Texas Instruments TMS320F28004 Series, Manual

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Advisory (continued) FPU: FPU-to-CPU Register Move Operation Preceded by Any FPU 2p Operation
Figure 3-2 shows the pipeline diagram of the issue if there is a stall in the E3 slot of the
instruction I1.
Instruction F1 F2 D1 D2 R1 R2 E W
R1 R2 E1 E2 E3
MPYF32 R6H, R5H, R0H
|| MOV32 *XAR7++, R4H
I2 F32TOUI16R R3H, R4H I2 I1
ADDF32 R3H, R2H, R0H
|| MOV32 *--SP, R2H
I4 MOV32 @XAR3, R6H I4 I3 I2 I1
I4 I3 I2 I1
I4 I3 I2 I1
I4 I3 I2 I1
I4 I3 I2 I1
I4 I3 I2
I1
( STALL )
I4 samples the result as it enters
the R2 phase, but I1 is stalled in
E3 and is unable to forward the
product of R5H*R0H to I4 (R6H does
not have the product yet due to a
design bug). So, I4 reads the old
value of R6H.
I4 I3 I2 I1
There is no change in the pipeline
as it was stalled in the previous
cycle. I4 had already sampled the
old value of R6H in the previous
cycle.
I4 I3 I2 Stall over
I3 I3 I2 I1
Comments
FPU pipeline-->
I1 I1
Figure 3-2. Pipeline Diagram of the Issue if There is a Stall in the E3 Slot of the
Instruction I1
Workaround Treat MPYF32, ADDF32, SUBF32, and MACF32 in this scenario as 3p-cycle instructions.
Three NOPs or non-conflicting instructions must be placed in the delay slot of the
instruction.
The C28x Code Generation Tools v.6.2.0 and later will both generate the correct
instruction sequence and detect the error in assembly code. In previous versions, v6.0.5
(for the 6.0.x branch) and v.6.1.2 (for the 6.1.x branch), the compiler will generate the
correct instruction sequence but the assembler will not detect the error in assembly code.
Example of Workaround:
MPYF32 R6H, R5H, R0H
|| MOV32 *XAR7++, R4H ; 3p FPU instruction that writes to R6H
F32TOUI16R R3H, R4H ; delay slot
ADDF32 R2H, R2H, R0H
|| MOV32 *--SP, R2H ; delay slot
NOP ; alignment cycle
MOV32 @XAR3, R6H ; FPU register read of R6H
Figure 3-3 shows the pipeline diagram with the workaround in place.
www.ti.com Silicon Revision B Usage Notes and Advisories
SPRZ439G – JANUARY 2017 – REVISED AUGUST 2022
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