80-Bus News


January–March 1982 · Volume 1 · Issue 1

Page 26 of 55

Free Memory!

That is, free as in make available. Oh well, with the price of 4116’s as low as they are now, I’m sure you can afford to buy them (and just be thankful that you haven’t got a ZX81… nearly fifty quid for 8K of RAM!). But I digress.

How much storage space have you got on your disks? According to my calculations, excluding the three system tracks, and the directory, a double sided single density, 5.25″ Pertec drive in a Henelec/​Gemini G805 System will put 148K onto each disk. My system will only put on 143K. Fiddle! Where’s my other 5K? I can access it using D-DOS and my Zap program, so why not with CP/M? A few moments with ZSID gave the answer. The byte in the BDOS which tells the system how much memory the disk will hold was set to 90H, that is 144. Increasing it to 95H gave me back all of my disk. My system was one of the very first Gemini CP/M systems sold, so maybe the fault has been corrected in later releases of the software. None the less, it may be worth your while checking your system. Just use STAT.COM on an empty disk, and see whether is states 143K or 148K free. (It was done deliberately to limit the track access on side 1 of the Henelec system as some read errors were occurring, you work out why!! Ed.) If you want to change it, the disk size byte can be found at the following locations in the Henelec/​Gemini CP/M 1.4:

16K System313EH
46K SystemA93EH

Obviously it’s better to change it in MOVCPM.COM, so that all future systems will be correct, but use a back-up copy!

Reset Switch

On my Nascom system, I’ve removed the reset switch from the keyboard, and put it on the front panel of the computer itself. In fact, I’ve not put on just one switch but two wired as in the diagram below. The idea is that switch 2 resets to 0000H and switch 1 resets to F000H. Switch 2 therefore cold boots CP/M and switch 1 resets NAS-SYS when that’s loaded, or gives a WBOOT jump in CP/M, at least, that’s the idea, so why doesn’t the Warm Boot work? Can anybody help?

– No reply. Watch my lips.... C.a.n. .a.n.y.b.o.d.y. .h.e.l.p.?.

Dr.Dark’s Disk Based NAS-SYS

In the second issue of Micropower (which isn’t as bad as I feared it might be), Dr. Dark described a method of adapting NAS-SYS 1 to run in a CP/M environment which is vastly superior to that used by CC Soft in their Disk Constructor Package (excepting that that is NAS-SYS 3). Using his method, it’s possible to adapt NAS-SYS 3 using the same instructions, except that the video address references are in a different position, as is the jump table. He has used (P)ut, (F)etch, (D) to exit to CP/M, and (Y) for a copyright notice. Altogether a very nice little patch, for the best debugger around.

The trouble is that I’ve already used F for Find, and NAS-SYS 3 uses P to display the stored user program registers. So, what do I use? I use D to jump into the disk sub-system, from where I have a choice of (R)ead, (W)rite, (N)as-sys, and ^C to exit to CP/M. An error takes me back to the disk sub-system, but a successful read or write returns me to NAS-SYS. The next job is to eliminate the code in NAS-SYS that’s duplicated in SYS. The changes from Chris Blackmores’ instructions for NAS-SYS 3 are:

Page 26 of 55