One answer that hasn't been covered is the need for separation of concerns due to the limitations of the C language.

Generally bootloaders are written in a mix of Assembly and C, with the very early boot stage in Assembly.

This is done to setup certain things like:

• allocating the C stack
• reading the stack pointer into the register
• reading the program counter into the register
• declaring reset vectors
• loading the second stage (initramfs) into RAM.

This is a very rough approximation of the steps taken and I am describing the ARM boot process, it is different again for x86 and other architectures.

However, the principle reason remains the same: allocating the C stack must be done from assembly.

One answer that hasn't been covered is the need for separation of concerns in regards to the startup code.

This is done to setup certain things like:

• allocating the C stack
• reading the stack pointer into the register
• reading the program counter into the register
• declaring reset vectors
• loading the second stage (initramfs) into RAM.

This is a very rough approximation of the steps taken and I am describing the ARM boot process, it is different again for x86 and other architectures.

However, the principle reason remains the same: allocating the C stack must be done from assembly.