SRAM macro sparing allows for full chip function despite the loss of one
or more SRAM macros. The controls and data flow for any single macro
within a protected group are made available to the spare or spares for
that group. This allows a defective or failed SRAM macro to be shut off
and replaced by a spare macro, dramatically increasing manufacturing
yield and decreasing field replacement rates. The larger the protected
group, the fewer the number of spares required for similar improvements
in yield, but also the more difficult the task of making all the controls
and dataflow available to the spare(s). In the case of the Level 2 Cache
chip for the planned IBM Z6 computer, there are 4 protected groups with
192 SRAM macros per group. Each protected group is supplanted with an
additional 2 spare SRAM macros, along with sparing controls and dataflow
that allow either spare to replace any of the 192 protected SRAM macros.