Wednesday, February 15, 2012

arrayForth notes #5 - Observations and hints

I've since improved the code in the previous entry. I've slimmed it down to 59 words (out of a max of 64). I am not sure I can get it small enough to include the wiring, but I'll look further at exemplar SPI code in the baseline for some more slimming tricks. One thing I am noticing is that literals (numbers) are very expensive. Each literal takes a whole word (18 bit cell) of RAM. This is the reason for such non-intuitive tricks as "dup or" instead of coding the literal "0". "Dup or" doesn't take up a whole RAM cell, so it can be packed with other instructions. Calls take up quite a bit of space too. If your word is shorter than a single 18 bit cell, you will do better just coding it inline rather than do a word call. Programming the GA144 in arrayForth means that you must become an expert in how words are compiled. You can escape this by using polyForth or eForth, but you lose the benefit of understanding how the GA144 actually works. I am still trying to get my arms around wiring, but I remain convinced that the true strength of the GA144 is basically as an FPGA killer for simple concurrent processes. Whereas the strength of a Silabs 8051 or ARM Cortex M is in the richness of peripherals, the GA144 doesn't benefit much from peripherals. It is an FPGA level Erlang. And, like Erlang, it has specific strengths. It's biggest strength is power efficient massive concurrency. I would like to see more I/O, but that only lulls me into the traditional concurrency perspective. I need to stop thinking about having dozens of sensors outputs tied to dozens of GA144 inputs. This isn't a strength. Most assuredly , it is my ability to model dozens of sensors concurrently without dealing with interrupts or global state machines -- that is it's biggest strength.

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