Re: 74LS612

From: Andre Fachat (
Date: 1998-04-16 14:27:00

Hi Ruud, 

I don't have the book here, but I try to clarify what I can remember
with the help from my homepage.

> 74LS612
> The 612 is an IC that enables you to expand your system with 8 extra
> addresslines. Notify the fact that I use the word "system" and NOT
> processor. The processor won't be aware of those extra addresslines. In
> case of the 6502/6510 the processor still only can address 64 KB.
> How do these 8 extra addresslines fit in the system? 

The problem that is confusing is that the C128 chip is called "MMU".
I wouldn't do so, but it is now fact. Therefore I would always
use C128-MMU or so when this chip is meant. For me the '610 qualifies
much more as an MMU. 

[I have even managed to do some write-protection on 4k block basis
with my design: Mapping the the MMU to the I/O space at a second address
differing by one address bit (of A4-11) and connecting this address line
to DX of the MMU (X a previously unused data line, after r/w handling, 
i.e. only on writes) gives another "address line" MOX. 
This is now used to disable the data bus '245 and to set the bus r/w 
line high when reset (only if no I/O is selected of course). 
Now writing to $eff0 (MMU register 0) maps a page in CPU $0*** read/write, 
while writing to $ef70 maps the same page write-protected :-)))
Now tell me this is not a 'real' MMU ;-) 
This schematic is not on the net so far.]

> The 74LS612 is a kind of MMU but WITHOUT the multiplexers. These have to be

"This means it provides the _address lines_ like the original CPU, only 
more of them and nothing else. The decoders/multiplexers that translate 
the address to the chip select lines must be ..."

> added by the user. Another difference is that the behaviour of the MMU is
> preprogrammed in the factory and cannot be changed. But with the 612 the
> user can program its behaviour any time. Speaking in MMU-terms this would
> mean that we could program the MMU in such away that BANK 0 behaves like
> BANK 3, BANK 3 and 1 as 1 and BANK 2 as 1 as well.

What does that mean?  I don't get the point here.
If I find it again and you like it I can dig out an old tex graphics
I produced to show how I used the '610 in my CS/A65.
On the left it has the 64k address space divided into 16 blocks a 4k,
on the right the real (larger) memory with pointers going from the 
CPU blocks to arbitrary memory blocks.

> The 612 has 16 registers of 12 bits each. The normal procedure is to
> address these registers by four addresslines. Four bits of the output are
> used to replace the original four addresslines, the other eight bits to
> form the extra eight addresslines. 
> Reconfiguring the original addresslines gives you the possebility to re-
> arange the internal configuration if the hardware. By example in a PET you
> could swap the the range $7000/$7FFF and $F000/$FFFF to test a new Kernal-
> ROM.

You should probably now define the terms "virtual memory" and "real memory",
with the CPU (virtual) memory address range going from 0-64k and real from
$000000 to $ffffff. 

> The extra eight lines can be used to add extra memory (RAM, ROM) or I/O in
> any way you want. A very nice example is to connect a complete 1541 to your
> own system by means of a small interface which replaces the original 6502.

Uh, I have to finally draw that schematics in xfig....
(I have a CS/A65 board that does that)

> Now you can reconfigure your own system in such a way that it sees the
> range $0000/$BFFF of the 1541 as its own and its former own area
                                ^ mapped e.g. in $01**** real memory
> $6000/$9FFF as area $C000/$FFFF. In this way you comfortably can test a new
              ^ from $00****
> Kernal for the 1541.
> Pinouts.
>           +---------------------+
>    RS2   -+  1               40 +-   +5V       
>           |                     |
>    MA3   -+  2               39 +-   MA2
>           |                     |
>    RS3   -+  3               38 +-   RS1
>           |                     |
>    CS    -+  4               37 +-   MA1
>           |                     |
>  STROBE  -+  5               36 +-   RS0
>           |                     |
>    R/W   -+  6               35 +-   MA0
>           |                     |
>    D0    -+  7               34 +-   D11
>           |                     |
>    D1    -+  8               33 +-   D10
>           |                     |
>    D2    -+  9               32 +-   D9
>           |                     |
>    D3    -+ 10               31 +-   D8
>           |      74LS612        |
>    D4    -+ 11               30 +-   D7
>           |                     |
>    D5    -+ 12               29 +-   D6
>           |                     |
>    MM    -+ 13               28 +-   NC
                                       C on some variants (I think my 
                                         own pinout description is wrong here
					 about the variants...)
>           |                     |
>    MO0   -+ 14               27 +-   MO11
>           |                     |
>    MO1   -+ 15               26 +-   MO10
>           |                     |
>    MO2   -+ 16               25 +-   MO9
>           |                     |
>    MO3   -+ 17               24 +-   MO8
>           |                     |
>    MO4   -+ 18               23 +-   MO7
>           |                     |
>    MO5   -+ 19               22 +-   MO6
>           |                     |
>    GND   -+ 20               21 +-   ME
>           |                     |
>           +---------------------+
> Pin functions:
> D0 thru D11       Connection to the databus when programming the registers
> RS0 thru RS3      Connection to the addressbus when programming the
>                   registers, normally A0 thru A3
> R/W               Read, active (H) and Write, active (L)
> STROBE            input to enter data into choosen register, active (L)
> CS                ChipSelect, active (L)
> MA0 thru MA3      connection to the addressbus during normal operation
                   ^would normally be A12-15 of a 6502
> MO0 thru MO11     The generated addresslines
                   ^would normally be A12-A23 of a 6502+MMU system
> MM                Map Mode input. When (H), MO0/MO3 reflect MA0/MA3,
		   ^ uhm, I think I once fell for this bug too. AFAIR it's
		     reflected at MO8-11, not MO0-3. Then MO0-7 would be low.
>                   MO4/MO11 are (L). When (L), MO0/MO11 reflect the 12 bits
>                   of the choosen register.
> ME                When (H), MO0/MO11 are tristate else active.
> NC                Not connected.

  C                 Latch clock input to save the output before changing
		    the address lines. '610 and '611 only.

You should probably have a look at the internal diagram, which makes
most things clear:
> 74LS610, 74LS611 and 74LS613.
> The 610 is an 612 plus an extra 12 bits latch. With the 612 MO0/MO11 always
> react on the behaviour of MA0/MA3 and MM. With the 610 you can freeze the
> output as long as you want. This is archieved with an extra inputline at
> pin 28, C (= Clock). A (H) on this input will the 610 behave like a 612, a
> (L) freezes the configuration.
> The 611 and 613 are the Open Collector versions of the 610 and 613.

Oops. :-)

> Connecting the 612 to a 6502.
> Connecting a 612 to a 6502 can be split up in two parts anyway: the memory-
> part and the I/O-part. 
> Memory-part.
> If you only want to reconfigure your system within its own 64 KB, then you
> only have to "cut" the addresslines A12 thru A15 and to place the 612 in
> the created gap. Practically this probably means you replace the processor
> by a small circuitboard with onboard the original processor, the 612 and
> some logic to perform I/O-operations with the 612.
> If you want to use the extra addresslines as well, you have to take care of
> the fact that the original hardware does not respond to addresses above
> $0FFFF. The original does not know the extra addresslines so to any decoder
> the address $0C000 is equal to $1C000, $2C000 and so on. IMHO the solution

 Probably better: when reading $1c000 or $2c000 etc. the original will
still feel addresses at $0c000, as it does not see the additional address lines

> for this problem is to disable Clock2 towards the original system. As far
> as I know all decoders and/or I/O-ICs in any Commodore use this signal to
> decide to become active or not.

Outch. I am not really sure about that. This would disable all timers,
make some of the chips (I think, the ones that are not static devices and
have a minimum clock frequency specified) loose memory.

I have used single-direction drivers for the address bus and a '245
for the data bus as connection between the CS/A65 busses (after
the MMU) and the original system. The '245 was disabled when the 
original system was not addressed (i.e. $x**** with x>0; well I actually use
$2**** for the emulation so it was x!=2, but you get the point) and also
the r/w line was driven high to the original system (read).
This is not perfect, however. In fact you would have to make the
address lines show a "safe" address to the original system, as otherwise 
accessing e.g. $1dc0d would read CIA 1 ICR in a C64, and we all know what 
this means...

But on the other hand, stopping the Phi2 is probably exactly what you
want to debug other software :-)

> I/O-part
> The decision to be made is where to place the decoder for the 612 itself:
> behind or before the 612. If you place it before the 612 it always will be
> accessible but the disadvantage is that it can get it the way when there is
> a need to remap certain areas to the area the 612 occupies. If you place it
> behind the 612 and you make a mistake with programming the 612, it is
> possible you cannot acces the 612 at all anymore meaning you are stuck with
> the momentary configuration unless you reset your system. 

in my CS/A65 I have used the "decoder on the CPU side" version, but there
is a way to disable it (and also to make the I/O area selected by this decoder
appear somewhere mirrored in the real address space (on the BIOS board,
description see below).

> Practical use.
> Andre Fachat built a 4 MHz 65C02 based computer, <A HREF=''>the CS/A65</A>,

Uh. It's running at 2MHz, but needs a 4MHz CPU to meet the timing.
The 4MHz has earlier address line outputs, which makes MMU mapping 
at 2Mhz possible. There _is_ some delay in the MMU (plus drivers, plus
bus delays safety margins, plus memory decoder delays...)!

> using the 74LS610. Some of the above info and ideas are based on his work.
> ---------------------------------------------------------------------------
> Questions:
> 1)    Why do you use a 610 as you don't have any specific use for the
>       onboard latch? (Answer: because I just happened to have one ???)

NO!!! The latch is used! (I was already looking for a place for this comment.
The internal RAM in the '610 is not dual-ported. With the /CS input 
the address lines for the internal RAM are selected, either MA0-3 or
RS0-3. The output of the RAM then reflects the value of the appropriate
register. Imagine addressing the MMU with a "decoder behind MMU" setup.
The MA0-3 lines are switched off, instead the register chosen with 
RS0-3 (normally A0-3) gives the output. This is possibly different
from the MA0-3-selected register, meaning that the MMU is suddenly 
deselected again.
Therefore I use Phi2 to time-share the MMU: Phi2 low maps the MA0-3
to MO0-7, and the latch (clocked with /Phi2) saves the output
value. Then only when Phi2 is high the /CS line is allowed to go low,
accessing a different MMU register. But this doesn't hurt anymore
because the latch has saved the output value the half cycle before.

> 2)    Why do you use IC16 to disconnect the 610 from the bus instead of
>       using the 610s own capability to do so?

I thought the IC16 (a 74LS245) can drive more devices on the bus. But 
this is indeed a valid question.

> 3)    You placed the decoder for the 612 before the 612 meaning you always
>       can access the 612 on the same address, $E800, whatever the
>       configuration is. On the other hand you say you use your CS/A65 to
>       test new ROMs for the 1541, meaning you need the whole range from
>       $C000 thru $FFFF. This is an conflicting situation. 
>       I noticed you have a inputline to disable IOSEL. Activating this line
>       when you address the area $C000/$FFFF when testing the 1541 ROM means
>       you are stuck to this configuration until a reset. 
>       How do you handle this particular situation?

On the BIOS board there is a configuration register. (IC4 and IC5)
With it I can enable the
/IOINH line on the CPU board, that disables I/O access on CPU $e800-$efff
no matter what the MMU says. 

But I can also enable the the mapping of
the I/O address space in real $0b000 of mapped memory address space
via the /EXTIO line. Real $0**** is mapped by the BIOS board and basically
IC2 on the BIOS board enables /EXTIO on the bus when Real $0b*** is addressed,
making the CPU board lower the /IOSEL line. [This is a (minor) design flaw.
/IOSEL should have been open-collector and instead of pulling /EXTIO 
the BIOS board should pull /IOSEL itself.... But it's the only design
flaw I have discovered so far :-) ]

So to test the VC1541, I would enable $0b*** I/O mapping, map real $0b***
to, say, CPU $9*** in CPU address space, and disable the CPU $E8** I/O mapping.
voila. But then, when testing the new VC1541 ROMs, I always used RESET to
return to OS/A65 anyway... :-)

> 4)    Why do you use 245s instead of the more power friendly 541?

Because I didn't know about the 541. Do they also have the same 
solder-friendly pinout (sometimes I even use '245 for uni-directional
drivers because of that)?

> At the moment I'm working out an idea of combining a 65816 and a 74LS610. 

I wonder if this is necessary. Esp. as the '610 is not really fast,
and when you want to have the 65816 running at speed....
I was thinking about using a 65816 at 8 MHz, with (Ah, I love those new
RAM chips...) _one_ 512k static RAM chip (expensive, but so what... :-)
and a divide-by-four cicuitry to use the CS/A65 memory/I/O at 2MHz.

It would indeed not be possible to do those nice mapping tricks with this
setup, but I don't want to use it for that anyway.

It might, however, be useful to map slower I/O stuff around when you are
more interested in speeding up/debugging original 8032/VIC20 or even
C64 programs. But remember, the delay in the MMU is not neglectable,
esp. when running with higher speeds.

I really have to dig out my old TeX file with the CS/A65 and OS/A65 1.3

hope it helps

Email address may be invalid. Use "fachat AT physik DOT tu-chemnitz DOT de"
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Andre Fachat, Institute of physics, Technische Universität Chemnitz, FRG

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