Altera Nios II Custom - Manual (Page 13)

Chapter 1: Nios II Custom Instruction Overview 1–9
Custom Instruction Types
January 2011 Altera Corporation Nios II Custom Instruction User Guide
Internal Register File Custom Instructions
The Nios II processor allows custom instruction logic to access its own internal
register file. This provides you the flexibility to specify if the custom instruction reads
its operands from the Nios II processor’s register file or from the custom instruction’s
own internal register file. In addition, a custom instruction can write its results to the
local register file rather than to the Nios II processor’s register file.
Custom instructions containing internal register files use readra , readrb , and writerc
signals to determine if the custom instruction should use the internal register file or
the
dataa , datab , and result signals. Ports a , b , and c specify the internal registers
from which to read or to which to write. For example, if readra is deasserted
(specifying a read operation from the internal register), the a signal value provides an
index to the internal register file. Ports a , b , and c are five bits each, allowing you to
address as many as 32 registers.
f For further details of Nios II custom instruction implementation, refer to the
Instruction Set Reference chapter of the Nios II Processor Reference Handbook.
Table 1–4 lists the internal register file custom instruction-specific optional ports. Use
the optional ports only if the custom instruction functionality requires them.
Figure 1–8 shows a simple multiply-accumulate custom logic block.
Table 1–4. Internal Register File Custom Instruction Ports
Port Name Direction Required Description
readra Input No
If readra is high, the Nios II processor supplies dataa ;
if readra is low, custom instruction logic reads the internal register file
indexed by a .
readrb Input No
If readrb is high, the Nios II processor supplies datab ;
if readrb is low, custom instruction logic reads the internal register file
indexed by b .
writerc Input No
If writerc is high, the Nios II processor writes to the result port;
if writerc is low, custom instruction logic writes to the internal
register file indexed by c .
a[4:0] Input No Custom instruction internal register file index
b[4:0] Input No Custom instruction internal register file index
c[4:0] Input No Custom instruction internal register file index
Figure 1–8. Multiply-accumulate Custom Logic Block
dataa[31..0]
datab[31..0]
writerc
result[31..0]
Multiplier Adder
D Q
CLR