From a2f5b2e08ad0e11aef1d116be9bcf8c8bc1559a0 Mon Sep 17 00:00:00 2001
From: Fabien Freling <public@ffreling.com>
Date: Mon, 10 Aug 2015 01:35:47 +0200
Subject: [PATCH] Fix signed integer parsing

---
 Makefile         |  3 +++
 _tags            |  1 +
 src/cpu.ml       | 13 +++++++++++--
 test/test_cpu.ml | 19 +++++++++++++++++++
 4 files changed, 34 insertions(+), 2 deletions(-)
 create mode 100644 test/test_cpu.ml

diff --git a/Makefile b/Makefile
index 2788214..40d7f17 100644
--- a/Makefile
+++ b/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
diff --git a/_tags b/_tags
index 46fd50f..46e4883 100644
--- a/_tags
+++ b/_tags
@@ -1 +1,2 @@
 true: package(unix), package(graphics), warn(A-40-42)
+<test/*>: package(ounit)
diff --git a/src/cpu.ml b/src/cpu.ml
index 102c11d..784b988 100644
--- a/src/cpu.ml
+++ b/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
diff --git a/test/test_cpu.ml b/test/test_cpu.ml
new file mode 100644
index 0000000..ea6790b
--- /dev/null
+++ b/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
+