Add more unit tests for Virtual Machine

test/VirtualMachineSpec.hs

@@ -612,3 +612,46 @@ spec = do
       let expected = Left "Attempt to pop from empty stack: tried to pop 1 elements, got 0"
       actual <- run input
       actual `shouldBe` expected 
+
+  describe "call" $ do
+    it "pushes return address and previous frame pointer to the stack" $ do
+      let input = " nop       \n\
+                  \ nop       \n\
+                  \ call &fun \n\
+                  \ nop       \n\
+                  \ halt      \n\
+                  \ fun: halt "
+      --                       ┌────┬─ fp points to the beginning of active stack frame
+      --                   1   0    │
+      --                       │    │
+      let expected = done [4, -1] 6 0
+      --                   ├───┤
+      --                   │   │
+      --                   │   └─ previous fp
+      --                   └───── return address (points to the next instruction after call)      
+      actual <- run input
+      actual `shouldBe` expected 
+    it "pushes stack frames in correct order for multiple calls" $ do
+      let input = "    nop     \n\
+                  \    call &f \n\
+                  \    halt    \n\
+                  \ f: nop     \n\
+                  \    push 1  \n\
+                  \    nop     \n\
+                  \    call &g \n\
+                  \    halt    \n\
+                  \ g: nop     \n\
+                  \    call &h \n\
+                  \ h: halt    "
+      --                       ┌──────────────────────┬─ fp points to the beginning of active stack frame
+      --                    6  5   4  3  2  1   0     │
+      --                       │                      │
+      let expected = done [14, 3, 10, 0, 1, 3, -1] 14 5
+      --                    ├──┤   ├──┤  │  ├───┤
+      --                    │  │   │  │  │  │   │
+      --                    │  │   │  │  │  └───┴─ call &f   
+      --                    │  │   │  │  └──────── push 1  
+      --                    │  │   └──┴─────────── call &g     
+      --                    └──┴────────────────── call &h
+      actual <- run input
+      actual `shouldBe` expected