Implement GOF in visa simulator

CHANGES.md

@@ -11,6 +11,8 @@
 
 ### Fixed
 
+- Fixed behavior of the `GOF` instruction in the visa simulator.
+
 ### Removed
 
 ## 0.2.5 (2024-02-09)

project/visa/lib/visa_simulator/src/memory.ml

@@ -25,6 +25,7 @@ end
 type t =
   { mutable register_R0 : int
   ; mutable register_R1 : int
+  ; mutable overflow_flag : bool
   ; output_device : Output_device.t
   ; memory : Ram.t
   }
@@ -33,6 +34,7 @@ type t =
 let create () =
   { register_R0 = 0
   ; register_R1 = 0
+  ; overflow_flag = false
   ; memory = Ram.create ()
   ; output_device = Output_device.create ~len:8
   }
@@ -88,7 +90,14 @@ let write t ~register_name ~address =
   return ()
 ;;
 
-let add t = t.register_R1 <- (t.register_R0 + t.register_R1) mod 256
+let add t =
+  let result = t.register_R0 + t.register_R1 in
+  t.overflow_flag <- result > 255;
+  t.register_R1 <- result % 256
+;;
+
+let overflow_flag t = t.overflow_flag
+let gof t = t.register_R1 <- (if t.overflow_flag then 1 else 0)
 let and_ t = t.register_R1 <- t.register_R0 land t.register_R1
 
 let switch t =

project/visa/lib/visa_simulator/src/memory.mli

@@ -63,3 +63,21 @@ val switch : t -> unit
 val cmp : t -> unit
 
 val not_ : t -> register_name:Visa.Register_name.t -> unit
+
+(** The [overflow_flag] function returns a boolean value indicating whether the
+    last arithmetic operation performed on the registers resulted in an
+    overflow. Currently, the only operation that can cause an overflow is
+    "add".
+
+    Specifically, the [overflow_flag] is initially set to [false]. It is updated
+    every time the [add] operation is performed: if the result of the addition
+    strictly exceeds 255, the flag is set to [true], indicating an overflow;
+    otherwise, it is reset to [false].
+
+    This function provides a way to check the current state of the overflow
+    flag. *)
+val overflow_flag : t -> bool
+
+(** [gof t] ("Get Overflow Flag") sets [R1] to the current value of the
+    [overflow_flag] (1 for true and 0 for false). *)
+val gof : t -> unit

project/visa/lib/visa_simulator/src/visa_simulator.ml

@@ -111,9 +111,7 @@ let execute_instruction t ~instruction =
     Memory.not_ t.memory ~register_name;
     return (increment_code_pointer t)
   | Gof ->
-    (* CR mbarbin: Gof is not used in the calendar, and implemented as Nop in
-       the simulator. It's supposed to set R1 with the overflow_flag of the last
-       register operation, that is set R1 to 1 iif [Add] produced a carry. *)
+    Memory.gof t.memory;
     return (increment_code_pointer t)
   | (Jmp { label } | Jmn { label } | Jmz { label }) as jump_instruction ->
     let%bind code_pointer =