Memory

Explaining the memory management in the kernel.

We have several tools to manage memory in the kernel, each with its own purpose and goal.

First, let's look at the memory layout of the kernel, to know where everything is located.

Then, we can look at the physical allocator which provide us with the raw memory from the hardware, which we can use later with the virtual mapper to map it into virtual memory in order to access it.

The heap in the kernel is implemented with heap allocator, and the PageAllocator will allocate pages using the virtual mapper.

Lastly we have the virtual space which is a very useful tool to get a virtual address for components that are not part of the kernel directly but we know where they are located in physical space. This includes, Memory mapped IO, ACPI structures, Multiboot structures, etc.

Memory pointer types

Another thing you might notice in the code is that we use u64 sometimes and usize other times. Here is what they mean:

• u64: This is a 64-bit unsigned integer, and it will be 64 no matter the platform. It is used to represent physical addresses. Because in x86 with CR0.PG bit enabled, the CPU can map 40-bit physical addresses to 32-bit virtual addresses. And thus it can be more than 32-bit.
• usize: This is a pointer-sized unsigned integer, and it will be 32 or 64 depending on the platform. It is used to represent virtual addresses, and it is the same size as a pointer on the platform.

Something tangent. For filesystem operations we only use u64, as hardware drives can have easily more than 4GB of space. without needing for the CPU to be 64-bit.