// 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/CPU.hdl

/**
 * The Hack CPU (Central Processing unit), consisting of an ALU,
 * two registers named A and D, and a program counter named PC.
 * The CPU is designed to fetch and execute instructions written in 
 * the Hack machine language. In particular, functions as follows:
 * Executes the inputted instruction according to the Hack machine 
 * language specification. The D and A in the language specification
 * refer to CPU-resident registers, while M refers to the external
 * memory location addressed by A, i.e. to Memory[A]. The inM input 
 * holds the value of this location. If the current instruction needs 
 * to write a value to M, the value is placed in outM, the address 
 * of the target location is placed in the addressM output, and the 
 * writeM control bit is asserted. (When writeM==0, any value may 
 * appear in outM). The outM and writeM outputs are combinational: 
 * they are affected instantaneously by the execution of the current 
 * instruction. The addressM and pc outputs are clocked: although they 
 * are affected by the execution of the current instruction, they commit 
 * to their new values only in the next time step. If reset==1 then the 
 * CPU jumps to address 0 (i.e. pc is set to 0 in next time step) rather 
 * than to the address resulting from executing the current instruction. 
 */

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:
    // CU
    cuDecEx(instruction=instruction[3..15],
        isAinstruction=isAinstruction, loadA=loadA, loadD=loadD, selAM=selAM,
        controlBits[0]=zx, controlBits[1]=nx, controlBits[2]=zy, controlBits[3]=ny,
        controlBits[4]=f, controlBits[5]=no, writeM=writeM);
    cuFet(instruction[3]=instruction[15], instruction[0..2]=instruction[0..2],
           zeroResult=zr, negativeResult=ng, loadPC=loadPC);
    
    // Registers
    Mux16(a=out, b=instruction, sel=isAinstruction, out=inA);
    ARegister(in=inA, load=loadA, out=A);
    DRegister(in=out, load=loadD, out=D);

    // ALU
    Mux16(a=A, b=inM, sel=selAM, out=AM);
    ALU(x=D, y=AM,
        zx=zx,
        nx=nx,
        zy=zy,
        ny=ny,
        f=f,
        no=no,
        out=out, zr=zr, ng=ng);
    And16(a=out, b=out, out=outM);

    // addressM
    And16(a=A, b=A, out[0..14]=addressM);
    // PC
    PC(in=A, load=loadPC, inc=true, reset=reset, out[0..14]=pc);
}