wip: CPU implementation

project-5-computer-architecture/hdl/CPU.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/5/CPU.hdl
+/**
+ * The Hack Central Processing unit (CPU).
+ * Parses the binary code in the instruction input and executes it according to the
+ * Hack machine language specification. In the case of a C-instruction, computes the
+ * function specified by the instruction. If the instruction specifies to read a memory
+ * value, the inM input is expected to contain this value. If the instruction specifies
+ * to write a value to the memory, sets the outM output to this value, sets the addressM
+ * output to the target address, and asserts the writeM output (when writeM = 0, any
+ * value may appear in outM).
+ * If the reset input is 0, computes the address of the next instruction and sets the
+ * pc output to that value. If the reset input is 1, sets pc to 0.
+ * Note: The outM and writeM outputs are combinational: they are affected by the
+ * instruction's execution during the current cycle. The addressM and pc outputs are
+ * clocked: although they are affected by the instruction's execution, they commit to
+ * their new values only in the next cycle.
+ */
+CHIP CPU {
+
+    IN  inM[16],         // M value input  (M = contents of RAM[A])
+        instruction[16], // Instruction for execution
+        reset;           // Signals whether to re-start the current
+                         // program (reset==1) or continue executing
+                         // the current program (reset==0).
+
+    OUT outM[16],        // M value output
+        writeM,          // Write to M? 
+        addressM[15],    // Address in data memory (of M)
+        pc[15];          // address of next instruction
+
+    PARTS:
+    
+
+    // Instructions.
+    // Decode the instruction bit.
+    // i xx a cccccc ddd jjj
+    // For C-Instruction the byte is stored as i(opcode) xx(notUsed) a cccccc(a/c = comp) ddd(destination) jjj(jump)
+    // i xxxxxxxxxxxxxxx
+    // For A-Instruction the i(opcode) is 0 and the rest is the 16(15)-bit value that is stored there.
+    
+    // Decode Instruction
+    Not(in=instruction[15], out=isAInstruction); // Flip it and if 1 it's an A Instruction
+    And(a=instruction[15], b=true, out=isCInstruction); // Check it with an And static true to check if both 1 then it's C instruction.
+    // Don't need decoding for the rest when A instruction.
+    // Just use instruction because instruction[15] will always be 0
+    // Decode C & Create Wires
+    // 1xA
+    And(a=instruction[12], b=true , out=a1);
+    // 6xC
+    And(a=instruction[11], b=true , out=c1);
+    And(a=instruction[10], b=true , out=c2);
+    And(a=instruction[9], b=true , out=c3);
+    And(a=instruction[8], b=true , out=c4);
+    And(a=instruction[7], b=true , out=c5);
+    And(a=instruction[6], b=true , out=c6);
+    // 3xD
+    And(a=instruction[5], b=true , out=d1);
+    And(a=instruction[4], b=true , out=d2);
+    And(a=instruction[3], b=true , out=d3);
+    // 3xJ
+    And(a=instruction[2], b=true , out=j1);
+    And(a=instruction[1], b=true , out=j2);
+    And(a=instruction[0], b=true , out=j3);
+
+    // aluOut is not made yet 
+    // A Register
+    // Should Load from ALU if 
+    And(a=isCInstruction, b=d1, out=loadAFromALU);
+    Mux16(a=instruction, b=aluOut, sel=loadAFromALU,out=aRegIn);
+    Or(a=isAInstruction, b=loadAFromALU, out=loadA);
+    Register(in=aRegIn, load=loadA, out=aRegOut);
+
+    // D Register
+    And(a=isCInstruction, b=d2, out=loadD);
+    Register(in=aluOut, load=loadD, out=dRegOut);
+    // a - instruction12 - a1
+    // a=0 → use A register
+    // a=1 → use M (inM from memory)
+    // ALU
+    Mux16(a=aRegOut, b=inM , sel=a1 , out=aluYin);
+    ALU(x=dRegOut,
+        y=aluYin,
+        zx=c1, nx=c2, zy=c3, ny=c4 , f=c5 , no=c6 , 
+        out=aluOut , 
+        zr=aluZrOut , 
+        ng=aluNgOut 
+    );
+    
+
+
+
+}