Fix signed integer parsing

Makefile

@@ -4,6 +4,9 @@ all:
 debug:
 	ocamlbuild -use-ocamlfind -I src oboy.d.byte
 
+check:
+	ocamlbuild -use-ocamlfind -I src -lflag -g test/test_cpu.byte --
+
 clean:
 	rm -rf _build
 	rm -f *.byte

_tags

@@ -1 +1,2 @@
 true: package(unix), package(graphics), warn(A-40-42)
+<test/*>: package(ounit)

src/cpu.ml

@@ -127,6 +127,13 @@ let pop_stack cpu mem =
   cpu.reg.sp <- (cpu.reg.sp + 1) mod 0xFFFF;
   merge_bytes low high
 
+let two_complement n =
+  (lnot n) + 1
+
+let signed_byte n =
+  if n land 0x80 <> 0 then -((two_complement n) land 0xFF)
+  else n
+
 
 
 (**
@@ -165,10 +172,12 @@ let run cpu (mem: Memory.map) =
 
   (* jump *)
   | '\x20' -> let n = read_pc_byte cpu mem in
-              let inst = sprintf "JR \tNZ, 0x%02X" n in
+              let s = signed_byte n in
+              let inst = sprintf "JR \tNZ, 0x%02X (%d)" n s in
               if cpu.flag.z = false then
               begin
-                inc_pc cpu (n - 1); inc_cycles cpu 12;
+                inc_pc cpu (s - 2); (* we read 2 bytes from PC, opcode and n *)
+                inc_cycles cpu 12;
               end else
                 inc_cycles cpu 8;
               inst

test/test_cpu.ml

@@ -0,0 +1,19 @@
+open OUnit2
+
+
+let test_signed test_ctx =
+  let n = 0xFA in
+  let s = Cpu.signed_byte n in
+  assert_equal s (-6)
+
+
+(* Name the test cases and group them together *)
+let suite =
+"suite">:::
+ [
+   "test signed">:: test_signed
+ ]
+
+let () =
+  run_test_tt_main suite
+