Soft-Core Processor Design - CiteSeer

Slowdown Over 29 Available Register Windows
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Number of Register Windows Available to the Programs
Figure 5.5 Performance vs. register file size for the SRAM system
number of register windows available is small. This is due to the higher memory latency, which
increases the overhead of the register file save and restore operations. The benchmark that suffers
the greatest slowdown when a small number of register windows are available is Fibo, on both
ONCHIP and SRAM systems. This is because the Fibo benchmark is a recursive algorithm with
very low computation, so the procedure calls dominate the run time. To understand why the
performance of recursive procedures follows the observed trends, a model of the register file
saving and restoring has to be developed.
When the CWP Manager is called on a register underflow, 8 global registers are first stored on
the stack, so that they may be used by the CWP Manager. The CWP Manager performs
operations in different register windows, so it can only use the global registers, because they are
visible in all register windows. Next, the Local and Input registers in a register window in which
the CWP Manager initially runs, and all the register windows from LO_LIMIT to HI_LIMIT are
stored on the stack. Finally, the Global registers are restored from the stack, along with the Input
registers of the register window at the position HI_LIMIT. Input registers are loaded with the
contents of the Output registers forwarded by the calling procedure before the window underflow
occurred, as required by the normal SAVE operation. After all the registers have been saved, the
CWP Manager returns the control to the program, with CWP at HI_LIMIT. Counting the number
59
Fibo
Non-recursive
by bytes
Recursive by
nibbles
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