2 ms·

What's the actual reason for real mode still being supported on modern processors in this day in age? Why didn't it die with the advent of AMD64 (aka x86-64)? W

• pwg 4y ago
  Most likely because for protected mode to function, there is a certain minimal amount of housekeeping data tables that need to be setup properly (i.e. LDT, GDT, IDT, etc), otherwise you'll just immediately take a double fault and the CPU will halt. Real mode exists today as a gateway to setting up all those housekeeping data tables so that once the "protected mode" switch is flipped on, the CPU will actually find code to execute.
• gpderetta 4y ago
  - There is likely very little to no cost in keeping it in - There might be even a cost in removing it. - Complexity is a barrier to entry to any competitor that want to produce compatible CPUs.
• toast0 4y ago
  When AMD64 came out, bios booting was dominant. You need real mode for that. In today's world, you could probably release an UEFI only cpu and few would notice. But I doubt it would save enough space to make a difference. And you'd open yourself to criticism from those few that still use real mode: this processor is fake, they'd say, because it has no real mode.
  • sgjohnson 4y ago
    > this processor is fake, they'd say, because it has no real mode. They could also claim that it’s not PC compatible. Because it literally wouldn’t be. Apple also achieved their dream of the Mac no longer being a PC with the release of M1.