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ly 6. The most expensive computers
had the widest control stores because
more complicated data paths used
more control lines. The least-costly
computers had narrower control
stores due to simpler hardware but
needed more microinstructions since
they took more clock cycles to execute
a System/360 instruction.
Facilitated by microprogramming,
IBM bet the future of the company
that the new ISA would revolutionize
the computing industry and won the
bet. IBM dominated its markets, and
IBM mainframe descendants of the
computer family announced 55 years
those with 8-bit data paths and as fast
as those with 64-bit data paths could
share a single ISA. The data paths are
the “brawn” of the processor in that
they perform the arithmetic but are rela-
tively easy to “widen” or “narrow.” The
greatest challenge for computer de-
signers then and now is the “brains”
of the processor—the control hard-
ware. Inspired by software program-
ming, computing pioneer and Turing
laureate Maurice Wilkes proposed
how to simplify control. Control was
specified as a two-dimensional ar-
ray he called a “control store.” Each
column of the array corresponded to
one control line, each row was a mi-
croinstruction, and writing microin-
structions was called microprogram-
ming. 39 A control store contains an
ISA interpreter written using micro-
instructions, so execution of a con-
ventional instruction takes several mi-
croinstructions. The control store was
implemented through memory, which
was much less costly than logic gates.
The table here lists four models
of the new System/360 ISA IBM announced April 7, 1964. The data paths
vary by a factor of 8, memory capacity
by a factor of 16, clock rate by nearly 4,
performance by 50, and cost by near-
