Specifications

Altera Corporation 8–49
October 2007 Nios II Processor Reference Handbook
custom
custom
custom instruction
Operation: if c == 1
then rC
←
f
N
(rA, rB, A, B, C)
else
Ø
←
f
N
(rA, rB, A, B, C)
Assembler Syntax:
custom N, xC, xA, xB
Where xA means either general purpose register rA, or custom register cA.
Example:
custom 0, c6, r7, r8
Description:
The
custom opcode provides access to up to 256 custom instructions allowed by the
Nios II architecture. The function implemented by a custom instruction is user-defined
and is specified at system generation time. The 8-bit immediate N field specifies which
custom instruction to use. Custom instructions can use up to two parameters, xA and
xB, and can optionally write the result to a register xC.
Usage: To access a custom register inside the custom instruction logic, clear the bit readra,
readrb, or writerc that corresponds to the register field. In assembler syntax, the
notation cN refers to register N in the custom register file and causes the assembler
to clear the c bit of the opcode. For example,
custom 0, c3, r5, r0 performs
custom instruction 0, operating on general-purpose registers r5 and r0, and stores the
result in custom register 3.
Instruction Type: R
Instruction Fields: A = Register index of operand A
B = Register index of operand B
C = Register index of operand C
readra = 1 if instruction uses rA, 0 otherwise
readrb = 1 if instruction uses rB, 0 otherwise
writerc = 1 if instruction provides result for rC, 0 otherwise
N = 8-bit number that selects instruction
313029282726252423222120191817161514131211109876543210
A B C ra rb rc N 0x32