The Cray-1S, announced in 1979, was an improved Cray-1 that supported a larger main memory of 1, 2, or 4 million words. The larger main memory was made possible through the use of 4,096 x 1-bit bipolar RAM ICs with a 25 ns access time. The Data General minicomputers were optionally replaced with an in-house 16-bit design running at 80 MIPS. The I/O subsystem was separated from the main machine, connected to the main system via a 6 MB/s control channel and a 100 MB/s High Speed Data Channel. This separation made the 1S look like two "half Crays" separated by a few feet, which allowed the I/O system to be expanded as needed. Systems could be bought in a variety of configurations from the S/500 with no I/O and 0.5 million words of memory, to the S/4400 with four I/O processors and 4 million words of memory.
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