Vintage COSMAC Elf is Pretty Close to Original

Popular Electronics was famous for the article introducing the Altair 8800 back in 1975 (well, the cover date was 1975; it really came out in late 1974). That was so popular (no pun intended), that they ran more computer construction articles, including the SWTPC 680 late in 1975. But in 1976 a very popular article ran on building a very simple computer called the COSMAC ELF. [Youtubba] had an Altair, but always wanted a “cute” COSMAC ELF. Now, forty-something years later, he finally got around to it. He made the very detailed video about his experience, below.

Surprisingly, he didn’t have to look very hard for too many of the components as most of them were available from Digikey. He had to get compatible RAM chips, the 1802 CPU and LED displays. He also couldn’t find a look-alike crystal, so he used a fake one and a hidden oscillator. The result looks awfully close to the original. He even did a nice front panel using Front Panel Express.

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Understanding DMA

In the world of computers, the central processing unit (CPU) is–well–central. Your first computer course probably explained it like the brain of the computer. However, sometimes you can overload that brain and CPU designers are always trying to improve both speed and throughput using a variety of techniques. One of those methods is DMA or direct memory access.

As the name implies, DMA is the ability for an I/O device to transfer data directly to or from memory. In some cases, it might actually transfer data to another device, but not all DMA systems support that. Sounds simple, but the devil is in the details. There’s a lot of information in this introduction to DMA by [Andrei Chichak]. It covers different types of DMA and the tradeoffs involved in each one.

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