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C-to-Hardware Mapping Reference
There are two types of exceptions to this rule:
■
■
Assignments that require zero logic elements in hardware
Assignments that use multiple registers to pipeline complex
arithmetic operations
The following sections discuss these exceptions.
Unregistered Operations and Assignments
Certain logical and bit-wise operations involving constants are trivial and
require no logic. In hardware, they are performed simply by
manipulating wires. Table 3–2 lists the applicable operators and
conditions. If an assignment consists solely of such operations, then its
result is not registered.
Table 3–2. Operators That Can Result in Unregistered Assignments
Operator
>>
<<
&
|
^
~
)
Description
Right bit-wise shift
Left bit-wise shift
bit-wise AND
bit-wise inclusive OR
bit-wise exclusive OR
bit-wise inversion
Type cast
Required Condition
Right-hand side is constant
Right-hand side is constant
Either operand is constant
Either operand is constant
Either operand is constant
Right-hand side is unregistered
Right-hand side is unregistered
The following assignment is an example of a zero logic-element
operation.
int masked_data = data_in & 0x000fffff;
The C2H Compiler generates no register for the variable masked_data,
because its value is represented simply by concatenating 12 bits of zeroes
with the lower 20 bits of data_in .
Additional examples of unregistered assignments:
shift_by_constant
or_with_constant
= data_in << 3;
= data_in | 0xf0f0f0f0;
invert_shift_and_consts = (~data_in & 0xff) << 8;
Altera Corporation
November 2009
9.1
3–3
Nios II C2H Compiler User Guide
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