Computer Hardware User's Guide

write access
2-cycle external bus
Pipeline Conflicts
8-16
Example 8–11. Busy External Port
STF R0,@DMA1
LDF @DMA2,R0
Pipeline Operation
PC
Fetch Decode Read Execute
n STF — — —
n+1 LDF STF — —
n+2 W LDF STF —
n+2 W LDF (nop) STF
n+2 W LDF (nop) (nop)
n+3 X W LDF (nop)
n+4 YXWLDF
Note: W, X, Y = Instruction representations
The second type of hold-everything conflict involves multicycle data reads.
The read has begun and continues until completed. In Example 8–12, the LDF
is performed from an external memory that requires several cycles to access.