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User’s Manual
IBM PowerPC 750GX and 750GL RISC Microprocessor
Programming Model
Page 98 of 377
gx_02.fm.(1.2)
March 27, 2006
Note: The PowerPC Architecture states that, in some implementations, the move-to FPSCR fields (mtfsf)
instruction might perform more slowly when only some of the fields are updated as opposed to all of the
fields. In the 750GX, there is no degradation of performance.
Floating-Point Move Instructions
Floating-point move instructions copy data from one FPR to another. The floating-point move instructions do
not modify the FPSCR. The CR update option in these instructions controls the placing of result status into
CR1. Table 2-17 summarizes the floating-point move instructions.
2.3.4.3 Load-and-Store Instructions
Load-and-store instructions are issued and translated in program order; however, the accesses can occur out
of order. Synchronizing instructions are provided to enforce strict ordering. This section describes the load-
and-store instructions, which consist of the following:
• Integer load instructions
• Integer store instructions
• Integer load-and-store with byte-reverse instructions
• Integer load-and-store multiple instructions
• Floating-point load instructions, including quantized loads
• Floating-point store instructions, including quantized stores
• Memory synchronization instructions
The 750GX provides hardware support for misaligned memory accesses. It performs those accesses within a
single cycle if the operand lies within a double-word boundary. Misaligned memory accesses that cross a
double-word boundary degrade performance.
For string operations, the hardware makes no attempt to combine register values to reduce the number of
discrete accesses. Combining stores enhances performance if store gathering is enabled and the accesses
meet the criteria described in Section 6.4.7, Integer Store Gathering, on page 234. Note that the PowerPC
Architecture requires load/store multiple instruction accesses to be aligned. At a minimum, additional cache
access cycles are required.
Although many unaligned memory accesses are supported in hardware, the frequent use of them is discour-
aged since they can compromise the overall performance of the processor.
Accesses that cross a translation boundary might be restarted. That is, a misaligned access that crosses a
page boundary is completely restarted if the second portion of the access causes a page fault. This might
cause the first access to be repeated. On some processors, such as the PowerPC 603, a TLB reload would
cause an instruction restart. On the 750GX, TLB reloads are done transparently, and only a page fault causes
a restart.
Table 2-17. Floating-Point Move Instructions
Name Mnemonic Syntax
Floating Move Register fmr (fmr.) frD,frB
Floating Negate fneg (fneg.) frD,frB
Floating Absolute Value fabs (fabs.) frD,frB
Floating Negative Absolute Value fnabs (fnabs.) frD,frB