Mander, R., Salomon, G. and Wong, Y. A PileMetaphor for Supporting Casual Organisationof Information. Proceedings of Human Factorsin Computing Systems CHI’92, pp 627-634,1992.

That is why building a Second Brain is a journey of personalgrowth. As your information environment changes, the way yourmind operates starts to be transformed.

Simultaneously, there was a revival of the old art of excerpting and the use of commonplace books. Yet, the latter were perceived no longer as memory aids but as true secondary memo-ries. Scholars, in turn, became increasingly aware that to address the informa-tion overload produced by printing, the best solution was to train a card index instead of their own individual consciousness.

We still do not understand how information practices from the worlds of learning, finance, industry, and administration cross-pollinated. From the fourteenth century onward, accountants developed complex instructions for note-taking to describe holdings and transactions, as well for the recording of numbers and calculations. By the seventeenth century, merchants, and indeed ship captains, engineers, and state administrators, were known to travel with trunks of memoranda, massive inventories, scrap books, and various ledgers and log books that mixed descriptive notes and numbers. By the eighteenth century, tables and printed forms cut down on the need for notes and required less description and more systematic numerical notes. Notaries also were master information handlers, creating archives for their legal and financial documents and cross-referencing catalogue systems.

Having made these points many times in the last few years, I've realized that the fundamental problem is in the mistaken belief that the type system has anything whatsoever to do with the storage allocation strategy. It is simply false that the choice of whether to use the stack or the heap has anything fundamentally to do with the type of the thing being stored. The truth is: the choice of allocation mechanism has to do only with the known required lifetime of the storage.

Now compare this to the stack. The stack is like the heap in that it is a big block of memory with a “high water mark”. But what makes it a “stack” is that the memory on the bottom of the stack always lives longer than the memory on the top of the stack; the stack is strictly ordered. The objects that are going to die first are on the top, the objects that are going to die last are on the bottom. And with that guarantee, we know that the stack will never have holes, and therefore will not need compacting. We know that the stack memory will always be “freed” from the top, and therefore do not need a free list. We know that anything low-down on the stack is guaranteed alive, and so we do not need to mark or sweep.

When a garbage collection is performed there are three phases: mark, sweep and compact. In the “mark” phase, we assume that everything in the heap is “dead”. The CLR knows what objects were “guaranteed alive” when the collection started, so those guys are marked as alive. Everything they refer to is marked as alive, and so on, until the transitive closure of live objects are all marked. In the “sweep” phase, all the dead objects are turned into holes. In the “compact” phase, the block is reorganized so that it is one contiguous block of live memory, free of holes.

If we’re in that situation when new memory is allocated then the “high water mark” is bumped up, eating up some of the previously “free” portion of the block. The newly-reserved memory is then usable for the reference type instance that has just been allocated. That is extremely cheap; just a single pointer move, plus zeroing out the newly reserved memory if necessary.

The idea is that there is a large block of memory reserved for instances of reference types. This block of memory can have “holes” – some of the memory is associated with “live” objects, and some of the memory is free for use by newly created objects. Ideally though we want to have all the allocated memory bunched together and a large section of “free” memory at the top.