464 PreAsic, C3 Ram configuration and ROM 7

[gerald]

I've recently put my hands on a 464 with a 40226 aka pre-asic aka CTRC4, which is also found on the 6128 cost down version.

I did not resist doing some tests regarding :
   - Playing with L4N6 pin (to have a 6128 behaviour)
   - Extension RAM management compared to 6128 and regular 464
   - ROM7 and ROMDIS handling (when in 6128 mode)

Results are :

L4N6 pin
While it is connected to GND on the 6128, it is left unconnected on the 464 and seems to be internally pulled up.
In 464 mode :
   - the /ROM2 signal is always high : ROM7 selection is not handled on board.
   - the /CAS1 signal is always high :  RAM extension is not handled on board.

In 6128 mode :
   - the /ROM2 signal is handled on board.
   - the /CAS1 is handled on board, with the expected result of getting garbage instead of base RAM when using RAM selection other than C0
   - CPC does get stuck after displaying the copyright message and before the "BASIC 1.0" message.
     However, plugging a DDI1, even not powered solve the problem.

Extension RAM management :
In 464 mode :
   - The RAMDIS signal IS taken into account. That mean that extension that does not force the MREQn will work. This include the SymbifaceII
   - The C3 mode is NOT supported, as on regular 464.

In 6128 mode :
   - Garbage all the way on extension RAM with no extension connected, as expected as no ram is there 
   - With an extension, C3 is working as expected
   - adding 41464 in the alternate footprint and connected to CAS1 instead of CAS0 may transform the 464 in 4128 

ROM7 and ROMDIS in 6128 mode
   - Well, no luck. ROMDIS does not prevent internal ROM7 to be selected
   - ROMDIS act as an output when ROM7 is selected (However I need to re-check that it is only happening on ROM 7)
   - ROMDIS act as the DDI ROMDIS as it is active even during ram access
[attach=2]
     
Bonus
   - on the following page, Gate Array and ASIC Pin-Outs - CPCWiki, pin 35 and 36 are undocumented.
   - on the PCB in the amendment service manual, these to pins goes to the FDC thru links : pin 35 to DATA, pin 36 to WINDOW
[attach=3]
   - Looks like amstrad was designing its own data separator and planned to use it on this version of the CPC. But As hinted in Arnold V specs - CPCWiki, it was not ready.

[arnoldemu]

   - CPC does get stuck after displaying the copyright message and before the "BASIC 1.0" message.
     However, plugging a DDI1, even not powered solve the problem.

/EXP signal

[gerald]

/EXP signal

Got it  .
When the DDI1 extension is not plugged, /EXP is pulled high, and /ROM2 signal is in fact ROM0 and replace the BASIC. As no ROM is connected, it just crash !

[Bryce]

Great work. Sounds like it works very similar to the full ASIC used in the plus.

Bryce.

[arnoldemu]

Got it  .
When the DDI1 extension is not plugged, /EXP is pulled high, and /ROM2 signal is in fact ROM0 and replace the BASIC. As no ROM is connected, it just crash !

Please "wiki" it.
EDIT: I've wikied it.

[gerald]

Please "wiki" it.
EDIT: I've wikied it.

You are too quick for me  !

[arnoldemu]

You are too quick for me  !

if I made any mistakes please correct them. I have copied and pasted your forum posts as close as possible and put your name at the top of the section.

[TotO]

The RAMDIS signal IS taken into account. That mean that extension that does not force the MREQn will work.

Thank you for confirming that, has I was not able to get the 464 pre-ASIC[nb]but supposed to work like 464 PLUS[/nb] version for my tests.

[TFM]

It can't even do RAM &C3     Lightyears off the ASIC or the GA in the 6128 even.

[Bryce]

It can't even do RAM &C3     Lightyears off the ASIC or the GA in the 6128 even.

According to the posts above: It can't do &C3 in 464 mode (for obvious reasons), but &C3 works fine when it's in 6128 mode.

Bryce.

[TFM]

?? Well, if I have that CPC464 and add 64 KB of RAM: Can it do it then?

[gerald]

?? Well, if I have that CPC464 and add 64 KB of RAM: Can it do it then?

No, unless you take your soldering iron and tie L4N6 to GND.
And /EXP as well 

[TFM]

Well, you may not like to read all the time about FutureOS. However if the mouse arrow leaves a smear at the screen, then &C3 doesn't work. If the mouse pointer works well without making traces, then &C3 works. I don't dare to ask you to check that...

[TotO]

Sure, as C3 is only useful for doing double buffering display. 

[Bryce]

Sure, as C3 is only useful for doing double buffering display. 

Double buffering is a very useful function and has saved many mediocre games since the 80's.

Bryce.

[TotO]

Sadly, not enough used on 6128 as I know.

[gerald]

Well, you may not like to read all the time about FutureOS. However if the mouse arrow leaves a smear at the screen, then &C3 doesn't work. If the mouse pointer works well without making traces, then &C3 works. I don't dare to ask you to check that...

Well, FutureOS is not required to check if C3 works or not 
But here you are :

[TFM]

Thanks. Result is obvious.