Hans-Martin Mosner wrote on Thu, 02 Aug 2007 08:29:54 +0200

When implementing Smalltalk hardware, what kind of object memory design would you prefer:

• object table vs. direct pointer

The great thing about the Mushroom-style virtually addressed caches (which Mario Wolczko is still trying to get Sun to adopt today) is that you get the flexibility of the object table and the performance of direct pointers. It is quite simple: you present to the cache an address with two parts, the object ID and the offset, and if there is a hit you get back the data directly. Only when there is a cache miss do you have to look up in some tables to translate the object ID into a physical address to be able to load the data into the cache.

Normally virtually addressed caches are considered a bad thing even though they are faster than physically addressed caches. But that is due to aliasing problems (two different cache lines might have copies of the same data) which make life difficult for C. Since Smalltalk doesn't have pointer math we don't have to worry about that.

http://www.wolczko.com/mushroom/index.html http://research.sun.com/projects/dashboard.php?id=13

• ref counting vs. generation scavenging cs -whatever-

In general I like whatever works in a distributed context. For a single workstation generation scavenging still seems to be the state of the art. Though these surveys are a bit old, they are still my main reference for garbage collection:

http://www.cs.utexas.edu/users/oops/papers.html#bigsurv (but see paper 12 as well) http://citeseer.ist.psu.edu/228113.html

• page-based virtual memory vs. object-based VM vs. none at all

Object-based is nicer. I like not only what the Mushroom guys did but am also a fan of LOOM and even OOZE. That said, my current design swaps whole groups of objects (perhaps I should call them islands?) from/to disk instead of individual objects. This is based on the fact that the observations that led to the design of the Amoeba operating system (size of memory vs size of files and speed of networks) are even more true today.

• in-band tags vs. out-of-band tags

Let me explain the last one. Smalltalk Implementations have typically had in-bad tags, i.e. the tags have only meaning in a word which is known to be an oop. The Burroughs machines had out-of-band tags, that is, every memory word had an additional tag. That way, you could even know whether a memory word contained uninterpreted bytes, a float value, or an "oop" (descriptor).

My own preference would be what you called out-of-band tags. For example, it is easy to implement a 36 bit processor in a modern FPGA (all internal memories are a multiple of 9 bits, not 8 like earlier chips) using ECC memory cards. The math and addresses would be 32 bits so you would have 4 bits for tags. When dealing with Flash, disks, networks and so on you could always compress/decompress your data and so the mismatch between 36 and 8 bits would not be a problem.

When using development boards it isn't so easy to do this. The ML401, for example, has 1MB of 36 bit ZBT SRAM but the 64MB of SDRAM is only 32 bits wide. And since my current focus is on making my project more Squeak (and C) friendly, RISC42 is a 32 bit processor. I am not sure that I would call Squeak's 1 bit tag and 31 bit data organization "in-band", however. It certainly is when seen from the viewpoint of the hardware or a few critical spots in the virtual machine, but for the rest of the system it looks as much as out-of-band as the Burroughs computers (the first real computer I ever used was a B6700, by the way) ever were.

Just out of curiosity: how much does one pay for such a kit? I don't know whether my wife would approve of a major investment into something of little perceived value :-)

to which Gary Fisher replied:

The Xilinx ML401 runs about US$500.

That is exactly why I got this one - there is a limit to how much electronics tourists can bring back to Brazil and that happens to be $500 :-) This allowed me to avoid the absurd import taxes (which add up to over 100%) that I paid on previous kits and the $100 Laptop.

A more modern kit like the ML501 costs twice as much but there are far cheaper options. The two leading FPGA companies have high end families (Virtex for Xilinx and Stratix for Altera) and low end ones (Spartan and Cyclone respectively) and there are several smaller players with many options. The Spartan 3E Starter Kit that Travis Kay mentioned in one of the first messages to this list, for example, only costs $149.

http://www.xilinx.com/xlnx/xebiz/designResources/ip_product_details.jsp?iLan...

You can get a good idea of what boards are available at:

http://www.fpga-faq.com/FPGA_Boards.shtml

I find the boards from Xess pretty interesting, though there are some other good companies out there. The problem with these companies is that they don't sell many boards each year, so their prices are rather high if you only look at what components are in each board. In contrast, some kits from the chip manufacturers are being sold below cost.

And David T. Lewis wrote:

Well, I don't have anything worthwhile to add, but I'm certainly interested in your discussions. Hope you don't mind folks just lurking and enjoying the read :)

That is the whole point of this list. Matthew and I were having a private discussion and realised that other people might be interested in it. Some might have something to add while others might just want to read this stuff. And which ones are which will probably change over time. Having a list that is archived will allow people who come in later (perhaps via a search) to read this.

-- Jecel