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projects/05/Memory.hdl

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+// This file is part of www.nand2tetris.org
+// and the book "The Elements of Computing Systems"
+// by Nisan and Schocken, MIT Press.
+// File name: projects/05/Memory.hdl
+
+/**
+ * The complete address space of the Hack computer's memory,
+ * including RAM and memory-mapped I/O. 
+ * The chip facilitates read and write operations, as follows:
+ *     Read:  out(t) = Memory[address(t)](t)
+ *     Write: if load(t-1) then Memory[address(t-1)](t) = in(t-1)
+ * In words: the chip always outputs the value stored at the memory 
+ * location specified by address. If load==1, the in value is loaded 
+ * into the memory location specified by address. This value becomes 
+ * available through the out output from the next time step onward.
+ * Address space rules:
+ * Only the upper 16K+8K+1 words of the Memory chip are used. 
+ * Access to address>0x6000 is invalid. Access to any address in 
+ * the range 0x4000-0x5FFF results in accessing the screen memory 
+ * map. Access to address 0x6000 results in accessing the keyboard 
+ * memory map. The behavior in these addresses is described in the 
+ * Screen and Keyboard chip specifications given in the book.
+ */
+
+CHIP Memory {
+    IN in[16], load, address[15];
+    OUT out[16];
+
+    PARTS:
+    DMux4Way(in=load,sel[0]=address[13],sel[1]=address[14] ,a=ram16k ,b=ram16k2,c=ramScreen);
+    Or(a=ram16k ,b=ram16k2 ,out=ram16end);
+    RAM16K(in=in ,load=ram16end ,address=address[0..13] ,out=out1);
+    Screen(in=in ,load=ramScreen ,address=address[0..12] ,out=out2);
+    Keyboard(out=outKey);
+    Mux4Way16(a=out1 ,b=out1 ,c=out2 ,d=outKey ,sel[0]=address[13],sel[1]=address[14] ,out=out);
+}