Altera Nios II Custom - Manual (Page 14)

1–10 Chapter 1: Nios II Custom Instruction Overview
Custom Instruction Types
Nios II Custom Instruction User Guide January 2011 Altera Corporation
When writerc is deasserted, the Nios II processor ignores the value driven on the
result port. The accumulated value is stored in an internal register. Alternatively, the
processor can read the value on the result port by asserting writerc . At the same
time, the internal register is cleared so that it is ready for a new round of multiply and
accumulate operations.
The readra , readrb , writerc , a , b , and c ports behave similarly to dataa . When the
custom instruction begins, the processor presents the new values of the readra ,
readrb , writerc , a , b , and c ports on the rising edge of the processor clock. All six of
these ports remain stable during execution of the custom instructions.
To determine how to handle the register file, custom instruction logic reads the active
high
readra , readrb , and writerc ports. The logic uses the a , b , and c ports as register
file indexes. When readra or readrb is asserted, the custom instruction logic ignores
the corresponding a or b port, and receives data from the dataa or datab port. When
writerc is asserted, the custom instruction logic ignores the c port and writes to the
result port.
All other custom instruction port operations behave the same as for combinational
and multicycle custom instructions.
External Interface Custom Instructions
Nios II custom instructions allow you to add an interface to communicate with logic
outside of the processor’s datapath. At system generation, conduits propagate out to
the top level of the SOPC Builder or Qsys system, where external logic can access the
signals.
By enabling custom instruction logic to access memory external to the processor,
external interface custom instructions extend the capabilities of the custom instruction
logic.
Figure 1–9 shows a multicycle custom instruction that has an external memory
interface.
Figure 1–9. Custom Instructions Allow the Addition of an External Interface
dataa[31..0]
datab[31..0]
clk
clk_en
reset
start
Conduit Interface
done
result[31..0]