Area neutrality. Buffer organizations
achieve area neutrality through narrower
buffers and additional buffer rows. The
number of sense amplifiers decreases linearly
with buffer width, significantly reducing
area because fewer of these large
circuits are required. We take advantage
of these area savings by implementing multiple
rows with latches far smaller than the
removed sense amplifiers. Narrow widths
reduce PCM write energy but negatively
impact spatial locality, opportunities for
write coalescing, and application performance.
However, the additional buffer
rows can mitigate these penalties. We examine
these fundamental trade-offs by
constructing area models and identifying
designs that meet a DRAM-imposed area
budget before optimizing delay and
energy.2
Buffer design space. Figure 2 illustrates
the delay and energy characteristics of
the buffer design space for representative
benchmarks from memory-intensive
scientific-computing applications.18-20 The
triangles represent PCM and DRAM baselines
implementing a single 2,048-byte
buffer. Circles represent various buffer
organizations. Open circles indicate organizations
requiring less area than the DRAM
baseline when using 12F 2 cells. Closed
circles indicate additional designs that become
viable when considering smaller 9F2
cells. By default, the PCM baseline
(see the triangle labeled ‘‘PCM base’’ in
the figure) does not satisfy the area budget
because of larger current-based sense amplifiers
and explicit latches.