Retro Computer Build Part 5 – Revised Address Decoder

So, after a couple of weeks since writing the first post about this Micro, I’m rethinking the address decoding and mapping of the same. Currently, address line A15 is being used to select between the lower 32K of memory and the upper 32K of memory. In the lower 32K of memory, I will page 32K block of RAM in and out, as needed. While the upper 32K of memory will be used for ROM and video RAM. I had previously suggested using a 3-8 line decoder to break up the upper 32K of memory into 4K chunks and then join a few of them to give 8K chunks. After a few days of writing the post about address decoding, I realised there were a few options available.

If I use a 2 to 4 line decoder (74HC139), I can use address lines A14 and A13 to break the upper 32K of memory into 4 x 8K blocks (which makes it easier for memory management). The upper 3 x 8K blocks can be used for firmware. I can potentially look at paging out one or two of these 8K blocks.

The lower 8K page can be sub divided into 4 x 2K blocks using another 2 to 4 line decoder (74HC139) on address lines A12 and A11. One of these 2K blocks can be used for basic video memory.  If I add a second or third 2K block this would give 4K or 6K of video memory, which would suffice for a basic 40 column x 25 line or 80 column by 25 line display (with some basic colours and cursor functions).

The lower 2K block can be sub divided into 8 x 256 byte pages for I/O. This is still allot of wasted address space for I/O, so what can be done to be more efficient with the memory usage?

We can use a programmable logic device , a “PLD”, to manage the control logic for the I/O. This way we can re-program the PLD at a future date to incorporate any additional changes to the I/O that we may want.

Using an Atmel ATF22LV10C will give us up to 10 outputs each, which can be individually assigned to one or more addresses. With up to 12 address inputs, the PLD makes it easier to map the control lines required. The unused pins on this device will be brought out to a header in case we need to use these pins at a later date. This is a  simpler solution to the multiple address decoders and additional logic ICs that potentially we were looking at.

When looking at the above revised memory map, we can see the I/O region is spread over 256 bytes of memory.

ATF22LC10C Datasheet

Years ago I used us a Hi-Lo Programmer that connected to my PC via an ISA card. This device was supplied with its own DOS program for taking Boolean logic and producing a map file which could be programmed into a PLD or a GAL. As I can’t use this anymore on modern PCs or a laptop, I need to find a more modern solution.

So I purchased a G540 programmer on eBay for very little money. The G540 was supplied with some adaptors for PLCC devices. A chip extractor was also supplied with this package. This programmer has a USB port, so it can connect to my laptop or desktop PC.

eBay G540 programmer

Atmel has a utility call WinCupl which allows you to write Boolean logic and create a map file which can be programmed into the 22LC10 PLD. I’ve never used this program before, so a little bit of study will be needed.


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