Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!cs.utexas.edu!uunet!amgraf!huver
From: huver@amgraf.UUCP (Huver)
Newsgroups: comp.sys.amiga
Subject: Re: DMA controllers and 32bit RAM
Summary: No DMA for LUCAS
Message-ID: <311@amgraf.UUCP>
Date: 26 Jun 89 07:31:21 GMT
References: <14391@ut-emx.UUCP>
Organization: Amgraf Inc.,  Kansas City
Lines: 43


To this query Chuck McManis and Dave Haynie had responded with info on DMA
I/O with 32-bit memory for the Amiga in general.  But let's look at the
specific setup the original poster has: an A1000 with LUCAS.

I dont' know how the DMA controller in question works, but a "fully
functional" Amiga 1000 DMA controller hanging off the expansion connector
must look for XRDY or *DTACK signals as memory response -- from expansion
and CHIP memory, respectively.

None of these signals are asserted by LUCAS.  XRDY is only on the A1000
expansion connector, not available inside the machine.  And LUCAS uses
*DTACK from the 68000 socket as an input signal, not an output.

More, Brad had said, in his FRANCES announcement mailing, that he did NOT
design the memory for 16-bit DMA support (FRANCES is designed to be a fixed
32-bit prot).  So all theories aside, no DMA is possible for the 32-bit RAM.

Suppose we redesign LUCAS for this.  It needs to recognize external bus
master request (this is aready there) and shut off on-board *DTACK decoding
logic, map the hi/lo-byte/word 68000 signals with address lines to proper
memory address, map the 32-bit data lines to 16-bit lines, and generate
*DTACK from fast memory *DSACKx.  But wait...

LUCAS interfaces with A1000 via the 68000 socket so it appears as a 68K to
Amiga.  We can't assert *DTACK because no 68000 ever asserts *DTACK.

The only way is negate *DTACK and hook it direcctly to XRDY on the expansion
connector.  So, LUCAS must now also manage XRDY fully -- all this work just
to support the slow (from LUCAS's perspective) 16-bit DMA.  Most of you
would agree: too much for too little gain.

With 16 MHz LUCAS, a driver code sitting in 32-bit memory doing programmed
I/O can run as fast as DMA -- once you step off LUCAS the speed drops to
max 7 MHz trasnfer.  Nothing can help.

The best answer is a 32-bit capable DMA for LUCAS (same is true for the
A2620 in an Amiga 2000).

------
Asking "Why can't I run UNIX on my Amiga?!" is like asking "Why can't I put
wife and four kids in my Porche?!"
-huver ...!uunet!amgraf!huver