General Computer Science 320201 GenCS I & II Lecture ... - Kwarc

(compileE(e,env) @ cdr, env’’)
end
c○: Michael Kohlhase 304
This completes the compiler for SW (except for the byte code generator which is trivial and an
implementation of environments, which is available elsewhere). So, together with the virtual
machine for L(VM) we discussed above, we can run SW programs on the register machine REMA.
If we now use the REMA simulator from exercise 17 , then we can run SW programs on our com- EdNote:17
puters outright.
One thing that distinguishes SW from real programming languages is that it does not support
procedure declarations. This does not make the language less expressive in principle, but makes
structured programming much harder. The reason we did not introduce this is that our virtual
machine does not have a good infrastructure that supports this. Therefore we will extend L(VM)
with new operations next.
Note that the compiler we have seen above produces L(VM) programs that have what is often
called “memory leaks”. Variables that we declare in our SW program are not cleaned up before the
program halts. In the current implementation we will not fix this (We would need an instruction
for our VM that will “pop” a variable without storing it anywhere or that will simply decrease
virtual stack pointer by a given value.), but we will get a better understanding for this when we
talk about the static procedures next.
Compiling the Extended Example: A while Loop
Example 471 Consider the following program that computes (12)! and the corresponding
L(VM) program:
var n := 12; var a := 1; con 12 con 1
while 2