More EEPROM / EPROM Questions

[Adam]

Hi Again Everyone,

I just wanted to say a quick thanks for the reponses so far, i'm new to
all this and am full of questions.

I was wanting to make some changes to the ROM of a Commodore 64 as I am
quite comfortable with assembly language, All the sites which give
guides on this mention 2764 & 27256 chips which are EPROMs only, I was
wanting to use EEPROMs insted.

I am able to get myself a smallish supply of Winbond W27C512-45 chips
which look ok for what I want to try and do. I just have a couple of
questions.. Looking at the datasheets for these chips they state that
they have 16 address lines (A0-A15), but other 27Cxxx have lines marked
A0-A13. Will these Windbond chips work in place in a system that only
uses A0-A13? I'm not quite clear on what the lines do.

Thanks once again for all the help,

Cheers,

Adam.

 

[Franc Zabkar]

Hi Again Everyone,

I just wanted to say a quick thanks for the reponses so far, i'm new to
all this and am full of questions.

I was wanting to make some changes to the ROM of a Commodore 64 as I am
quite comfortable with assembly language, All the sites which give
guides on this mention 2764 & 27256 chips which are EPROMs only, I was
wanting to use EEPROMs insted.

I am able to get myself a smallish supply of Winbond W27C512-45 chips
which look ok for what I want to try and do. I just have a couple of
questions.. Looking at the datasheets for these chips they state that
they have 16 address lines (A0-A15), but other 27Cxxx have lines marked
A0-A13. Will these Windbond chips work in place in a system that only
uses A0-A13? I'm not quite clear on what the lines do.

Thanks once again for all the help,

Cheers,

Adam.

Just tie the unused address lines to ground. Otherwise connect them to
a 2-way header and jumper them high or low. The two extra bits will
allow you to partition your EEPROM/EPROM into four sections, each with
different code, effectively giving you four C64 machines in one. Note
that the C64 uses ROMs (23xx) so you will need to make an adapter
socket. I've actually replaced all three (?) ROMs in a Commodore FDD
with a single EPROM in this way.

BTW, how can you be "comfortable" with assembly language if you don't
understand addressing?

- Franc Zabkar

 

[Adam]

Just tie the unused address lines to ground. Otherwise connect them to
a 2-way header and jumper them high or low. The two extra bits will
allow you to partition your EEPROM/EPROM into four sections, each with
different code, effectively giving you four C64 machines in one. Note
that the C64 uses ROMs (23xx) so you will need to make an adapter
socket. I've actually replaced all three (?) ROMs in a Commodore FDD
with a single EPROM in this way.

BTW, how can you be "comfortable" with assembly language if you don't
understand addressing?

- Franc Zabkar

Hi Franc,

I am comfortable in the software side i.e, I can code in assembly, but
I have no idea what address lines do on an EPROM having never touched
the hardware side before. BTW, I should mention that the only assembly
I have done is on finished products such as a C64 an a PC, I have not
done any low level stuff such as prototype boards and the like since
uni.

Thanks for your response.

Adam.

 

[Adam]

Hi Chris,

Thanks! That is really quite simple!

So if the EPROM chip does have extra address lines then they can be just
pulled to ground because the CPU can't use them as it doesn't have enough
address lines anyway, is that right?

Cheers,

Adam.

 

[Lord Garth]

Hi Chris,

Thanks! That is really quite simple!

So if the EPROM chip does have extra address lines then they can be just
pulled to ground because the CPU can't use them as it doesn't have enough
address lines anyway, is that right?

Cheers,

Adam.

That is correct, otherwise their state is undetermined.

 

[Lionel]

Hi Again Everyone,

I just wanted to say a quick thanks for the reponses so far, i'm new to
all this and am full of questions.

I was wanting to make some changes to the ROM of a Commodore 64 as I am
quite comfortable with assembly language, All the sites which give
guides on this mention 2764 & 27256 chips which are EPROMs only, I was
wanting to use EEPROMs insted.

I am able to get myself a smallish supply of Winbond W27C512-45 chips
which look ok for what I want to try and do. I just have a couple of
questions.. Looking at the datasheets for these chips they state that
they have 16 address lines (A0-A15), but other 27Cxxx have lines marked
A0-A13. Will these Windbond chips work in place in a system that only
uses A0-A13? I'm not quite clear on what the lines do.

The newer chips have more memory, so they have more address lines. You
don't need the extra space, so as long as you ground (or pullup) the
unneeded lines (A14-A15 or whatever), it'll work correctly. Just make
sure that you program your code into the part of the ROM that matches
however you hadwire the extra address lines.
Alternatively, you could replace two 27256s or eight 2764s with one of
your 512s, which would be a lot more elegant.