SH7750_08 Datasheet, PDF (328/1162 Pages) Renesas Technology Corp – 32-Bit RISC Microcomputer

Section 8 Pipelining
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.
Rev.7.00 Oct. 10, 2008 Page 244 of 1074
REJ09B0366-0700