PURPOSE: To improve the capacity of a network control program (NCP) by preparing two kinds of large and small buffers and using them efficiently.

CONSTITUTION: A buffer is constituted of a control area and a data area, and a buffer control pointer is provided on the control area. In a size designating part designated by a buffer control pointer, a buffer size is written in advance. When it is desired to secure a buffer, a program issues a GETBUF macro- instruction. When a buffer whose actual use quantity is small is enough, although a large buffer is used in the course of an operation, this system is constituted so that a small buffer is secured, when securing the buffer in the next time. On the contrary, when although a small buffer is used, it is short, and it has the chain of the second buffer and thereafter, a large buffer is secured, when securing the buffer in the next time. Also, when one of the numbers of large and small buffers is short, the number of buffers is changed to match the operation by unifying or dividing the other buffer.