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From: fan@ucla-cs.UUCP
Newsgroups: comp.os.minix
Subject: Re: EMS spec
Message-ID: <5664@shemp.UCLA.EDU>
Date: Thu, 23-Apr-87 14:29:11 EST
Article-I.D.: shemp.5664
Posted: Thu Apr 23 14:29:11 1987
Date-Received: Sat, 25-Apr-87 06:41:24 EST
References: <497@gouldsd.UUCP>
Sender: root@CS.UCLA.EDU
Reply-To: fan@CS.UCLA.EDU (Roy Fan)
Organization: UCLA Computer Science Department
Lines: 35
Keywords: so how *does* it work ?

----------

	I am also thinking about creating an EMS ramdisk instead of
using system ramdisk.  I have the book, but I didn't have the disks
yet.  If I read the book correctly, I think that in order to implement
an EMS ramdisk:

(1)	the EMS ramdisk can have a maximum of 1MB since the pointer
they use is only 16 bits (the segment).  This doesn't seem right
though, there might be something I miss.

(2)	you will need an EMS driver that will map the logical memory
into the 64K physical memory.  For a very simple ramdisk driver, I was
thinking of mapping the first bank to the beginning of the logical
memory, and the rest 3 banks after that.  That way, you will have a
minimum of 48K to transfer.  And if the process requests more than
48K, you could break them into parts.

(3)	I don't know who called the ramdisk initialization routine.
But what it should do is (a) record how large is the RAMDISK (b) don't
take any memory for ramdisk.

	I have JRAM-3 memory board, and the memory mapping is very
simple.  I was thinking about doing a direct hardware access without
going through the EMS driver.  on the JRAM-3, you could map a 64K
segment anywhere on the 1M physical address.  Thus, I could map 128K
on D segment and the E segment.  And the mapping is very simple, too.
All is involved is to POKE a one-byte value into the control address
(F800:0000 is the default).  MOV    F800:0000, 23H    is all that's
needed.  (I didn't bother with MOV  AX,F800   MOV DS,AX).


Roy Fan
University of California    fan@cs.ucla.edu
Los Angeles