我在stata中用append命令合并两个数据，一个438MB，一个406MB，命令如下
use 1.data
append using 1.data
    出现了如下错误：
      op. sys. refuses to provide memory
    Stata's data-storage memory manager has already allocated 1600m bytes and it just attempted to allocate another 16m bytes.  The
    operating system said no.  Perhaps you are running another memory-consuming task and the command will work later when the task
    completes.  Perhaps you are on a multiuser system that is especially busy and the command will work later when activity quiets down.
    Perhaps a system administrator has put a limit on what you can allocate; see help memory.  Or perhaps that's all the memory your
    computer can allocate to Stata.
r(909);
我将stata的memory设置成3.5g，运行的时候还是出现这个错误，请问是什么原因呢？能有什么解决的办法吗？