The dual-run-length codes are the important technique for test data compression.Test compression has two steps:first

the don’t-care bits in the test data are filled with 0 or 1s and the test data are divided into run sequences;second

every run in the sequences is converted to the compression code according to the given encoding algorithm.However

all the former existing papers focus on the second step

ignoring the importance of the first step thus to lose a certain potential compression ratio.In this paper

we address the importance of don’t-care bits filling to test data compression ratio and propose a novel algorithm

which fills don’t-care bits according to the selecting codes to achieve the higher compression ratio.This algorithm can be used with many dual-run-length codes without impacting on the decoder structure or the chip implementation flow.For the mainstream dual-run-length codes

the compression ratio is improved by 6%-9%.