Unsigned addition really performs subtraction.
Posted: Thu Jul 29, 2021 8:29 pm
I am working on finding out the physical address range that I need to add to my page tables to access the provided framebuffer.
When I add the size of the framebuffer (Which I calculated as pitch * height) to the address of the framebuffer, the result appears to be a subtraction instead.
The calculated size of my framebuffer is 0x384000 bytes, and GRUB says that the start address is at 0x40000000. However, when I add these two numbers, I get 0x3fc7c000.
My first thought was that I was (somehow) getting a negative number for the size of my framebuffer (Or, at least the sign bit was set because of the multiplication result being large enough.) however, both my integers are `unsigned long long`and `unsigned long`, and I even checked with (x & (1 << 63)) and the sign bits are not set on either. Even if the sign bits are set, they should not be added as a signed integer.
What do you guys think is going on here? Am I doing something wrong?
This is the github repo: https://github.com/wireboy5/64bitOS
Thank's for your consideration!
When I add the size of the framebuffer (Which I calculated as pitch * height) to the address of the framebuffer, the result appears to be a subtraction instead.
The calculated size of my framebuffer is 0x384000 bytes, and GRUB says that the start address is at 0x40000000. However, when I add these two numbers, I get 0x3fc7c000.
My first thought was that I was (somehow) getting a negative number for the size of my framebuffer (Or, at least the sign bit was set because of the multiplication result being large enough.) however, both my integers are `unsigned long long`and `unsigned long`, and I even checked with (x & (1 << 63)) and the sign bits are not set on either. Even if the sign bits are set, they should not be added as a signed integer.
What do you guys think is going on here? Am I doing something wrong?
This is the github repo: https://github.com/wireboy5/64bitOS
Thank's for your consideration!