nand2tetris/NOTES.md

NOTES

RAM8

trick is to figure out how to mux twice based on address, once for setting the load bit and once for picking the correct output bus.

RAM64

Interesting part here is that the MSB|LSB decision is arbitary. You could use the LSB to pick the RAM8 module, and use the MSB as the address within the RAM8, and it would still work. The caller for RAM64 doesn't care how you use the "complete address". It just cares that you return the same thing for that address.

RAM512

I used the premise of the previous note and decided to index the RAM64 moduls by the LSB instead of the customary MSB. It still works 🤘

Fill.asm

Figured out that my RAM16K implementation was wrong while working on this. The rough pseudocode would be:

int r0=*screen;
while(true) {
  int color = 0;
  if (*kbd > 0) {
    color = -1;
  }

  // This sets an entire row of pixels to color
  // each row has 32 registers (512/16) that we set to color
  *r0 = color;
  *r0+1 = color;
  *r0+2 = color;
  *r0+3 = color;
  *r0+4 = color;
  // and so on
  *r0+31 = color;

  // if we are on the last row
  if (r0-24575 <=0) {
    r0 = *screen;
  }
}

So every "cycle" of the loop, we are coloring an entire row. The row is decided by R0, which is set to @SCREEN at the start. So if you press a key while we are on the middle of the loop (say 120th row), everything from that row onwards would get painted in black, and then the loop resets r0=*screen once we cross the limits. The next iteration of the loop then starts filling the white pixels we'd left in the previous iteration. I kept the smallest paint unit as the row, but it doesn't really matter that much. The only difference is that I'm reading kbd a total of 256 times to paint the screen. Reading once per register also would work, and reading once per "screenfill" would also work. But that changes the 'delay' b/w your keyboard press and the screen fill start. I thought per row was a good compromise. assembly: Since our assembly doesn't have any functions, writing Assembly is much tougher than I thought it would be. All state is global, there are no functions, and you can only jump using goto.