Posted by
ivant on
Dec 01, 2019; 9:36am
URL: http://nand2tetris-questions-and-answers-forum.52.s1.nabble.com/ALU-Failing-to-Load-tp4033963p4033967.html
You should read the excellent
Hardware Constructor Survival Kit post, which describes most of the pitfalls and how to overcome them.
In addition to that, I think your implementation is not correct logically. The specification of the ALU chip is:
/**
* The ALU (Arithmetic Logic Unit).
* Computes one of the following functions:
* x+y, x-y, y-x, 0, 1, -1, x, y, -x, -y, !x, !y,
* x+1, y+1, x-1, y-1, x&y, x|y on two 16-bit inputs,
* according to 6 input bits denoted zx,nx,zy,ny,f,no.
* In addition, the ALU computes two 1-bit outputs:
* if the ALU output == 0, zr is set to 1; otherwise zr is set to 0;
* if the ALU output < 0, ng is set to 1; otherwise ng is set to 0.
*/
// Implementation: the ALU logic manipulates the x and y inputs
// and operates on the resulting values, as follows:
// if (zx == 1) set x = 0 // 16-bit constant
// if (nx == 1) set x = !x // bitwise not
// if (zy == 1) set y = 0 // 16-bit constant
// if (ny == 1) set y = !y // bitwise not
// if (f == 1) set out = x + y // integer 2's complement addition
// if (f == 0) set out = x & y // bitwise and
// if (no == 1) set out = !out // bitwise not
// if (out == 0) set zr = 1
// if (out < 0) set ng = 1
I admit they are a bit ambiguous, but you should be able to work it out from the expected function.
For example, if you have both zx and nx set (1), then the x input should first be "set" to 0, and then negated, which would produce 1111111111111111. That is, the nx should operate on the output of zx. The same for the zy/ny.
The way you compute the AddXY seems wrong. Perhaps you meant to use Add16 instead of the third Or16?
And the ng bit computation seems very convoluted and I can't tell if it's right or wrong. It should be 1 if the out is negative, 0 otherwise. How do we tell if a number is negative?