Xilinx Answer #1669 : ** XABEL-CPLD: Out Of Memory error or long compile times (@CARRY directive)

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** XABEL-CPLD: Out Of Memory error or long compile times (@CARRY directive)

Record #1669
Product Family: Software

Product Line: Data I/O

Product Part: XABEL-CPLD

Problem Title:

** XABEL-CPLD: Out Of Memory error or long compile times (@CARRY directive)

Problem Description:
Urgency: Standard

General Description:
Designs that have wide with arithmetic functions, 16 bit adders
for example, may take a long time to compile or cause an
Out of Memory error to appear.

Solution 1:

Use the @CARRY compiler directive to limit the bit width of
arithmetic functions.

Syntax: @carry <expression>;

Use: The @CARRY directive specifies how adders, counters, and comparators are generated, enabling you to reduce the amount of logic required for processing large arithmetic functions.

         For example, for a 16-bit adder, an @CARRY statement
         with an expression that results in 4 would divide
         the 16-bit adder into four 4-bit adders, creating
         intermediate nodes, thereby reducing the amount of
         logic generated. @CARRY 4 limits the lookahead
         carry by generating chains of 4-bit adders and
         comparators for all subsequent adder and comparator
         equations.

         If the @CARRY directive is not used or @CARRY 0 is
         specified, full lookahead carry is generated.

Example: Equations
         @CARRY 4; " generate adder chain
         [s16..s0] = [.x., a15..a0] + [.x., b15..b0];

End of Record #1669 - Last Modified: 12/27/99 17:12

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