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User’s Manual
IBM PowerPC 750GX and 750GL RISC Microprocessor
Programming Model
Page 84 of 377
gx_02.fm.(1.2)
March 27, 2006
2.2.3.3 Time-Critical Floating-Point Operation
For time-critical applications where deterministic floating-point performance is required, the FPSCR bits must
be set with: the non-IEEE mode enabled, the floating-point exception masked, and all sticky bits set to one.
With these settings, the 750GX will not cause exceptions nor generate denormalized numbers, either of
which slows performance.
2.2.3.4 Floating-Point Storage Access Alignment
The 750GX does not provide hardware support for floating-point storage that is not word aligned. In these
cases, the 750GX invokes an alignment exception, and it is left up to software to break up the offending
storage access operation appropriately. In addition, some non-double-word-aligned storage accesses will
suffer a performance degradation as compared to an aligned access of the same type.
In general, floating-point single-word accesses should always be word aligned and floating-point double-word
accesses should always be double-word aligned. The frequent use of misaligned accesses is discouraged
since they can compromise the overall performance of the processor.
2.2.3.5 Optional Floating-Point Graphics Instructions
The 750GX implements the graphics instructions Store Floating-Point as Integer Word Indexed (stfiwx),
Floating Select fsel(.), fres(.), and frsqrte(.). For Floating Reciprocal Estimate Single A-Form (fres), the esti-
mate is 12 bits of precision. For Floating Reciprocal Square-root Estimate A-Form (frsqrte), the estimate is
12 bits of precision with the remaining bits zero.
.
Table 2-5. Floating-Point Operand Data-Type Behavior (Page 1 of 2)
Operand A
Data Type
Operand B
Data Type
Operand C
Data Type
IEEE Mode
(NI = 0)
Non-IEEE Mode
(NI = 1)
Single denormalized
Double denormalized
Single denormalized
Double denormalized
Single denormalized
Double denormalized
Normalize all three Zero all three
Single denormalized
Double denormalized
Single denormalized
Double denormalized
Normalized or zero Normalize A and B Zero A and B
Normalized or zero
Single denormalized
Double denormalized
Single denormalized
Double denormalized
Normalize B and C Zero B and C
Single denormalized
Double denormalized
Normalized or zero
Single denormalized
Double denormalized
Normalize A and C Zero A and C
Single denormalized
Double denormalized
Normalized or zero Normalized or zero Normalize A Zero A
Normalized or zero
Single denormalized
Double denormalized
Normalized or zero Normalize B Zero B
Normalized or zero Normalized or zero
Single denormalized
Double denormalized
Normalize C Zero C
Single quiet not-a-number
(QNaN)
Single signaling not-a-
number (SNaN)
Double QNaN
Double SNaN
Don’t care Don’t care QNaN
1
QNaN
1
1. Prioritize according to Chapter 3, “Operand Conventions,” in the PowerPC Microprocessor Family: The Programming Environ-
ments Manual.