Nascom News­let­ter

be quite as expected – each EPROM location within a block will remain at an address relative to a 16k block address, ie. a typical 16k block would be C000 D000 E000 and F000 for positions 1 2 3 4 respectively if however the block address started at B000 then they would run as follows for 1 2 3 4 – D000 E000 F000 and then B000 respectively – does that make sense.

I think that is all you need to know with regard to addressing – except that it is quite O.K. to only access part of a page without any side effects.

Modifications To Provide Soft­ware Paging

First the bad news – you will need to buy a CMOS IC type MC4053 or CD4053 or similar (about 50p) and, if you haven’t got one fitted, a 24-way dil plug to save B........g OOPS, messing up the address selection dil socket.

With reference to Fig 2 – STICK the IC with its legs SKYWARD so that pin 1 of the IC is next to pin 24 of the dil plug. Pins 1, 4, 6, 7, & 8 can the be (bent) straight and soldered directly to the dil plug as shown. The ramainder of the connections can then be made – I mould suggest using THIN single stranded wire for this job au there is NOT a LOT ROOM and solder bridges can occur all too easily. Then wire ping 9 & 10 to IC32 pins 15 & 7 respectively these wires pick up the latched addresses from the FF port bits 2 & 3 (bit 1 turns ON or OFF the card – hence the odd page address). Finally connect pin 16 of your I.C. to. +5v which is conveniently available on the link blocks and is shown in fig 1. When using 2732 it as essential to use WAIT STATES as very few if any are fast enough to go at 4 MHz without, I have therefore shown the link fitted.

That’s it folks it should all happen by typing O FF 03 to select page (2) 03 etc. and remember pressing reset will select page (1) 0l. Selecting new pages will CRASH the computer if any $UIN or $UOUT jumps are operating so type N FIRST to disable them otherwise you will have to do a RESET anyway and you also risk SCRIBBLING UPON YOUR PROGRAMS, if in doubt press RESET and reselect your page.

Final Notes

In my own system and that shown 8k of EPROM on page (4) 07 has been replaced by the BASIC ROM (link to R on the dil plug) this can be altered simply by moving that wire link to the apropriate bank select. With a little ingenuity or another BIT A LOGIC the additional EPROM locations could be paged in or put at another address. If all this paging is getting complicated “Why not write a MENU program to select the pages and start the progs for you and load them into RAM when and if required” – I did and it took 1k to do it all!!!!!!!!.