4 ms·

The pdp-11 had a very primitive virtual memory system. It was not page based. So you had to swap in and out entire processes (with seperate instruction and data

by phicoh 5y ago

The pdp-11 had a very primitive virtual memory system. It was not page based. So you had to swap in and out entire processes (with seperate instruction and data you could leave the program in memory, swap out data, and swap in data from another process running the same program). Later pdp-11 models could have a few processes in ram at the same time.

monocasa 5y ago
Only for very low end systems. Most PDP-11s running Unix had something like the KT-11D MMU providing true paging hardware.
- phicoh 5y ago
  You are right. After I wrote my text, I realised that there must have been some kind of paging. Of course, with 8 Kbyte pages in a 64 Kbytes it is not the same granularity as today. Imagine unmapping one page to keep the stack separate from the heap. That's 1/8th of your already small address space. But the part that is also missing the ability to restart instructions after a page fault. If I remember correctly some old discussion, that may have been possible for many instructions but not some popular instructions. I found an online copy of the reference manual which states: "It is not generally possible to recover from these aborts." So paging yes, but not in the modern sense where you can recover from a page fault. Which is key to many aspects of a modern unix system. For example, copy-on-write forks.
  - monocasa 5y ago
    The 8KB pages could also be broken down into 128 byte blocks, so that you could map a (contiguous) portion of a page, and not have to use the whole 8KB. You could also use the same feature to give yourself guard 'pages' (but really guard blocks). It specifically supported the ability to grow down for stacks and guards. And the abort issue is around restarting indirect addressing modes, which will result in repeats of accesses. Technically this is an issue if you were pulling pointers directly out of MMIO regions, but it's one of those "doctor it hurts when I do this." "Then don't do that" kind of situation. If you're getting real cute with advanced addressing using MMIO regions to bounce out to other regions, do that in the kernel (as you probably would be anyway). Then you can do all the fun virtual memory tricks.

by phicoh 5y ago

monocasa 5y ago

Only for very low end systems. Most PDP-11s running Unix had something like the KT-11D MMU providing true paging hardware.

phicoh 5y ago
You are right. After I wrote my text, I realised that there must have been some kind of paging. Of course, with 8 Kbyte pages in a 64 Kbytes it is not the same granularity as today. Imagine unmapping one page to keep the stack separate from the heap. That's 1/8th of your already small address space. But the part that is also missing the ability to restart instructions after a page fault. If I remember correctly some old discussion, that may have been possible for many instructions but not some popular instructions. I found an online copy of the reference manual which states: "It is not generally possible to recover from these aborts." So paging yes, but not in the modern sense where you can recover from a page fault. Which is key to many aspects of a modern unix system. For example, copy-on-write forks.
- monocasa 5y ago
  The 8KB pages could also be broken down into 128 byte blocks, so that you could map a (contiguous) portion of a page, and not have to use the whole 8KB. You could also use the same feature to give yourself guard 'pages' (but really guard blocks). It specifically supported the ability to grow down for stacks and guards. And the abort issue is around restarting indirect addressing modes, which will result in repeats of accesses. Technically this is an issue if you were pulling pointers directly out of MMIO regions, but it's one of those "doctor it hurts when I do this." "Then don't do that" kind of situation. If you're getting real cute with advanced addressing using MMIO regions to bounce out to other regions, do that in the kernel (as you probably would be anyway). Then you can do all the fun virtual memory tricks.