Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!cs.utexas.edu!husc6!frooz!sward
From: sward@cfa.HARVARD.EDU (Steve Ward)
Newsgroups: comp.sys.ibm.pc
Subject: AT Bus Mastership Control: How does it work?
Keywords: AT, Bus, Master, Handshaking
Message-ID: <226@cfa.HARVARD.EDU>
Date: 30 Oct 89 18:51:32 GMT
Organization: Harvard-Smithsonian, Cambridge, MA
Lines: 40


How does the AT Bus (8/16-bit memory and I/O expansion slots) work
with respect to multiple bus masters?  In particular, please describe
the bus signal protocol in detail.

I have found only a couple of sentences on this topic even though I have
examined about 12 published MSDOS, PC and AT technical references of
various sorts - all claiming to contain technical hardware information.
It appears I need some net help on this one.  Here is what I have found:

The AT Bus supports multiple bus masters through use of normal DMA
handshaking (DRQ, -DACK handshaking) and the -Master signal line.

This isn't too specific.  Here is my own best guess as to what this
means:  A potential bus master requests the AT bus by asserting DRQx
and thus requesting the bus through normal DMA handshaking means.  When
the bus is available and granted to the requester, the -DACKx signal
is asserted and detected by the bus requester, which in turn asserts
the -Master signal line then deasserts DRQx.  As long as the  -Master
signal line is asserted the requester maintains bus mastership even
though the DMA cycle has quickly come and gone.  In my scenario the
DMA is used strictly to arbitrate bus access while the -Master signal
is used to maintain bus control.  For this to work the DMA arbitration
logic would not grant DMA bus access (assert any -DACKx) until the
-Master signal line was deasserted.

Am I close?  What is the accurate truth?

I am also assuming that address lines and other control and bus signal
lines (other than data) will be tri-stated by the motherboard upon the
assertion of the -Master signal line, since the bus master would take
control of all bus signal lines.

Any and all help is appreciated.  I'll summarize to the net.

Thanks in advance.

Steven Ward     ward@cfa.harvard.edu

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