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From what I understand, SRAM is more expensive than DRAM, but takes less support circuitry. That doesn't matter though, since the Z80 includes everything needed to refresh and support DRAM, doesn't it. That's why a lot of computers in the 80's (considering the C64, the ZX Spectrum, etc) all used DRAM instead of SRAM.

Back to the Galaksija, so here are two alternatives than a Z80 + SRAM combination:

  • Intel 8080 + SRAM (advantage: a cheaper CPU)

  • Z80 + DRAM (advantage: cheaper memory)

My understanding could easily be off! I haven't a clue about this kind of thing yet.

So why was SRAM chosen in the Galaksija design? Could have been supply issues in Yugoslavia, which was rather different from a lot of Europe. Or was there a technical reason to do with the way the machine is built?

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Many low cost computers of the era used SRAM instead of DRAM, including the ZX80, TRS-80 Model 100 and VIC-20. There were a few advantages.

  • Much simpler to interface to, which is particularly important for hobby machines where the complexity of DRAM makes the PCB or hand writing significantly more complex. The Galaksija used a single sided PCB that already needed many wire jumpers.

  • SRAM is much easier for beginners to understand in terms of timing. The address and data buses don't need to be multiplexed, the timing is trivial.

  • The computer can be halted and the CPU single stepped for ease of debugging. At the time debugging software was very limited and expensive and people would single step their CPUs and watch the bus activity. Due to needed constant refreshing you can't single step a Z80 CPU with DRAM.

  • The Galaksija used software to generate the video display. To incorporate DRAM refresh requires the refresh cycled to be timed to interlace with the display drawing code, complicating the design significantly.

  • At the time SRAM was more easily available and even cheaper than DRAM in some instances, simply because it was an older technology and very widely used. I don't know the situation in Yugoslavia at the time, but given that DRAM based home computers were relatively new in the west I'd think availability and cost was a big factor.

The Z80 + SRAM is a classic combination for hobby computers of the era (and even now), and based on what Wikipedia says about the development of the machine it seems likely that the designer used a tried and tested and straightforward scheme.

  • 1
    "Many low cost computers of the era used SRAM instead of DRAM" Not realy. While the TRS-80 did use DRAM not SRAM (and even predates the Galaksija by 6 years - a whole era back then), the reasons for the other where outside the SRAM vs. DRAM issue. The VIC 20 got it's SRAM because Commodore had a huge inventory of outdated SRAM, otherwise to be thrown away. In contrast the ZX80 was designed to go with the absolute minimum to lower cost, DRAM was thus out of the picture. – Raffzahn Dec 21 '18 at 11:21
  • Clarification, the TRS-80 Model 100 used SRAM. – user Dec 21 '18 at 11:25
  • TRS-80 Moel 100 is of 1983, but againl a special case, as the Kyotronic 85 was designed for extreme low power usage to extend battery life. It further used rather expensive 8 KiB SRAM - 40 bucks each in 1983 (making RAM 2.5 times more expensive than using 6116, but ~4 times lower power). So The M100 is not realy making a case either. – Raffzahn Dec 21 '18 at 11:49
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    The ZX80 also reuses its refresh cycle for video fetch, albeit from ROM (third-party trickery aside). So it might be that the simplification achieved there outweighs the cost deficit when the thing has only 1kb anyway? – Tommy Dec 21 '18 at 12:54
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    If the computer had memory mapped video, then no particular suffering was required since the memory had to be accessed for video purposes anyway. It was just a question of arranging video memory so the video caused the RAM refresh. The Apple II did that with discrete IC's ! – Harper Dec 22 '18 at 1:41
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This is a question that can't get a definitive answer, as design decisions, especially in such minor areas are not driven by singular logic - or logic at all.

First of all, in 1983 noone would have used a 8080 anymore. If at all, it would have been an 8085 (*1).

Secondly, by 1983 the price difference was marginal at best - especially when considering that it is meant as a home built computer where the builder acquires the parts - both priced below 4 USD. So why not use the better CPU (*2)?

The same is true for the RAM. DRAM only gets less expensive for larger sizes. By late 1983, 2 KiB 6116 were about 4 USD apiece, while a 16 KiBit 4116 cost 1.50 USD or 12 USD for a set. While the prices for sure were different in Yugoslavia, the general ratio should have been quite similar. The Galaksija was meant to run with 2 KiB RAM as a standard (minimum) configuration - offering two more RAM location to expand to whopping 6 KiB (*3).

More importantly, DRAM needs, as mentioned, support circuitry for address multiplexing and refresh. So while the refresh part is simplified on the Z80, the multiplexer is still needed (*4) - plus a satisfying clock source to do so. Thus DRAM increases complexity of a design. Not exactly a good thing when targeting a self builder hobbyist market (*5).

Last but definitively not least, the Galaksija was supposed to use a single sided PCB. Each and every intersection of traces thus needs soldering a wire to bridge either. A lower chip count will in next to all cases result in less wires and less intersection, thus making the job of soldering way less frustrating.


*1 - Single 5V supply is a definitive plus for any design.

*2 - At least as assumed by the general public.

*3 - Keep in mind, it uses character based graphics, so no need for large screen buffers, 512 Bytes is all that is needed. A Spectrum in contrast needs already more than 6 KiB for the display file alone.

*4 - Now, if Zilog had this as well build in, it would have made a killer feature.

*5 - For the implied question, why next to all mass produced machines use DRAM, size is the most obvious answer. Eight 4164 give a full 64 KiB (and 41464 even reduce it to two). At that size it's way cheaper than SRAM. Sheer volume and factory production vs home build also changes the rules.

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