, a logical operator can connect only two logical expressions (except for NOT which is a unary logical operator). Example:((x==y)==a[3]) is not valid SIL expression because (x==y) is a logical expression, while a[3] is an arithmetic expression and "==" operates only between two arithmetic expressions/strings. 7. Assignment Statement The SIL assignment statement assigns the value of an expression to a variable, or an indexed array of the same type. String values must be enclosed in double quotes. Strings have a size of at most 16 characters including ‘\0’ at the end(User need not append ‘\0’. It is done by the compiler). Type errors must be reported at compile time. The general syntax is as the following: = ; Example: q[3]=(x==y); t=TRUE; are both valid assignments to boolean variables. 8. Conditional Statement The SIL conditional statement has the following syntax: if then Statements else Statements endif; The else part is optional. The statements inside an if-block may be conditional, iterative, assignment, or input/output statements, but not the return statement. 9. Iterative Statement The SIL iterative statement has the following syntax: while < Logical Expression > do Statements endwhile; Standard conventions apply in this case too. The statements inside a while-block may be conditional, iterative, assignment, or input/output statements, but not the return statement. 10. Return Statement The main body as well as each function body should have exactly one return statement and it should be the last statement in the body. The syntax is: return ; The return value of the function is the value of the expression. The return type should match the type of the expression. Otherwise, a compilation error should occur. The return type of main is integer by specification. 11. Input/Output SIL has two I/O statements – read and write. The syntax is as the following: read (<IntegerVariable/StringVariable>); write (<Arithmetic Expression/StringVariable>); The read statement reads an integer value from the standard input device into an integer variable or an indexed array variable. The write statement outputs the value of the arithmetic expression into the standard output. Note that input/output operations are not allowed on boolean type. Example: read (a [x]); write (7(5+a[9]);
- System Calls The experimental operating system supports the following 10 system calls:
- integer Create(string fileName);
- integer Open(string fileName);
- integer Write(integer fileDescriptor, string buffer, integer numWords);
- integer Seek(integer fileDescriptor, integer lseek);
- integer Read(integer fileDescriptor, string buffer, integer numWords);
- integer Close(integer fileDescriptor);
- integer Delete(string fileName);
- integer Fork();
- integer Exec(string fileName);
- integer Exit(); Refer chapter 8 of Machine Specification for implementation details of system calls.
- An Example SIL Program The following SIL program calculates and prints out the factorial of the first n numbers, value of n read from standard input. decl integer factorial(integer n); enddecl integer factorial (integer n) { decl integer rvalue; enddecl begin if (n==1) then rvalue = 1; else rvalue = n * factorial (n-1); endif; return rvalue; // Note only one RETURN statement is allowed. end } integer main( ){ // Main definition should always begin like this decl integer n,i ; enddecl begin read (n); i = 1; while ( i <= n) do write ( factorial(i)); i = i + 1; endwhile; return 1; // Any integer value may be returned end }