+The block files access the disk via the system's normal
+buffering mechanism and may be read and written without regard to
+physical disk records. There is also a `raw' interface
+which provides for direct transmission between the disk
+and the user's read or write buffer.
+A single read or write call results in exactly one I/O operation
+and therefore raw I/O is considerably more efficient when
+many words are transmitted. The names of the raw files
+conventionally begin with an extra `r.'
+.PP
+In raw I/O counts should be a multiple of 512 bytes (a disk sector).
+Likewise
+.I seek
+calls should specify a multiple of 512 bytes.
+.SH "DISK SUPPORT"
+The origin and size (in sectors) of the
+pseudo-disks on each drive are as follows: