U. Glaeser

FIGURE 6.6 Scope of register renaming.
Scope of Register Renaming
The scope of register renaming indicates how extensively the processor makes use of renaming. In this
respect we distinguish between partial and full renaming. Partial renaming is restricted to one or to only
a few instruction types, for instance only to FP-instructions. This incomplete form of implementation
was typical for the introductory phase of renaming, at the beginning of the 1990s (see Fig. 6.1). Examples
of processors using partial renaming are the Power1 (RS/6000), Power2, PowerPC 601, and the Nx586,
as shown in Fig. 6.6. Of these, the Power1 (RS/6000) renames only FP-loads. As the Power1 has only a single
FP-unit, it executes FP-instructions in sequence, so there is no need for renaming floating point register
instructions. Power2 introduces multiple FP-units, consequently it extends renaming to all FP-instructions,
whereas the PowerPC 601 renames only the Link and count register. In the Nx586, which includes an
integer core, renaming is restricted obviously only to FX-instructions.
Full renaming covers all instructions including a destination register. As Fig. 6.6 demonstrates, virtually
all recent superscalar processors employ full renaming. Noteworthy exceptions are Sun’s UltraSparc line
and Alpha processors preceding the Alpha 21264.
Layout of the Rename Buffers
Overview
Rename buffers establish the actual framework for renaming. From their layout we point out three
essential design aspects—the type and the number of the rename buffers provided as well as the number
of the read and write ports, as shown in Fig. 6.7.
Types of Rename Buffers
The choice of which type of rename buffers to use in a processor has far reaching impact on the
implementation of the rename process. Given its importance, we will outline the various design options.
To simplify our presentation, we initially assume a common architectural register file for all data types
processed. We later extend our discussion to the split register scenario that is commonly employed.
© 2002 by CRC Press LLC
Partial renaming
Renaming is restricted
to particular
instruction types
A few early superscalar
processors, such as
Power1 1
Power2 2
Power-PC 3
Nx586 4
Comments:
1
Scope of register renaming
(RS/6000, 1990)
(1993)
601 (1993)
(1994)
Trend
Full renaming
Renaming comprises
all eligible
instruction types
The indicated superscalar
lines beginnig with
PowerPC 603 (1993)
PA 7200 (1995)
Pentium Pro (1995)
R10000 (1996)
K5 (1995)
MII (1997)
Most notable exceptions
are former
Alpha processors and
Sun's UltraSparc line
The Power1 renames only FP-loads.
2
The Power2 extends renaming to all FP-instructions.
3
The Power PC 601 renames only the Link and count
register
4
Since the Nx586 has an integer core, it renames only
FX-instructions.