Problems and Solutions

This section summarizes the major problems encountered when compiling Pascal to Java bytecode and their solutions. Full treatment is given in the subsequent chapters.

No call-by-reference in Java

To create references to variables, they are compiled into arrays of one element. The reference to the array can then be passed in case the variable has to be passed by reference. See section 9.1.

No nesting in Java

Because there is no nesting in Java or in the bytecode, nested procedures and functions have to be made global. This also means that local variables and constants have to be compiled in such a way that nested procedures and functions can still access the local variables and constants of the procedure or function they are declared in. This has to be done by passing the local variables as call-by-reference parameters to every nested procedure and function. Local constants are made global, which saves passing them around. See section 9.2.

No symbolic labeling in the bytecode

All jumps in the bytecode jump to an offset. An algorithm is designed that sets internal labels from which the offsets for the jump instructions are resolved once they are known. See section 10.6.

No non-local goto

In the bytecode the goto instruction can only jump inside a frame. This causes a problem with a goto statement in Pascal that causes a non-local jump. This problem is solved by using an exception to pass the target label and letting the exception handler complete the jump. See section 11.3.