Read-Only Memory (ROM) is a type of non-volatile memory, meaning the data stored in it persists even when power is removed. The key characteristic of ROM is that, as the name suggests, it is "read-only" in the sense that once data is written to it during the manufacturing process, it cannot be modified, or only can be modified under very specific conditions (e.g., electrically erasable ROM types like EEPROM or Flash memory).

Why can't ROM be written to normally?

ROM is designed to be write-protected by its nature for several reasons:

1. Manufacturing Process: ROM is programmed during the manufacturing process. Data is burned or etched into the memory at the factory, and once this process is complete, it's designed to be permanent. In traditional ROM types, no mechanism exists for users or devices to change the stored data after manufacturing.

2. Hardware-Level Protection: ROM chips typically lack the hardware circuits or mechanisms necessary to support writing. This is in contrast to writable memory like RAM or Flash memory, which have mechanisms that can be triggered electronically to allow data modification.

3. Purpose of ROM: ROM is mainly used to store firmware or essential system software that must remain unchanged, like the BIOS in a computer or the bootloader in a device. It’s meant to provide stable, secure code that doesn’t need to change unless under very controlled circumstances.

Can ROM be reverse-engineered to make it writable?

There are some ways to "modify" ROM, but these involve extreme techniques, and they don’t generally make ROM writable in the usual sense:

1. EPROM (Erasable Programmable ROM): This type of ROM can be erased and reprogrammed using ultraviolet (UV) light. EPROMs have a small transparent window on the chip that allows for erasing the data using UV light, but this process is tedious and requires specialized equipment. Once erased, new data can be written to it using a programmer.

2. EEPROM and Flash Memory: These are "electrically erasable" ROM types, meaning data can be erased and rewritten through electrical signals. While not "true" ROM, they are sometimes used in place of ROM in modern devices because they allow for rewriting. EEPROMs are slower and have a limited number of write/erase cycles compared to Flash memory, but they are more flexible.

3. Physical Attacks: In theory, with enough time, money, and expertise, one could attempt to physically manipulate or reverse-engineer a ROM chip at a hardware level using tools like electron microscopes or scanning tunneling microscopes. This would involve extracting the contents of the ROM and then rewriting or replacing the chip with new data. However, this is a complex and expensive process, and most people wouldn't have the resources to do this on a large scale.

Why don't we use read-only memory for all security purposes?

While it sounds appealing to use read-only memory (ROM) to secure important code and prevent tampering (like with a firmware program that cannot be overwritten), there are several reasons ROM is not a universal solution:

1. Flexibility: ROM is very inflexible. If your system needs to be updated with new features, bug fixes, or patches, you cannot modify the ROM once it’s written. Using ROM for everything would make it impossible to respond to security vulnerabilities, software bugs, or other updates after a device is deployed. Flash memory or EEPROM is much more versatile for this reason.

2. Risk of Obsolescence: Devices that use ROM cannot easily be upgraded. If the code stored in ROM is vulnerable, outdated, or inefficient, you would be stuck with it unless you physically replace the ROM chip, which is impractical in many cases.

3. Cost and Practicality: While ROM might prevent software-based tampering, it’s not practical for everyday applications, especially in consumer electronics where updates and patches are often needed. ROM is more costly and harder to manage compared to rewritable memory types like Flash.

4. Secure Boot: A more modern approach to secure systems is to use "secure boot" or "trusted execution environments" in which the system verifies the integrity of code at boot-time. Secure boot ensures that only signed, trusted code is loaded, and any modification to the boot process or operating system would result in a failed boot. This method balances security with flexibility.

5. Advanced Attacks: While ROM could prevent direct overwriting of critical code, it doesn’t prevent sophisticated attacks like side-channel attacks, physical attacks, or vulnerabilities in the hardware itself. Security is not just about making data unreadable or unmodifiable; it’s also about responding to emerging threats.

Conclusion

In short, read-only memory (ROM) is designed for specific use cases where data integrity and immutability are crucial. It provides a layer of security, but it also comes with significant drawbacks in terms of flexibility and practical use. For most general applications, especially ones requiring updates or flexibility, writable memory types like Flash or EEPROM are preferred. For highly secure environments, a combination of hardware security (such as TPMs and secure boot) and encrypted storage is often used.

So while ROM could prevent hackers from easily overwriting critical software, it isn’t the best choice for systems that need regular updates or flexibility.