[Phase4] Update README.md

phase4/README.md

@@ -6,3 +6,29 @@
 
 **12111611 Ruixiang Jiang**
 
+
+
+## Design
+
+### Procedure Call
+
+#### Active Variables
+
+Before function call, we only need to store the active variables before invoking the function
+
+The active variables are stored in the set `active_vars`, which is easy to maintain
+
+The set will be updated only in three cases:
+
+- `x := ...`
+- `PARAM x`
+- `READ x`
+
+We only need to add `x` to `active_vars` while translating the TACs
+
+When we get TACs like `IF x [op] y GOTO label`, we **don't** need to add `x` and `y` to `active_vars`, because `x` and `y` must be already declared in the form of the above three cases
+
+Finally, clear the set after getting a new function TAC `FUNCTION f :`
+
+
+