The Z80 was introduced, as I understand it, at a clock speed of 2 MHz. Later, Z80-based computers at 3.5 MHz started showing up.
In what year did this faster clock speed become available? (In production quantity, not just samples.)
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The Zilog Z80 (2.5 MHz) was introduced in 1976.
In 1977, the Z80A (4 MHz) was available for $65 for the ceramic package and $59 for the plastic version.
The Z80B (6 MHz) was available in August, 1980 (p. 18) if not earlier.
The Z80H (8 MHz) was introduced in October, 1982 (p. 1).