Path: utzoo!attcan!uunet!husc6!sri-unix!dbeck
From: dbeck@unix.SRI.COM (Douglas K. Beck)
Newsgroups: comp.sys.mac.programmer
Subject: Transfer problems
Message-ID: <18251@sri-unix.SRI.COM>
Date: 18 Jun 88 16:38:51 GMT
Reply-To: dbeck@unix.sri.com (Douglas K. Beck)
Organization: SRI, Menlo Park, CA.
Lines: 15

Here is a question for some fine detain maven.  My current
project is being done with Lightspeed C.  One of the tasks
is to move 16 megabytes of data from a National Semi add in
board in my Mac II to a parallel output port.  The task requires
that a new long word be delivered in one microsecond or under.
Looking at my 020 book lead me to believe that the instruction
move.l (a3)+,(a4) could be done in 7 cycles.  The MacII clock
has a 64ns period thus the basic instruction *should* execute in
447ns.  I constructed an assembly language loop that consisted of
16 of the move instructions and two loop update instructions which
contribute about 10 percent overhead, with the entire loop body
residing in the cache.  The execuition speed I obtain is 1.22us
per longword transfer. Where did I go wrong?  A demoralized mind
wants to know.
dbeck@unix.sri.com  doug beck