Four bit adder: Difference between revisions
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The first is <code>4 pack reverse unpack</code>. <code>4 pack</code>, takes the top 4 arguments off the stack and puts them into a nest. <code>reverse</code> reverses the order of the items in a nest, and <code>unpack</code> does the opposite of <code>4 pack</code>. If the stack contained <code>1 2 3 4</code> before performing <code>4 pack reverse unpack</code>, it would contain <code>4 3 2 1</code> afterwards.
This is necessary, because moving four items from the stack to the ancillary stack <code>temp</code> using <code>4 times [ temp put ]</code> and then bringing them back one at a time using <code>temp take</code> will reverse their order, so we preemptively reverse it to counteract that. It is desirable for the task for arguments to <code>4bitadder</code> to be in most significant bit first order so that the intent of, for example, <code>1 1 0 0 1 1 0 0 4bitadder</code> is immediately obvious.
The same reasoning applies to the second instance of <code>4 pack reverse</code>; <code>witheach</code> iterates over a nest, placing each item in the nest (from left to right) on the stack before performing the word or nest that follows it. The <code>fulladder</code> within the nest needs to operate from least to most significant bit, as per the diagram in the task description.
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