3 ms·

The CPU contains essentially no hardware that is there only to support real mode, which is simply an documented combination of configurations of various protect

by dfox 7y ago

The CPU contains essentially no hardware that is there only to support real mode, which is simply an documented combination of configurations of various protected mode relevant mechanism plus changed behavior of exactly one microarchitectural operation, which involves replacing bunch of nontrivial microcoded logic with simple 4 bit left shift.

bonzini 7y ago
There is hardware that is there only to support v86-mode and v86 mode extensions though. V86 mode really really should go away...
- dfox 7y ago
  In fact there isn't any hardware to speak of (3 bits worth of architectural registers and bunch of trivial logic related to that?) to support vm86 mode, and that is to some extent the consequence of how true real mode is implemented. The combination of paging enabled (which is not strictly necessary), protected mode enabled, 16b code segment and real mode style segment loads gets you vm86 mode (complete with weird stuff on stack of exception handler vectors), in fact when you look at vm86 entry gate descriptor, this is exactly what it does. The microarchitecture essentially does not care about that. edit: there is no documented direct way to flip the segment/selector bit, which is the reason why you need documented vm86 mode. The hardware always calculate the linear address according to whatever is in shadow descriptor of segment/selector register, difference between real/vm86 and protected mode is what happens when you store something into there.
PopePompus 7y ago
But it's not just a matter of wasting transistors and silicon. The ancient x86 modes are like old legacy code in a software package you have to maintain. They add complexity. Intel hasn't exactly been cranking out an endless series of bug-free chips lately. It might behoove them to make a line of chips which are pure x86-64.
- tedunangst 7y ago
  How many of the recent intel vulnerabilities would have been prevented by jettisoning real mode?
bogomipz 7y ago
Could you elaborate on this a little more? What exactly is "the changed behavior of exactly one microarchitectural operation"? Which operation?

by dfox 7y ago

bonzini 7y ago

There is hardware that is there only to support v86-mode and v86 mode extensions though. V86 mode really really should go away...

dfox 7y ago
In fact there isn't any hardware to speak of (3 bits worth of architectural registers and bunch of trivial logic related to that?) to support vm86 mode, and that is to some extent the consequence of how true real mode is implemented. The combination of paging enabled (which is not strictly necessary), protected mode enabled, 16b code segment and real mode style segment loads gets you vm86 mode (complete with weird stuff on stack of exception handler vectors), in fact when you look at vm86 entry gate descriptor, this is exactly what it does. The microarchitecture essentially does not care about that. edit: there is no documented direct way to flip the segment/selector bit, which is the reason why you need documented vm86 mode. The hardware always calculate the linear address according to whatever is in shadow descriptor of segment/selector register, difference between real/vm86 and protected mode is what happens when you store something into there.

PopePompus 7y ago

But it's not just a matter of wasting transistors and silicon. The ancient x86 modes are like old legacy code in a software package you have to maintain. They add complexity. Intel hasn't exactly been cranking out an endless series of bug-free chips lately. It might behoove them to make a line of chips which are pure x86-64.

tedunangst 7y ago
How many of the recent intel vulnerabilities would have been prevented by jettisoning real mode?

bogomipz 7y ago

Could you elaborate on this a little more? What exactly is "the changed behavior of exactly one microarchitectural operation"? Which operation?