PDP-11 bootup

On Thu, Mar 26, 2009 at 9:20 PM, John Floren <slawmaster at gmail.com> wrote: It's been a long time since I've used those ROMs, so I can't say for sure (though disks on the RQDX _will_ be DU disks), but since you've been pulling things out, you probably have to take a moment to ensure that you have a complete grant chain. As has been discussed on this list many times before, Qbus and Unibus are not like ISA or PCI busses that most folks are used to. You have to have "grant cards" to pass grant signals across empty bus slots, but since Qbus grant cards are somewhat rare (I only have 1 or 2 myself), they are typically used for making room around hot cards or for temporary changes to the bus. Normally, you move all the cards up so that the grant signals are contiguous. In a BA23 chassis, that means that you have to pay attention to which slots are Q22-only and which slots have a "CD" slot on the right. In the case of a BA23, the top three slots are CD slots and the bottom slots are Q22 in a serpentine fashion. I'm not going to draw the slot priority - it's documented well elsewhere including the list archives. Suffice it to say that if you don't know what that means, you have some reading to do. The consequence of not having continuous grant is that non-interrupt and non-DMA things like boot roms will work, but operating systems will not. You will be able to read the boot block off of the boot medium, but then the machine will mysteriously "freeze". It's not frozen, it's stuck in a loop, waiting for the boot device to interrupt, but since there's a gap in the bus, the interrupt will never come. There used to be configuration guides to show how to load up Qbus machines, but I wouldn't know where to point people now. Generally speaking, it's CPU up top, memory next, then fast devices that can't wait (ethernet and TK50-type tape controllers), then comms (because they are slow), and then disk (because they are buffered and can wait). Unlike a PC, it totally matters what order things are in because the interrupt and DMA priority isn't determined by jumper, it's determined by what cards are closer to the CPU (which is why the grant chain matters). So that's the long of it. The short of it is... don't expect a running Qbus machine to run after you pull anything out unless you reorganize it or at least stuff in grant cards into the blank spots. If you can find a suitable Minicomputer handbook that covers at least the 11/23 if not the 11/73, you should be in good shape - it's all covered in the front few chapters. The only difference from model to model is what particular memory, comms and disk cards are described in detail. The generalities about how the Qbus works didn't change from the mid-1970s when it originally came out - just specific things like grant-chain patterns with different backplanes, termination, number of address lines, etc. Once you get the right configuration (CPU, memory, disk, serial, boostrap...), I'm sure you'll have a fun robust system. There's just a little work to do on the front-end to get there. It's not as straightforward as firing up a modern machine, but it's fun once you get past that part. -ethan