Thread: 68k details
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Old 26 August 2018, 12:24   #213
Megol
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Spent some time reading old magazines and other sources of information of the great dark ages of computing (;p).

The short of it: there were no 70ns DRAM in 1979 when the 68000 was released, there were no 70ns DRAM in 1984 (that I found). There were 80ns DRAM however - specified suitable to mainframes(!!) and optimized for low access times, no exactly fitting the market the 68k was in, seems to be the equivalent of RLDRAM.

Up to at least 1984 there were no fast page mode, only page mode. Fast memories had a minimum cycle time of about 230ns with full RAS/CAS, down to about 120ns in page mode (there were however at least one chip with an 100ns page mode timing in 1984).
Most of the DRAM was much slower.

The page size varied of course with density with 64 to 128 entries relatively common with 256 entries on the higher scale.

So there's that. A reasonable 68000 designer wouldn't design for a memory standard over 6 years in the future, they'd design for something that could be built using available and expected components. Combine that with the constraints and design goals they had and making a memory optimized design isn't realistic IMO.

The 68000 cost about $500 when released, the 8086 cost about $87.
In 1984 the cheapest 1MiB card I could find in the limited time spent was about $2000 BTW.

Data sources: Byte magazine, Mostek databooks, Texas Instruments databooks, misc. searches. Bitsavers and archive.org for the win.
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