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After completing the course, I thought it would be fun to go back and draw and simulate all the logic gates and chips visually, since the supplied Hardware Simulator does not do this. By “visually”, I mean you see switches for the inputs, and light bulbs for the outputs, and the connections between pins are color-coded to show low or high signals, and you can create your own custom chips made of sub components.
I decided to use the third party program Logicly, because the user interface is really easy and beautifully designed.
I was able to construct the entire Hack chip set, and simulate running a program. Here are some interesting things I ran into:
(1) Instead of RAM16k, I only went up to RAM8. So the RAM memory is 8 16-bit registers. This is because in trying to construct RAM64, I noticed the file load time was getting too long, and I realized that for simulation purposes, 8 registers is all I needed.
(2) Instead of ROM32k, I built a simple ROM chip with only 8 16-bit instructions (for the same file performance reasons above). The 8 fixed instructions I put in are:
0 0000000000000010 @2 // A = 2
1 1110110000010000 D=A // D = 2
2 0000000000000000 @0 // A = 0
3 1110011111001000 M=D+1 // RAM[0] = 2 + 1 = 3
4 1111000010010000 D=D+M // D = 2 + RAM[0] = 2 + 3 = 5
5 1110001100001000 M=D // RAM[0] = 5
6 0000000000000110 @6 // A = 6
7 1110101010000111 0;JMP // goto 6
(3) I completely ignored Screen and Keyboard
(4) The DFF chip is built-in for nand2tetris, but I decided to build it myself in Logicly. And this is where I learned a huge lesson. What happened is I constructed DFF myself using NAND gates feeding into themselves, using a classic configuration. But later when I built and simulated the Program Counter chip (PC), I noticed the light bulbs were going haywire. While the clock input was high, the PC was incrementnig wildly. This is when I realized the DFF that I built was LEVEL triggered, not EDGE triggered. Positive EDGE triggered means the output can only change at the moment when the input changes from low to high. LEVEL triggered means the output can change as long as the level is high, even during the short pulse from the clock. I fixed my DFF to be positive edge triggered, and things predictably worked as it should, perfectly synchronized, ticks and tocks.
(5) I used only 1 oscillating clock input for the entire computer (in the top-most Computer). This means chips like Bit and Register have a CLK input switch which feed off of the top-most oscillating clock. Note that in the nand2tetris HDL files, it is sort of implied that there is only ONE clock driving all the sequential chips.
(6) The Logicly program does not allow for grouping multiple pins together, e.g. I had to painstakingly drag 16 pins for a 16-bit connection, whereas in many cases it would be easier to group them and treat them like one pin. Alas, Logicly does not support this (at least not in version 1.11.4)
In the end, it was very satisfying to watch my CPU literally light up as it processed an instruction, and to see how all the fetching of instructions is driven and synchronized by the oscillating clock. Using the simple Hack program above, I could see visually all the instructions and values pumping through the system, one by one, automatically! It makes your chips comes alive. Well worth doing this, either after the course like I did, or perhaps while your are working on and debugging your chips.
You will definitely gain a better understanding of the difference between combinational and sequential chips, and of how the clock signal works with positive edge triggering, and how that drives the process. Highly recommended!
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