Yeah my bad. Was working on my own OS where I *did* map 0x0 in VM. I just realised that other OS's don't tho.
The state of Twitter programmers
James Reed
Brody Torres
@71139247
ultracringe
Logan Clark
>assuming it will crash
And that's how you get fucked by undefined behavior.
Kevin Jenkins
The first 1KiB is IVT
0x400 is BDA
and so on
Accessing REAL memory around 0x0 is a very stupid idea
Isaac Miller
>0x0 has to be readable
>Reading 0x0 and therefore NULL is legal.
#include
#include
int main()
{
int a = *((int*) NULL);
fprintf(stderr, "Value of a=%x\n", a);
return 0;
}
There's nothing preventing you from doing that. :)
Here's a fun fact, the macOS loader has a segment that is called SEG_NULL which maps the lower part of memory as non-writable and non-readable. I found this out when messing with the loader code on macOS and trying to reverse engineer a way of making minimal Mach-O binaries.
Blake Evans
IVT and BDA is only relevant when running in 16-bit real-mode. IVT becomes the IDT and BDA can be safely overwritten in protected/long mode.
Interesting.
Carter James
Holy shit nobody fucking cares, take your outrage bait back to /b/ or /trash/
Kayden Fisher
SMM is still running when in protected mode and requires EBDA. it is mapped ~500k
Gavin Fisher
what is this creature
Ayden Allen
That's a gender-fluid mutt