On 2018-05-06 15:07, Noel Chiappa wrote: Right. I would probably say that no time-sharing OS with any kind of protection between processes will allow direct access to the hardware. My point being that (I think we agreed) pages are invisible to the process, while segments are very visible. And here we talk from a hardware point of view. So why would it not be significant? It's what the whole definition was about. That is not the same thing as being visible. The OS gives you some mechanisms or services. The OS might base that design on restrictions imposed by the underlying hardware, but the underlying hardware is in fact totally invisible. A user process use services provided by the OS, and does not try to manipulate, or is even aware of the underlying hardware. Yes. But a user program running under some OS with protection, would not allow you to access the hardware anyway. Anything like the MERT segments can in fact be provided with both types of memory management. And the program will not be aware of which hardware is involved, or how to set this up. The OS deals with all of that. There might be some constraints on where the segment can appear in your virtual address space, but that does not change the fact that the service can be provided by the OS with either hardware. Or, to take RSX PLAS directives as an example. When you create a region (which is the equivalent to a segment), it can have alignment on either a 64B resolution, or a 512B resolution. And the OS will try to accomodate. And where it appears in your virtual memory space, and what offsets to use and so on can be requested, but the OS will then give you something as close as it can to this, based on hardware restrictions. All of this is so similar to mmap() that we could in fact be having this exact discussion based on mmap() instead, if that were to help. I don't see you claiming that every machine use a segmented model, but running any modern Unix on any hardware will give you this exact same tools. Read up on the mmap() system call, if you don't already know it. Now, would you say that this system call makes the hardware directly visible and manipulated by the user program? Johnny -- Johnny Billquist || "I'm on a bus || on a psychedelic trip email: bqt(a)softjar.se || Reading murder books pdp is alive! || tryin' to stay hip" - B. Idol