16K of Basic Rom

[Freya]

This is a bit of a strange question so I'll try and expain a bit.

Could Sinclair have added a 16K Basic Rom to the ZX81?

I mean I know they could do almost anything to it in theory but given the existing hardware in the zx81,would there be road blocks to making a 16K Rom work.

For instance are things mapped into the upper 8K that can't be moved.

Also I assume thechip socket is physically too small for a 16K Rom?
It needs all those extra address lines?

What do you think about it?

Freya

[1024MAK]

Simple answer: yes Sinclair could have used a 16K ROM in the ZX81/TS1000.

Actually, the existing address decoding allows for a 16K ROM. There is absolutely nothing on a standard machine that prevents a 16K ROM from being used apart from one missing address line (A13). Missing as in it does not go to the socket. But this can be added with a patch/jumper wire.

In practice, if you want to use an EPROM, there is also the minor problem of not all the socket connections being completely correct for 16K EPROM chips. But this is a problem for 8K EPROM chips too.

Mark

[Paul]

And in practice there is sponzy
A 16K ROM that turns a ZX81 into a black and white Spectrum.

[Freya]

Some of the boards even have a larger rom socket!
Was this so a 16k rom couldbe added at a later date or something else?

See Below:

https://www.sinclairzxworld.com/downloa ... hp?id=4703

http://gja.space4me.com/zx81/motherboard-closeup.jpg

[Moggy]

All socketed ZX81's had this 28 pin socket not just some.

My own theory, based on address line connections on this socket, is that initially 28 pin EPROMS were used at the testing stage before the final 24 pin PROMS were written/manufactured.
A couple of connections on the socket/board allow a 2764 EPROM to be used with only 2 pin lifts/rewires needed so would have been a quick useful way to test the ROM code.

Just my take on it nothing more.

[1024MAK]

I don’t have exact details to hand, but before the 2764 and later series of EPROM chips became the most common types, there were a number of different EPROM manufacturers making chips with various similar pin-outs, but with some important differences for the same memory capacity. Hence you had to be careful if you wanted to use a chip from a different manufacturer.

Mask ROM chips were also available in a number of different pin-outs. Some of which could have certain pins customised (the company ordering could select one out of a number of options for the control pins for example).

Mask ROM chips also were often available in a smaller chip package compared to the equivalent capacity EPROM, because they need less power and control pins. Also, with a Mask ROM chip, the order of the address pins is less important, as the manufacturer can arrange the data to suit the PCB track layout.

Mark

[Moggy]

I seem to remember that Texas Instruments used a non standard three voltages for some of the 27xx series chips before falling into line with other manufacturers.

Some info here...

https://www.applefritter.com/content/ch ... rough-2764

[Freya]

I thought it was an interesting idea that Sincl.air could have made an additional 16K rom for the zx81, especially when 16k roms became cheaper like when they were making the Spectrum etc.

I have a feeling this might have been a popular add on for the zx81 and wouldn't have been that hardto do.
They could put back in those read and data commands and a few other new commands maybe and then maybe in the rest ofthe space there could have been an assembler and a hex editor and a dissasembler perhaps.

What code would you like to have in a 16K rom?

I guess only 8k of it would be new if it replaced the existing rom.