80960SA datasheet(9/39 Pages) INTEL

9 / 39 page

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

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

procedure calls can be made without having to

access the procedure stack resident in memory.

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.

Figure 4. Multiple Register Sets Are Stored On-Chip

r15

ONE OF FOUR

LOCAL

CACHE

LOCAL REGISTER SET

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