Re: Why we like abstraction
Posted by cadet1620 on Oct 20, 2012; 5:34pm
URL: http://nand2tetris-questions-and-answers-forum.52.s1.nabble.com/Why-we-like-abstraction-tp1914023p4025361.html
This image shows the effect of this bug on bits 0 and 1 when computing
x=0
y=1
zx = nx =1
zy = ny = f = no = 0
The result is
out = 0xFFFF should be 0!
--Mark
(Note: trailing "-" on a signal name means inverted signal.)
URL: http://nand2tetris-questions-and-answers-forum.52.s1.nabble.com/Why-we-like-abstraction-tp1914023p4025361.html
This ALU has a bug in it. It does not properly compute the AND operation because of a bad optimization in the implementation of bits 1..15. Carry out from the full adder is used as the input to the "f" multiplexor. 15 additional Nand gates will be required to invert the half adder carry out to get the correct multiplexor input.
This image shows the effect of this bug on bits 0 and 1 when computing
x=0
y=1
zx = nx =1
zy = ny = f = no = 0
The result is
out = 0xFFFF should be 0!
--Mark
(Note: trailing "-" on a signal name means inverted signal.)
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