Implement DEC (absolute, X).

hainesr · Oct 24, 2020 · 80169b2 · 80169b2
1 parent a4c9724
commit 80169b2
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diff --git a/lib/mos6502/cpu_instructions.rb b/lib/mos6502/cpu_instructions.rb
@@ -594,6 +594,12 @@ def instructions
           compare(@a, @memory.get(absolute(@x)))
         },
 
+        # DEC (absolute, X)
+        0xde => lambda {
+          address = absolute(@x)
+          @memory.set(address, dec(@memory.get(address)))
+        },
+
         # CPX (immediate)
         0xe0 => lambda {
           compare(@x, next_byte)

diff --git a/test/cpu_instructions_test.rb b/test/cpu_instructions_test.rb
@@ -1303,6 +1303,27 @@ def test_0xdd
     assert(@cpu.zero?)
   end
 
+  def test_0xde
+    @cpu.load!(
+      [
+        0xa9, 0x01, 0xa2, 0x08, 0x9d, 0x92, 0x53,
+        0xde, 0x92, 0x53, 0xde, 0x92, 0x53
+      ]
+    )
+    @cpu.step
+    @cpu.step
+    @cpu.step
+    assert_equal([0x01], @cpu.dump_memory(0x539a, 1))
+    @cpu.step
+    assert_equal([0x00], @cpu.dump_memory(0x539a, 1))
+    refute(@cpu.negative?)
+    assert(@cpu.zero?)
+    @cpu.step
+    assert_equal([0xff], @cpu.dump_memory(0x539a, 1))
+    assert(@cpu.negative?)
+    refute(@cpu.zero?)
+  end
+
   def test_0xe0
     @cpu.load!([0xa2, 0x20, 0xe0, 0x10, 0xe0, 0x30, 0xe0, 0x20])
     @cpu.step