INMC 80 News


May–September 1981, Issue 4

Page 16 of 71

Dateline Algeria

from Richard Bateman

Dear INMC80,


I agree with S.C. Willmott that the black flashes are irritating. To get rid of them should be an easy matter of connecting the BUSREQ line to the video blanking, so that the Z80 does not access the video memory during the display time. This would cause the Z80 to work a little slower than it does at the moment, but it is only like having a 2MHz clock. Note, it could still suffer from the dreaded black flasher , but only if the Z80 was making an access to the screen at that moment. The loss of speed can be calculated as:

loss%= (1–(16*14*48)/(320*64)
= 1–(time displaying)/(total frame time)
= approx 50%

This would be equivalent to a clock rate of 2MHz. The lines must be tied FALSE so that they don’t do funny things while the bus is idle. I have not tried this but it should work.


With regard to Mr Parkinson’s documentation, I think that he has got it about right, except that as his toolkit tends to be disabled by any sort of reset, and has to be reloaded, (this is a serious drawback), a mention of how to effect a warm start would be a help. (You ain’t read the instructions, execute an address three bytes less than the top address used by ‘Toolkit’, and it’ll warm start. Ed.) On the toolkit, a few unwelcome suggestions are as follows. The find command could be made to translate the codes by using the BASIC itself. If the string was entered as a line, say line 0, then let the Find do its comparison. After the command, the line could be deleted. Long lines could be handled by looking for a “\” as the first character, and then including the previous line as the start of the basic line, using a modified INLIN. It is possible to keep making suggestions, but then it gets a bit out of hand. The renumber and cross reference are worth it alone, with the others as a bonus. The find needs to use reserved words, and a warm start should be enough to make an issue 2.


I have installed a RAM64 board onto my N2, which means that I now have 96k on line. The page mode is installed on both RAM B and the RAM64. The RAM64 worked first time and without problems, but the RAM B started giving problems for the first time in a year. I have not tracked down the problem yet, but sometimes it works and some times it doesn’t. One of those! Since I only have a 3A psu, it may be loss of power, but I will try putting heavy wires from the psu to the bus.

An interesting problem arises when I try to use BASIC. The BASIC initialises Ok but as soon as I hit a key, it crashes. Particularly when I have the Toolkit loaded. It could be something to do with the IO select line used by the page mode hardware. It is surprising that Nascom used 128 ports! (What? – Ed.)

A weakness of the N2 memory map means that the workspace of a lot of the firmware is not, and cannot be put onto a page. This is a serious draw back to the system as designed. Another drawback is the inability to use DMA to the closed page. When (if) I design my DMA disk interface to run with my (also yet to be designed) super disk system, I won’t be able to swap closed pages to provide a disk cache memory. I can’t think what I would use it for..............

That aside, the page mode is a bit limited in its use because the system turns off whole boards at a time. That means it is necessary to copy the contents across from one page to the other of portions that are common. Also, addresses occupied by firmware can’t be got at, such as the bottom 4k, the top 8k. A slight modification to the board would allow a substantial increase in flexibility. If each card had its own port number, and the 8 bit latch was used to enable either blocks of memory (in 16K

This is an OCR’d version of the scanned page and likely contains recognition errors.

Page 16 of 71