Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!uunet!microsoft!brianw
From: brianw@microsoft.UUCP (Brian WILLOUGHBY)
Newsgroups: comp.sys.apple
Subject: Re: Really small question
Summary: MVP cycle timing
Message-ID: <10040@microsoft.UUCP>
Date: 22 Dec 89 02:09:26 GMT
References: <kZTLjCG00WB7Q=4bJa@andrew.cmu.edu> <9542@microsoft.UUCP> <50690@srcsip.UUCP>
Reply-To: brianw@microsoft.UUCP (Brian WILLOUGHBY)
Organization: Microsoft Corp., Redmond WA
Lines: 32

shankar@src.honeywell.com (Subash Shankar) writes:
>MVP takes 7 cycles per byte, and my understanding was that DMA only
>takes one cycle per byte (perhaps two since the address and data lines
>are shared).  

I may have to review the W65C802/816 data sheets, but I thought the 7 cycles
occurs ONCE for setting up the MVP instruction.  This includes fetching the
opcode and the two bank bytes as well as a few internal setup cycles.  Then
you have one cycle for each byte access until the move is complete.

Also, concerning cycles per byte moved, you're thinking of single direction DMA
- i.e. peripheral to memory or memory to peripheral.  If you want a memory to
memory DMA, you'll need two accesses per byte, one to read from the source
address and one to write to the destination address.  These two addresses can
be different.

The 6502 memory access cycle only uses (reads or writes) data at the very end
of the cycle.  The final clock edge is used to latch the data into RAM or into
the CPU depending upon the direction of data transfer.  The first half of the
cycle is used for address setup, and the second half is used to allow the data
lines to settle.  Thus the new sharing of the data lines to extend the address
bus to 24 bits does not lengthen the memory access cycle (in fact it is shorter
since the processor is now running at higher clock rates than before).  On a
related note, the Apple ][ only used half of the 1 MHz cycle time for CPU
accesses.  50% of the 1 MHz clock was devoted to video address and video data.
Moral, there is plenty of time - until you get up to 13 MHz, that is.

Brian Willoughby
UUCP:           ...!{tikal, sun, uunet, elwood}!microsoft!brianw
InterNet:       microsoft!brianw@uunet.UU.NET
  or:           microsoft!brianw@Sun.COM
Bitnet          brianw@microsoft.UUCP