// 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/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 and reads 0. 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 lectures and the book.
 */

// * 4-way 16-bit multiplexor:
// * out = a if sel = 00
// *       b if sel = 01
// *       c if sel = 10
// *       d if sel = 11

 // 0 - 16383 - Ram 16K
 // 16384 - 24575 Screen 8K
 // 24576 - 24576 Keyboard (1 word)
 
 // 16383 = 0(01)1111111111111 Ram 16k
 // 16384 = 0(10)0000000000000 Screen 8k
 // 24576 = 0(11)0000000000000 Keyboard

CHIP Memory {
    IN in[16], load, address[15];
    OUT out[16];

    PARTS:
    // Ram16K
    Mux(a=load, b=false, sel=address[14], out=loadRam);
    Mux(a=true, b=false, sel=address[14], out=isRam);
    // Screen
    Mux(a=false, b=load, sel=address[14], out=loadScreen);
    Mux(a=false, b=true, sel=address[14], out=isScreen);
    // Keyboard
    Mux(a=false, b=true , sel=address[13], out=keyboard1);
    Mux(a=false, b=true, sel=address[14], out=keyboard2);
    // isKeyboard
    And(a=keyboard1, b=keyboard2, out=isKeyboard);

    // Logic
	RAM16K(in=in, load=loadRam, address=address[0..13] , out=outRam);
    Screen(in=in , load=loadScreen, address=address[0..12], out=outScreen);
    Keyboard(out=keyboardOut);

    // Output logic
    Mux4Way16(a=outRam, b=outRam, c=outScreen, d=keyboardOut , sel=address[13..14] , out=out);
}