mov	u.base,r1 / address of data is in r1
	neg	r3 / 512 - file offset (mod512.) in r3 (i.e., the number
		   / of free bytes in the file block
	cmp	r3,u.count / compare this with the number of data bytes to
		           / be written to the file
	blos	2f / if less than branch. Use the number of free bytes
		   / in the file block as the number to be written
	mov	u.count,r3 / if greater than, use the number of data bytes
		           / as the number to be written
2:
	add	r3,u.nread / r3 + number of bytes xmitted during write is
		           / put into u.nread
	sub	r3,u.count / u.count = no. of bytes that still must be
		           / written or read
	add	r3,u.base /  u.base points to the 1st of the remaining data
		          / bytes
	add	r3,*u.fofp / new file offset = number of bytes done + old
		           / file offset
	rts	r0