HD6417750RF240DV Datasheet, PDF (296/1132 Pages) Renesas Technology Corp – Renesas 32-Bit RISC Microcomputer SuperH™ RISC engine Family / SH7750 Series

Section 8 Pipelining
SH7750, SH7750S, SH7750R Group
operation, never occurs. For example, when FADD follows FDIV with no dependency between
FP registers, FADD is not stalled even if both instructions update the cause field of FPSCR.
Anti-flow dependency can occur only between a preceding double-precision FADD, FMUL,
FSUB, or FTRV and a following FMOV, FLDI0, FLDI1, FABS, FNEG, or FSTS. See figure 8.3
(g).
If an executing instruction locks any resource—i.e. a function block that performs a basic
operation—a following instruction that happens to attempt to use the locked resource must be
stalled (figure 8.3 (h)). This kind of stall can be compensated by inserting one or more instructions
independent of the locked resource to separate the interfering instructions. For example, when a
load instruction and an ADD instruction that references the loaded value are consecutive, the 2-
cycle stall of the ADD is eliminated by inserting three instructions without dependency. Software
performance can be improved by such instruction scheduling.
Other penalties arise in the event of exceptions or external data accesses, as follows.
• Instruction TLB miss
• Instruction access to external memory (instruction cache miss, etc.)
• Data access to external memory (operand cache miss, etc.)
• Data access to a memory-mapped control register.
During the penalty cycles of an instruction TLB miss or external instruction access, no instruction
is issued, but execution of instructions that have already been issued continues. The penalty for a
data access is a pipeline freeze: that is, the execution of uncompleted instructions is interrupted
until the arrival of the requested data. The number of penalty cycles for instruction and data
accesses is largely dependent on the user's memory subsystems.
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Sep 24, 2013