[N8VEM: 18710] MMU for N8VEM

[Frank Vanderydt]

Hi there,

I'm studying the schematic for N8VEM and i'm wondering if the Z80 can
address as much as ram as the Z180 ? I see that U12 74LS174 has 2 free data
bits unused (d4 and d5).. so 1MB ram would be possible ?
Is this possible or is cp/m limiting this ?

I hope anybody can answer me.

Sincerely,

Frank

[d***@public.gmane.org]

Hi Frank,<br/>
<br/>
the Z80 can only address 64kByte RAM, not more. The N8VEM-MMU is an banking device to switch RAM-segements into the physical address space.<br/>
<br/>
Here from the data sheet of the z180:<br/>
<br/>
Based on a microcoded execution unit and an advanced CMOS <br/>
manufacturing technology, the Z80180, Z8S180, Z8L180 (Z8X180) is an <br/>
8-bit MPU which provides the benefits of reduced system costs and low <br/>
power operation while offering higher performance and maintaining <br/>
compatibility with a large base of industry standard software written <br/>
around the ZiLOG Z8X CPU.<br/>
Higher performance is obtained by virtue of higher operating frequencies, <br/>
reduced instruction execution times, an enhanced instruction set, and an on-chip memory management unit (MMU) with the capability of <br/>
addressing up to 1 MB of memory<br/>
<br/>
A0&ndash;A19. Address Bus (Output, Active High, 3-state). A0&ndash;A19 form a 20-<br/>
bit address bus. The Address Bus provides the address for memory data <br/>
bus exchanges, up to 1 MB, and I/O data bus exchanges, up to 64K. The <br/>
address bus enters a high impedance state during RESET and external bus <br/>
acknowledge cycles. Address line A18 is multiplexed with the output of <br/>
PRT channel 1 (TOUT, selected as address output on RESET) and address <br/>
line A19 is not available in DIP versions of the Z8X180.<br/>
<br/>
Best regards<br/>
<br/>
Wolfgang<br> <br> <div name="quote" style='margin:10px 5px 5px 10px; padding: 10px 0 10px 10px; border-left:2px solid #C3D9E5; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;'> <div style="margin:0 0 10px 0;"> <b>Gesendet:</b>&nbsp;Samstag, 20. September 2014 um 19:48 Uhr<br/> <b>Von:</b>&nbsp;&quot;Frank Vanderydt&quot; &lt;phreakske-***@public.gmane.orgm&gt;<br/>
<b>An:</b>&nbsp;n8vem-/***@public.gmane.org<br/>

<b>Betreff:</b>&nbsp;[N8VEM: 18710] MMU for N8VEM
</div>
<div name="quoted-content">
<div>Hi there,<br/><br/>I&#39;m studying the schematic for N8VEM and i&#39;m wondering if the Z80 can address as much as ram as the Z180 ? I see that U12 74LS174 has 2 free data bits unused (d4 and d5).. so 1MB ram would be possible ?<br/>Is this possible or is cp/m limiting this ? <br/><br/>I hope anybody can answer me.<br/><br/>Sincerely,<br/><br/>Frank<br/></div>

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[John Coffman]

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Z80 board real estate on SBC v2 may be the limiting factor.<br>
<br>
FYI:&nbsp; the Z180 Mark IV can address on-board 512K RAM plus 4Mb RAM on
a 4MEM board (current) or 15Mb additional RAM on an external memory
board (designed, but not yet fabricated).&nbsp; 4MEM and external memory
are mutually exclusive.<br>
<br>
--John<br>
<br>
<br>
<br>
On 09/20/2014 10:48 AM, Frank Vanderydt wrote:
<blockquote
cite="mid:1590c87b-b0b7-4a5a-bbc9-9ce908e0baf4-/***@public.gmane.org"
type="cite">
<div dir="ltr">Hi there,<br>
<br>
I'm studying the schematic for N8VEM and i'm wondering if the
Z80 can address as much as ram as the Z180 ? I see that U12
74LS174 has 2 free data bits unused (d4 and d5).. so 1MB ram
would be possible ?<br>
Is this possible or is cp/m limiting this ? <br>
<br>
I hope anybody can answer me.<br>
<br>
Sincerely,<br>
<br>
Frank<br>
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[Frank Vanderydt]

So normally 512KB should be sufficient for a Z80 design.
I already have the N8 so i know about the possibilities of the Z180, a very
interesting chip compared to the Z80.

The answer of John seems what i wanted to know.
I think there's no limitation as long as the bios or software addresses
enough lines to swap out the memory.

I know some old arcade games addressed a lot more rom as 1MB :)

[James Moxham]

Hi Frank,

The Z80 can only address 64k directly. However you can set up as much
external memory as you like. In hardware it is a matter of adding more
latches, or using unused data bits as you suggest. In software it needs a
few routines to access the external memory. CP/M with a ram disk added, or
MP/M.

Hacking hardware can get a bit complicated though. eg the N8VEM uses
blocks of 32k, and for MP/M you would want smaller blocks so that more
memory is available for the user. On a FPGA design you can experiment with
these things in software, and do things that would take quite a few
latches, such as splitting the memory up into 4k blocks, and any block can
be mapped into any part of the 64k Z80 space. This is very useful for
MP/M. However, if you just want a bigger ram disk, the 32k block size is
fine and yes, you could use those free data bits.

Happy hacking :)

James Moxham

Post by Frank Vanderydt
Hi there,
I'm studying the schematic for N8VEM and i'm wondering if the Z80 can
address as much as ram as the Z180 ? I see that U12 74LS174 has 2 >free
data bits unused (d4 and d5).. so 1MB ram would be possible ?
Is this possible or is cp/m limiting this ?
I hope anybody can answer me.
Sincerely,
Frank
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[Frank Vanderydt]

Thanks James,

[Alan Cox]

CP/M doesn't gain much from over 128K or maybe a shade more with giant
disks. The rest can perhaps be used as ramdisc.

4k pages also mean lots more clocks per switch and lots more gates.