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Color Classic won't boot with FPU installed


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Modern RAM speed is specific because accesses can't happen too soon and shouldn't happen too long after data is ready. The speeds have to do with clocking of the data in and out of memory sequentially after the first location is accessed. Old RAM used to just care about overall speed  - if your computer requires 80 nanosecond or faster, then that's the only measurement that matters, for the most part (this is before EDO).

 

Completely static chip designs can run at any speed, or can even be held in a stopped state. Chip designs with dynamic components (registers, caches, whatever, implemented using capacitors) can only be clocked as slow as the safe minimum speed, but since all Motorola m68020 / m68030 / m68881 / m68882 chips sold at speeds from 16 MHz (or 12 MHz, for the early '020) to 50 MHz, all chips will run safely at 16 MHz without issues.

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I'm late replying, sorry @johnklos.

Translating this to my very limited understanding of RAM: The specific speed of modern DDRn RAM is determined by the RAM-internal time management necessary to bring the data to the in-out-pins [and additionally handling refresh cycles?]. Is this a reasonable interpretation?

 

About my 68882 I bought in 1992: So it may even be rated for higher speeds. I'll look that up when I'm burying it out of the storage boxes pile.

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Modern DDR memory runs communication to the computer's bus at many gigahertz (whatever the memory is rated for), but in reality dynamic RAM is still not all that much faster than it ever has been. Memory access time in the late 1980s was typically in the range of 60 to 100 nanoseconds. The fastest DDR4 memory in the world can only manage access in around 8 nanoseconds.

 

That represents a speed increase of eight to ten times faster, while density has increased ten thousand fold. This is why DDR exists - you pipeline reads and writes so that you're waiting that full 8 nanoseconds as infrequently as possible. At 4 GHz, your cycle time is .25 nanoseconds, so waiting for memory could take up to 32 clock cycles!

 

To answer your question, the specific speed is the speed that the memory controller and memory on the DIMM can pipeline continuous operations.

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