4 ms·

> Pointers (capabilities, aka selectors) can have not only a base address, but a hardware enforced limit There are only 8k possible pointers in the LDT, plus 8

• StillBored 3y ago
  Sure, 40 years later, but for comparison my computer in 1990 had 1MB ram. Its replacement had 8M iirc a year or two later. I remember in the later 1990's having a problem with my socket7 computer because the caches couldn't physically tag more than 64M of ram, so everything above that was uncached. Linux of the mid 1990's would print a half dozen lines when one typed 'ps'. A limitation of 8 thousand different protection ranges would have been a lot for a program utilizing a few hundred KB of actual data and coming from a system were it was a PITA to access a data structure > 64K. It might not have been enough to do a super fine grained implementation, but it was more than enough for the time period, and had any significant OS's used it in a meaningful way I'm sure it would have been extended when limitations here hit, as was everything else in the following products. Oh, and also one could have reloaded the GDT, or swapped some number of LDTs at some boundary if needed. It wouldn't have really been much more disruptive in the 1980s than switching the page tables on task switch, like every modern OS.
• aforwardslash 3y ago
  8k LDT descriptors per task. Each LDT table also needed an entry in GDT, limiting the amount of tasks to also another 8k. So 64k descriptors total. Thing is, most ia32 operating systems dont actually use the existing hardware model for multitasking, and instead use simpler approaches that cater to the flat memory model.