![](http://datasheet.mmic.net.cn/230000/V4ECFUM_datasheet_15625205/V4ECFUM_130.png)
6-14
ColdFire CF4e Core User’s Manual
For More Information On This Product,
Go to: www.freescale.com
Operand Execution Pipeline (OEP)
Figure 6-4. EMAC-Specific OEP Sequence Stall
In Figure 6-4, the OEP stalls the store-accumulator instruction for 3 cycles: the depth of the
EMAC pipeline minus 1. The minus 1 factor is needed because the OEP and EMAC
pipelines overlap by a cycle, the EX stage. As the store-accumulator instruction reaches the
EX stage where the operation is performed, the just-updated accumulator 0 value is
available.
As with change/use stalls between accumulators and general-purpose registers, introducing
intervening instructions that do not reference the busy register can reduce or eliminate
sequence-related store-MAC instruction stalls. In fact, a major benefit of the EMAC is the
addition of three accumulators to minimize stalls caused by exchanges between the
accumulator(s) and the general-purpose registers.
6.3.5 FPU-Specific OEP Sequence Stalls
One FPU-related read-after-write register data hazard merits mention involving OEP
register file operands sourced to the FPU (for example,
fpGEN Ry,FPx
instruction). If
the source Ry register is modified by a previous instruction, the FPU operation stalls for 1
cycle, so an executeResult-to-OC2 forwarding datapath is not required. This restriction
allows the register operand sourced directly from the OEP register file to the FPU relatively
early in the OC2 cycle. The stall occurs in OAG. Consider the following sequence:
sub.l <ea>y,d0
fadd.l d0,fp0
The source operand d0 is modified immediately before the FPU uses it. The fadd.l detects
the pending register update in its DS stage, forcing a 1-cycle stall in OAG. This stall delays
the OC2 stage for fadd.l until the updated d0 value is available from the OEP register file.
mac
mac
mac
mac
mac
mov
mov
mov
mov
accumulator 0
mov
mac
mac
mac
mac
mov
old
new
Three-cycle
regBusy stall
DS
OAG
OC1
OC2
EX
EMAC EX1
EMAC EX2
EMAC EX3
EMAC EX4
mov
F
Freescale Semiconductor, Inc.
n
.