3 ms·

The reason is that the term "fault" is used is because on the PDP range of computers (the first computers to run Unix) when a instruction (op code) fails it cre

The reason is that the term "fault" is used is because on the PDP range of computers (the first computers to run Unix) when a instruction (op code) fails it creates a "fault". In the case of accessing a segment that no longer exists, or beyond the length of the segment, etc the instruction that tried to execute but failed is said to have "faulted", or suffered a "fault". So an instruction that faults due to a memory segment issue is called a "segment fault" and one that faults due to a memory page issue is called a "page fault". In the case of segment, the name stuck even though almost all modern CPUs have pages rather than segments.

tankenmate 6y ago
This leads on to demand paging, when an instruction tries to access a page that isn't loaded in memory (say for example the page has been "paged out" to swap), then when the instruction faults, it traps into the kernel page handler, and the kernel then determines that yes, the program does own that page and that it is indeed in swap, the kernel will then "page in" that page, i.e. read it from the swap partition and put it in memory. Once the IO has completed and the page is in memory then kernel will reschedule the process and start the execution from the instruction that previously failed. Now that the page is in memory the instruction should execute normally and the process will continue unaware that some of its memory wasn't in RAM but now it is. [EDIT] fixed a typo.
gumby 6y ago
"Faulting" was a general term (has it fallen out of favor?) when an instruction didn't execute properly and wasn't specific to DEC machines. Hence page faults, as you note. Neither PDP-7 (original Unix machine) nor PDP-11 had segmented memory. Segmentation was fundamental in the design of Multics, and I suspect the term, if not the signal itself of course, was carried over from there. Memory segmentation was pretty common back then, though far from universal. By the way there were a lot of machines under the "PDP" umbrella with some vary different architectures. PDP-1, -7, and -9 had 18-bit words; PDP-5, -8 and -12 had 12-bit words; the PDP-6, -10 (and renamed from PDP -20) had 18-bit addresses and 36-bit words; and the PDP-11/LSI-11 had 16-bit words (this was later pumped up into the 32-bit VAX line). Unix only really ran on the 11s (and VAXes which BTW gave us the MIPS metric) apart from the one PDP-7 development machines. We used a variety of Ones on all those machines, often homegrown. In those days it was quite common for a company to write their own OS and programming language just for their computers, and for their customers to do the same!

tankenmate 6y ago

This leads on to demand paging, when an instruction tries to access a page that isn't loaded in memory (say for example the page has been "paged out" to swap), then when the instruction faults, it traps into the kernel page handler, and the kernel then determines that yes, the program does own that page and that it is indeed in swap, the kernel will then "page in" that page, i.e. read it from the swap partition and put it in memory. Once the IO has completed and the page is in memory then kernel will reschedule the process and start the execution from the instruction that previously failed. Now that the page is in memory the instruction should execute normally and the process will continue unaware that some of its memory wasn't in RAM but now it is. [EDIT] fixed a typo.

gumby 6y ago

"Faulting" was a general term (has it fallen out of favor?) when an instruction didn't execute properly and wasn't specific to DEC machines. Hence page faults, as you note. Neither PDP-7 (original Unix machine) nor PDP-11 had segmented memory. Segmentation was fundamental in the design of Multics, and I suspect the term, if not the signal itself of course, was carried over from there. Memory segmentation was pretty common back then, though far from universal. By the way there were a lot of machines under the "PDP" umbrella with some vary different architectures. PDP-1, -7, and -9 had 18-bit words; PDP-5, -8 and -12 had 12-bit words; the PDP-6, -10 (and renamed from PDP -20) had 18-bit addresses and 36-bit words; and the PDP-11/LSI-11 had 16-bit words (this was later pumped up into the 32-bit VAX line). Unix only really ran on the 11s (and VAXes which BTW gave us the MIPS metric) apart from the one PDP-7 development machines. We used a variety of Ones on all those machines, often homegrown. In those days it was quite common for a company to write their own OS and programming language just for their computers, and for their customers to do the same!