You don’t need to load code into RAM to execute it

This will be a basic fact to some, but you don’t need to load code into RAM to execute it. You can execute code straight from ROM.

In fact, this is how most computer systems boot up. After the CPU finishes initializing, it starts executing at a specific physical address which is generally mapped to some kind of Boot ROM.

(On x86, this first instruction is located at 0xFFFFFFF0, which is interestingly almost completely at the top of memory. The code there then needs to contain a jump to the rest of the actual boot code. (Source: Intel 64 and IA-32 Architectures Software Developer’s Manual, Vol 3A Section 9.1.4)

I believe ARM systems are different and the start address can vary.)

The Boot ROM β€” like the name suggests β€” is not RAM. It’s ROM. It’s a totally separate device on the memory bus offering nonvolatile storage. It’s mapped into physical memory using the mesh of digital logic that implements the physical memory mapping. (More:

The CPU is generally not aware of what specific device is on the other end of the memory bus, servicing reads and writes. During instruction fetch, it simply issues reads to the memory bus, receives instruction data, then executes it. The data can transparently come from RAM, ROM, or potentially even some other device, provided it is fast enough.

The reason this was unintuitive to me, is because until recently I’ve only ever done “normal” programming, where programs are loaded from disk into memory before running them. This is the domain of probably 99% of programmers. And it’s not even just limited to userspace application programmers; even kernel developers have their code loaded into RAM before its run. It’s usually only the developers of very early stage bootloaders and microcontroller firmware developers that need to be aware of the CPU running code from locations other than RAM.


Wikipedia booting

Any thoughts?