Memory.hdl

// 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:
    // Put your code here:

//for deciding what is loaded
DMux(in=load, sel=address[14], a=loadRAM, b=loadOther); //14 bits needed for RAM, therefore, 15th bit used to access screen or keyboard
DMux(in=loadOther, sel=address[13], a=loadSCREEN, b=null); //14th bit is difference between 24575 and 24576th word 

//RAM 
RAM16K(in=in, load=loadRAM, address=address[0..13], out=RAMout);

//SCREEN
Screen(in=in, load=loadSCREEN, address=address[0..12], out=screenOut);


//KEYBOARD
Keyboard(out=KEYBOARDout);

//for deciding what comesout 
Not(in=address[14], out=ram); //if the 15th bit is 0, then 'ram' will be positive, and cues that ram should be outputed
Not(in=address[13], out=screen); //if the 14th bit is 0, then the address is within the range of SCREEN, and screen will be selected.
 
Mux16(a=KEYBOARDout, b=screenOut, sel=screen, out=screenOrKeyboard);
Mux16(a=screenOrKeyboard, b=RAMout, sel=ram, out=out);



}