/ u8 -- unix

rtap: / read from the dec tape
	asr	r1 / divide the i-number by 2
	sub	$4.,r1 / (i-number/2)-4  r1
	mov	r1,cdev / cdev now has device number
	jsr	r0,bread; 578. / read in block thats in *u.fofp

wtap:
	asr	r1 / divide i-number by 2
	sub	$4.,r1 / r1 = i-number minus 4
	mov	r1,cdev / this is used as the device number
	jsr	r0,bwrite; 578. / write block (u.fofp) on dec tape
				/ Maximum

rrk0:
	mov	$1,cdev / set current device to i., disk
	jsr	r0,bread; 4872. / read block from disk (maximum block
				/ number allowed on device is 4872.)
				/ - (u.fofp) contains block number

wrk0:
	mov	$1,cdev / set current device to 1; disk
	jsr	r0,bwrite; 4872. / write block (u.fofp) on disk

rrf0:
	clr	cdev / set current device to 0., fixed head disk
	jsr	r0,bread; 1024. / read block (u.fofp) from fixed head
				/ disk (max. block number allowed on
				/ device is 1024.)

wrf0:
	clr	cdev / set current device to 0., fixed head disk
	jsr	r0,bwrite; 1024. / write block '(u.fofp)' on fixed head
				 / disk

bread: / read a block from a block structured device
	jsr	r0,tstdeve / error on special file I/O (only works on
			   / tape)
	mov	*u.fofp,r1 / move block number to r1
	mov	$2.-cold,-(sp) / "2-cold" to stack
1:
	cmp	r1,(r0)	/ is this block # greater than or equal to
			/ maximum block # allowed on device
	bhis	1f / yes, 1f (error)
	mov	r1,-(sp) / no, put block # on stack
	jsr	r0,preread / read in the block into an I/O buffer
	mov	(sp)+,r1 / return block # to r1
	inc	r1 / bump block # to next consecutive block
	dec	(sp) / "2-1-cold" on stack
	bgt	1b / 2-1-cold = 0?  No, go back and read in next block
1:
	tst	(sp)+ / yes, pop stack to clear off cold calculation
	mov	*u.fofp,r1 / restore r1 to initial value of the
			   / block #