Using Intermediate Expressions

You can use intermediate (constant) expressions in the declarations section to reduce the number of output pins required to implement multi-level functions. Intermediate expressions can be useful when a module has repeated expressions. In general, intermediate expressions

decrease the number of output pins required, but

increase the amount of logic required per output

A constant expression is interpreted by the ABEL compiler as a string of characters, not as a function to be implemented. For example, for the following sets of Declarations and Equations

Declarations
TMP1 = [A3..A0] == [B3..B0];
TMP2 = [A7..A4] == [B7..B4];

Equations
F = TMP1 & TMP2;

the compiler directly substitutes the declarations into the equations, creating

F = (A7 !$ B7) & (A6 !$ B6) & (A5 !$ B5)
& (A4 !$ B4) & (A3 !$ B3) & (A2 !$ B2)
& (A1 !$ B1) & (A0 !$ B0);

In contrast, if you move the constant declarations into the equations section, in the following manner:

Declarations
TMP1,TMP2 pin 18,19

Equations
TMP1 = [A3..A0] == [B3..B0];
TMP2 = [A7..A4] == [B7..B4];
F = TMP1 & TMP2;

the compiler implements the equations as three discrete product terms. This will produce the following result:

TMP1 =(A3 !$ B3) & (A2 !$ B2) & (A1 !$ B1) & (A0 !$ B0);

TMP2 =(A7 !$ B7) & (A6 !$ B6) & (A5 !$ B5) & (A4 !$ B4);

F = TMP1 & TMP2;

The first example (using intermediate expressions) requires one output with 16 product terms. The second example (using equations) requires three outputs with less than 8 product terms per output. In some cases, the number of product terms required for both methods can be reduced during optimization.