In DECsystem-10, the segment of user virtual address space be ginning at zero. It contains the job data area and I/O buffers. The length of the low segment is stored in location. JBREL of the job data area.When writing two-segment programs, it is advisable to place data locations and impure code in the low segment.; 在DECsystem-10中,用户虚地址空间中从0地址 开始的存储空间段,包括作业数据区和输入输出缓冲区。低段的长度存放在作业数据区的JBR EL单元上。当写入两段程序时,可以把数据单元和非纯代码放在低段上。