Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!pasteur!ucbvax!RADC-TOPS20.ARPA!GUBBINS
From: GUBBINS@RADC-TOPS20.ARPA (Gern)
Newsgroups: comp.sys.zenith.z100
Subject: Re: Memory and RamDisks on the Z100
Message-ID: <12383621063.8.GUBBINS@RADC-TOPS20.ARPA>
Date: 19 Mar 88 18:22:11 GMT
References: <7668@oberon.USC.EDU>
Sender: usenet@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 56

It is a Vicious LIE that MS-DOS can only access 640K.   No! No! NO!
MS-DOS can access at least 1MByte (maybe higher?).  The architecture
of the machine sets the upper limit.  IBM-PCs set that limit IN 
HARDWARE to 640K, some clones (Z-150 is one) can be altered to let
MS-DOS access 704K unless you have an EGA card (which is addressed 
starting at 640K).  MS-DOS on the Z-100 can run up to 1MByte Limit
by turning off the Monitor ROM (top 32K addressed) and the video is
already addressed by PHANTOMed address space.    With 768K on the
motherboard, a Z-205 card (WITH the switches SET PROPERLY To start
at paragraph C000), the unmodified Z-100 will state 992K of usable
RAM under MS-DOS.  If you could turn off the ROM before MS-DOS boots
(an OUT instruction can turn off the ROM), you would have 1MByte of
usable RAM.

Now the catch is:  The Video shares 192K of address space with 
'real' RAM (on the Z-205 or such) starting at 768K.   The video
is SUPPOSED to be normally off unless code is actually using the
video RAM.   You turn the video RAM on (any video RAM access will
now PHANTOM in, turning off the 'real' RAM), use the video RAM,
then turn it off (The video PHANTOMs away, and the 'real' RAM can
be addressed normally).   Well behaved programs (Compiled ZBASIC
programs are not in this catagory) have no problems with this.  Bad
programs (violating ZDS recomendations for video access) turn
the Video on in the beginning, and off at the end.  If the program
is ended (CTRL/C, etc) without the clean-up, it leaves the video
on and it effectively swaps video bit plane data with your RAM disk
(why RAM Disk, is next) unless you are running the MDISK from ZDS
with patches made by Rob Logan that check for this.  All this stuff
will be posted in time....

Sionce most programs access the video during execution, the program
MUST not be executing in the address space shared by the video RAM.
Turning on the video while executing in the shared address space
will cause the program to 'page' (PHANTOM in S-100 terms) itself
out of existence (it will start executing in the video RAM).  The
safest way to prevent this curently is to set up a MDISK that
occupies the addresses of the video space at the least and tell
MS-DOS the start of the MDISK is the Top of its RAM (use RAMLIM.COM
in the PD Lib).    The ROM PHANTOMs in as well, unless turned off.
I have not played with turning off the ROM during MDISK access yet,
but it would allow for 32K more RAM for the MDISK.

I run at home (have winnie) with 768K for MS-DOS and 224K for MDISK
(Z-205) and at work (dual floppy) 480K for MS-DOS and 512K MDISK
(224k of 512K is Z-205).

It will all be covered in time in the ZUpGrade Series.   I also
want to point out that the ZUpGrade Series will explain old motherboard
upgrades without the cost of the FBE (a fine product, though) kit,
I don't like at all the idea of the FBE Z-205 card upgrade in the
way it accesses RAM by removing motherboard RAM (Rob Logan has a 
much better way) and I am working on a non-Z-205 way that is much
cheaper still.

Cheers,
An overworked Gern
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