Fix signed integer parsing

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