couldn't find the definition

[shannon]

	There is a file mbuf.h in the directory \rtems-ss-20030417\cpukit\libnetworking\sys,in which there are the following code:
/*
 * Mbufs are of a single size, MSIZE (machine/machparam.h), which
 * includes overhead.  An mbuf may add a single "mbuf cluster" of size
 * MCLBYTES (also in machine/machparam.h), which has no additional overhead
 * and is used instead of the internal data area; this is done when
 * at least MINCLSIZE of data must be stored.
 */

#define	MLEN		(MSIZE - sizeof(struct m_hdr))	/* normal data len */
But I couldn't find the definition of MSIZE and it isn't in the file machine/machparam.h. Who can tell me where it is?