Donkey Kong Country 2 and Open Bus

Most of them. Static RAM was (and still is) more expensive since it needs more transistors and chip area per bit stored. It it, however, also much easier to interface since it doesn't need refresh circuitry. This is why you see it in the earliest designs, and also why you see it in so many hobbyist designs. It's also why you tend to see it in the video systems even if the rest of the machine uses DRAM. Dealing with DRAM refresh while reading out the whole memory chip sequentially (while also having a second port to read/write from the CPU!) starts making things very complicated.

But still DRAM is what you would use for a "real" system. Wozniak's design for the Apple II used a clever hack where the system actually runs at 2 MHz with an effective CPU rate of 1 MHz. Any read from a DRAM row will refresh the entire row. Approximately every other cycle the video system steps incrementally through memory, refreshing as it goes.

Same with the VIC-II and the 6510 in the Commodore 64. The video chip is given the main character role for the bus, stopping the CPU from moving forward if it needs cycles for video generation or DRAM refresh.

The clever thing about the Apple II is that there are no refresh cycles. Woz laid out the screen buffer in memory in such a way that simply scanning out the screen will touch every single row of DRAM.

This is more about saving chips than saving cycles, since the Apple II was implemented entirely with 74 series logic. A more traditional approach that used spare cycles during horizontal blanking would have required several more chips.

It does mean that the layout of the Apple II's screen memory is somewhat insane. Those DRAM chips needed to be refreshed every 2ms, and it takes 16ms to scan out a whole screen. Every 8th of the screen needs to be spread out across all 128 rows.

The Sinclair ZX80 and ZX81 have static RAM internally, which you wouldn't expect for a) a computer that's designed to be as cheap as possible and b) uses a Zilog Z80 which has built-in refresh circuitry.

The reason is that the designers saved a few chips by repurposing the Z80's refresh circuit as a counter/address generator, when generating the video signal. Specifically, it uses the instruction fetch cycle to read the character code from RAM, then it uses the refresh cycle to read the actual line of character data from the ROM. The ZX80 nominally clocks the Z80 at 3.25MHz, but a machine cycle is four clocks (two for fetch, two for refresh), so it's effectively the same speed as a 0.8125 MHz 6502.

I wrote a long section here about how the ZX80 uses the CPU to generate the screen and the extra logic that involves, but it was getting too long :) The ZX81 is basically just a cost-reduced ZX80 where all the discrete logic chips are moved into one semi-custom chip.

Doing this makes external RAM packs more expensive too. You couldn't use the real refresh address coming from the Z80 because the video generator would be hopping around a small range of addresses in the ROM, rather than covering the whole of RAM (or at least each row of the DRAM). The designer has two options:

1. Use static RAM in the external RAM pack, making it substantially more expensive for the RAM itself; 2. Use DRAM in the external RAM pack, and add extra refresh circuitry to refresh the DRAM when the main computer is using the refresh cycle doing its video madness.

I think most RAM packs did the second option.