//John Barr
//testing registers with combinational logic
// Register4 is the PC.  s = 0 will take the value of the PC from start[4]
// s=1 will take the pc from an incremented value
// bits 1 and 0 of the pc are used to specify the write register and the read register.
// So if you begin with s=0, start=0 you will reset the PC
// If you then set s=1, the pc will begin incrementing.
// Note that even though the PC will increment from 0 to 15, only bits 1 and 0 are used
// as input to the Register44.  So Register44 access will cycle from 0 to 1 to 2 to 3
// and back to 0.  
// To test, set s=0, start=0, load = 1 and in=some value.  Go through a complete cycle.
// Now set s=1.  Set in=value 2.  go through a complete cycle.
// leave s=1, set in = value3.  go through a complete cycle.
// leave s=1, set in = value 4.  go through a complete cycle.
// Now leave s=1 and set load=0.  As you go through cycles, you will see your
// values reappear as the PC value increases (and the lower two bits go from 0 to 3).
CHIP pcExample {
	IN in[4], load,s, start[4];
	OUT outBus[4], pc[4];
	PARTS:
	Mux24(a=start, b=pcInc, s=s, out=muxOut);
	Register4(in=muxOut,load=true, out[0]=r0,out[1]=r1,out[2]=r2,out[3]=r3);
	Or(a=r0, b=false, out=pc[0]);
	Or(a=r1, b=false, out=pc[1]);
	Or(a=r2, b=false, out=pc[2]);
	Or(a=r3, b=false, out=pc[3]);
	Register44(in = in, add[0]=r0,add[1]=r1, load = load, chipselect=true, out=outBus);
	add4(a[0]=r0, a[1]=r1, a[2]=r2, a[3]=r3, b[0]=true, b[1]=false, b[2]=false, b[3]=false, cin=false, sum=pcInc);

}