Patrick Kramer wrote:
> > On Fri, 19 Mar 1999, Alex Wulms wrote:
> >
> > > This mystery can be solved quite simply if you look at the
> > specifications of
> > > the MSX slot. In a cartridge slot, you have three signals, just for the
> > > convenience of the hardware designer:
> > > CS1: This signal becomes active if the Z80 addresses memory in page 1
> > > CS2: This signal becomes active if the Z80 addresses memory in page 2
> > > CS12: This signal becomes active if the Z80 addresses memory in page 1
> > or 2
> >
> > I already knew this signals, but I never understood how to create a ROM in
> > pages 0 and 3.
> Well, you don't need these signals then.
> Example: if you want to have a 16K ROM in page 0, you would have to
> select this ROM whenever the slot is selected, AND address space 0000-3fff
> is addressed. This means A14 and A15 MUST be low.
> So as a _CS for the ROM you would have _SLTSL OR A14 OR A15.
Well ... The signals CS1 CS2 and CS12 only exists with the only
purpose
of avoiding the mirror effect that you guys are discursing about ...
these signals are generated by a half part of a 74LS139(the same wich
outputs
the main slot select signals in it's other circuit) and then "filtered"
by a logic with
OR ports to ensure they will be enabled on the read cicles ONLY. Then
these signals are
for ROM only
Megaroms doesn´t need to use them because they have a decoder inside to
get read and write
CS signals (they need to generate write addressing to the registers
isn't is ?).
It's possible to make a rom reside on the page 0 by connecting it's pin
ChipSelect (/CS)
to the SLOTSEL signal and connecting the OutputEnable(/OE) to the Z80
/RW (Read enable).
I made a DOS 2 cartridge myself using a DOS2 eprom wich was given for
me by a friend ...
I used a 74ls00 , a 74ls74 and a 74ls32 .... I used the 74ls32 and
74ls00 to make a simple
two bit megarom and the 74ls00 generates a CS12 wich is enabled during
both write and read
cicles. What I mean with this is that you don't need of the CSx signals
exept if you are
planning to make a small cartridge with a 27128 (uses CS1 or CS 2) or a
27256 (Uses CS12).
> Well, only use _SLTSL as a _CS for the RAM. There is no need for
> page-related chip selects as all pages are addressed.
Surely ! Memory mappers and simple mainrom cartridges go on this rule
!!!
> (A14 and A15 are used)
Every time if you need to make memory mapper control ... exept if YOU
WANT the mirror effect ! =)
I also made a simple memory mapper with S-Ram ... If anyone have
interest I may publish my
schematics ... =)
Greetings from Brasilia , Brazil
P.S.: I'm SURE tham ANY MSX can run both types of ROM on the page 0 (AB
or CD types)
Anyone tried to do a rom wich works in the page 3 ????
-------------------
Vic_Viper , The Gradius Space Ship (C) 1985 Konami (Go play on a near
arcade)
-------------------
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