We now come to a description of the cards that make up the system. The photo above shows you what one looks like. Not very exciting, is it. Each card measures 15"x15" and has two little levers, which are truly fantastic, to assist in removal and insertion. (Anyone who's played with the guts of a PDP-11 will know what a pain in the botty it is trying to insert and remove their cards. This is where I let out a big belly-laugh and gloat, although there's no way in hell I'd ever turn down a PDP that was offered to me!) The whole card is covered with a big black sheet of fibre-glass to insulate the electronics from the card above. Removing this sheet makes the card a whole lot more interesting to look at, and this is what I did.
As I said in the previous section, "Checking It Out", the Nova 3/12 has twelve slots in the card cage. Below is a list of which card I have in which slot. Forgive me if it looks a bit weird on your browser, I really suck at tables.
|Nova 3 CPU
The heart of any computer system is the Central Processing Unit (CPU) and the Nova is no exception. These days the processor is a single chip (the Pentium 4 is also a pretty decent stove) but the Nova 3 uses an entire circuit board. If you think that's rather extravagant, the DECsystem 10 has a CPU the size of a refrigerator! The Nova 3 was special as it was the first Nova to have a stack pointer. In previous models you had to emulate one by using an accumulator as an index register. The Nova line has probably got one of the sexiest instruction sets I've ever seen, and I'm not just saying that because I own one. If you dont belive me, check out Carl Friend's Nova Instruction Set Summary and drool at its elegance!
The CPU is not microcoded as all instructions are performed in hardware. Although the board contains a lot of small ROMs, these are used to lower the chip count by emulating functions previously performed by standard logic gates (one must remember that this machine was made before there were programmable logic devices such as PALs and FPGAs).
What kind of a cryptic name is that for a card? It doesn't give us any indication as to what its function is, does it? Anyhow, this card allows three options depending on what chips are installed in the circuit board. One could have hardware multiply/divide instructions, a parity generator/checker, and a memory managment unit (with or without memory protection). The board in my machine only has the last, and in my view most important, option - the Memory Management and Protection Unit (MMPU). The memory management section extends the memory address space from 32k words to 128k words (in case you're unaware, a word is 16 bits or 2 bytes) and assigns blocks of memory to different user programs. The memory protection feature, as the name suggests, offers protection from any other user program writing over the assigned block. If another program tries to use this protected part of memory, the memory protection unit signals the operating system to kill the offending program. Sounds rather gruesome but it creates a lovely mental image, doesn't it?
This is one of three 16k word core memory boards that reside in my system. For those of you who don't know what core memory is, Carl Friend gives a nice explaination as to how it works as does the Yet Another PDP-11 Page. One thing that's really sexy about core memory is that the contents aren't lost after the power goes out. I spent 10 minutes entering a program via the front panel once and was able to demonstrate the same program to a friend a week later without going through the tedious ritual of having to re-enter it.
As you can see, there's a metal cover in the middle of the board to protect the memory from being damaged. I could have removed it to photograph the ferrite cores, but had've I damaged them in the process I'd be violently beating my head against the wall in utter embarassment. As you can see in the photos directly above, the enamel coated wire that is woven through the cores is very fine and fragile. I thought it wiser not to take the risk since finding a replacement board is not simply a matter of going to the local PC shop and saying "Hey geezer, got any of these?"
From what I can gather, this board is the interface for a cassette unit. Now I don't know how many people out there have ever loaded games from cassette on a Commodore-64, but it's very slow, tedious, and an experience I'd rather not relive. It would be a different story if it were big fat reels of tape, then I could sit and watch them spin in fascination. Thankfully, this card serves the additional purpose of being a terminal interface.
UPDATE: Bruce Ray sent me the following addition:-
The "Cassette board" [4075/+ series] was an exellent general purpose board for the Nova systems because - in addition to provisions for a "cassette tape" interface - it supported a standard TTY with strappable baud rates and an RTC. All interfaces were optional(!), so boards could be only partially populated. The Cassette interface is not what most people imagine, that it, it is not an interface to a standard cassette unit. DG made a mag tape drive that to the programmer looked identical to a 9-track tape drive, but the physical medium was the cassette form factor.
Upon further inspection of the board I discovered that, besides the TTY interface, it also had the Real-Time Clock option that Bruce mentioned. Yay!
This card allows four additional terminals to be connected, allowing a total of five simultaneous users to have access to the machine. That's right boys and girls, the Nova 3 is a multi-user machine!
Just as the name suggests, this card controls the disk cartridge unit. What more can I say?