TABLE OF CONTENTS How can you create constrained-random tests that can be easily modified? There are several tricks you can use.
Most constraint examples in this book use constants to make them more readable. In Example 6-25, size is randomized over a range that uses a variable for the upper bound.
class bounds; rand int size; int max_size = 100; constraint c_size { size inside {[1:max_size]}; }endclass By default, this class creates random sizes between 1 and 100, but by hanging the variable max_size, you can vary the upper limit.
You can use variables in the dist constraint to turn on and off values and anges. In Example 6-26, each bus command has a different weight variable.
typedef enum (READ8, READ16, READ32) read_t;class ReadCommands; rand read_t read_cmd; int read8_wt=1, read16_wt=1, read32_wt=1; constraint c_read { read_cmd dist {READ8 := read8_wt, READ16 := read16_wt, READ32 := read32_wt}; }endclass By default, this constraint produces each command with equal probability. If you want to have a greater number of READ8 commands, increase the read8_wt weight variable.
