From d67f57b7527d9a420cfc3ffae7629a8b5d427920 Mon Sep 17 00:00:00 2001
From: Thiago Auler dos Santos <thiagoauler@gmail.com>
Date: Thu, 16 Nov 2017 20:12:06 -0200
Subject: [PATCH] implementing relative addressing mode

---
 src/6502.c        | 12 ++++++------
 src/inc/opcodes.h |  3 ++-
 src/opcodes.c     | 37 ++++++++++++++++++++-----------------
 3 files changed, 28 insertions(+), 24 deletions(-)

diff --git a/src/6502.c b/src/6502.c
index 6ef6820..db64b35 100644
--- a/src/6502.c
+++ b/src/6502.c
@@ -9,12 +9,6 @@ void init()
 {
     // pc is set using 0xFFFC
     pc = read_word(0xFFFC);
-    
-    write_mem(0xD010, 'T');
-    write_mem(0xD010, 'E');
-    write_mem(0xD010, 'S');
-    write_mem(0xD010, 'T');
-    write_mem(0xD010, 'S');
 }
 
 void fetch()
@@ -65,6 +59,12 @@ void decode()
             address_mode = absolute_y;
         }
     }
+    
+    if ((ir & 0b00011111) == 0b00010000)
+    {
+        // adjust the addressing mode for branch intructions
+        address_mode = relative;
+    }
 }
 
 void execute()
diff --git a/src/inc/opcodes.h b/src/inc/opcodes.h
index 67a5962..3c73ac8 100644
--- a/src/inc/opcodes.h
+++ b/src/inc/opcodes.h
@@ -44,7 +44,8 @@ enum address_mode
     absolute_y  = 0b110,
     absolute_x  = 0b111,
     indirect_x,
-    zero_page_y
+    zero_page_y,
+    relative
 };
 
 typedef enum opcodes_table oct;
diff --git a/src/opcodes.c b/src/opcodes.c
index 79b63ad..12ddf7c 100644
--- a/src/opcodes.c
+++ b/src/opcodes.c
@@ -75,6 +75,17 @@ void fetch_operand()
             operand = read_byte(address);
             pc = pc + 1;
             break;
+        case relative:
+            address = read_byte(pc);
+            if ((address >> 7) == 0)
+            {
+                address = pc + address;
+            }
+            else
+            {
+                address = pc - address;
+            }
+            break;
     }
 }
 
@@ -141,10 +152,9 @@ void bcc()
     // branch on carry clear
     if(!C_IS_SET)
     {
-        address = read_byte(pc);
-        address = address + pc;
+        fetch_operand();
         pc = address;
-    }
+	}
 }
 
 void bcs()
@@ -152,8 +162,7 @@ void bcs()
     // branch on carry set
     if(C_IS_SET)
     {
-        address = read_byte(pc);
-        address = address + pc;
+        fetch_operand();
         pc = address;
     }
 }
@@ -163,8 +172,7 @@ void beq()
     // branch on result zero
     if(Z_IS_SET)
     {
-        address = read_byte(pc);
-        address = address + pc;
+        fetch_operand();
         pc = address;
     }
 }
@@ -179,8 +187,7 @@ void bmi()
     // branch on result minus
     if(N_IS_SET)
     {
-        address = read_byte(pc);
-        address = address + pc;
+        fetch_operand();
         pc = address;
     }
 }
@@ -190,8 +197,7 @@ void bne()
     // branch on result not zero
     if(!Z_IS_SET)
     {
-        address = read_byte(pc);
-        address = address + pc;
+        fetch_operand();
         pc = address;
     }
 }
@@ -201,8 +207,7 @@ void bpl()
     // branch on result plus
     if(!N_IS_SET)
     {
-        address = read_byte(pc);
-        address = address + pc;
+        fetch_operand();
         pc = address;
     }
 }
@@ -217,8 +222,7 @@ void bvc()
     // branch on overflow clear
     if(!V_IS_SET)
     {
-        address = read_byte(pc);
-        address = address + pc;
+        fetch_operand();
         pc = address;
     }
 }
@@ -228,8 +232,7 @@ void bvs()
     // branch on overflow set
     if(V_IS_SET)
     {
-        address = read_byte(pc);
-        address = address + pc;
+        fetch_operand();
         pc = address;
     }
 }