X86 address space Layout

X86_64 is defined to support physical memory addresses up to 52 bits long. Because M5 uses 64 bit integers for addresses, every physical address has 12 extra bits which aren't directly accessible to the software running on the simulated CPU. M5 uses those bits to differentiate between different physical address spaces which are allocated for various purposes. In order to maximize the space given to the actual address portion of the address, the bits that select the address space grown down from the MSB as they're allocated. In effect, the index of the address space is just written into the most significant bits of the address in reverse, with it's least significant bit in the addresses most significant bit. The currently defined address spaces are the following:

• 0x0000000000000000 - Physical memory.
• 0x8000000000000000 - IO ports.
• 0xC000000000000000 - PCI config space.
• 0x2000000000000000 - The registers of all the local APICs. Each one gets a 2 page chunk of this space.
• 0xA000000000000000 - Interrupt messages. Each APIC gets a portion of this space as well. The various addresses work like memory mapped registers and allow the APICs to send each other IPIs.