80960SA
purpose registers provided in other popular micro-
processors. The term global refers to the fact that
these registers retain their contents across
procedure calls.
The local registers, on the other hand, are procedure
specific. For each procedure call, the 80960SA
allocates 16 local registers (r0 through r15). Each
local register is 32 bits wide.
1.1.4
Multiple Register Sets
Although programs may have procedure calls nested
many calls deep, a program typically oscillates back
and forth between only two to three levels. As a
result, with four stack frames in the cache, the proba-
bility of having a free frame available on the cache
when a call is made is very high. In fact, runs of
representative C-language programs show that 80%
of the calls are handled without needing to access
memory.
If four or more procedures are active and a new
procedure is called, the 80960SA moves the oldest
local register set in the stack-frame cache to a
procedure stack in memory to make room for a new
set of registers. Global register g15 is the frame
pointer (FP) to the procedure stack.
Global registers are not exchanged on a procedure
call, but retain their contents, making them available
to all procedures for fast parameter passing.
To further increase the efficiency of the register set,
multiple sets of local registers are stored on-chip
(See Figure 4). This cache holds up to four local
register frames, which means that up to three
procedure calls can be made without having to
access the procedure stack resident in memory.
ONE OF FOUR
LOCAL
REGISTER SETS
REGISTER
CACHE
LOCAL REGISTER SET
r
0
31
0
r
15
Figure 4. Multiple Register Sets Are Stored On-Chip
5
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