ROM versus disk loading
I will explore the concept via the practical examples of cartridge-based and disk-based video games console.
When you insert a disk into an Xbox, the game data is typically read from the disk and transferred into the console’s memory by the OS. If the game is installed to the hard drive, this initially involves transferring substantial amounts of data to speed up future access. When you insert a cartridge into a console like the SNES, the data is not transferred in the same way. Instead, the cartridge contains a ROM chip that, when inserted, integrates directly with the motherboard. From the operating system’s point of view, this ROM chip behaves as an extension of the system’s memory architecture.
The primary advantage of a ROM-based game is significantly reduced latency. The game data can be accessed as rapidly as any other data moving along the buses of the motherboard. With a disk, the data must first be read into a buffer and then transferred to a memory address where it can be accessed by the CPU.
With a ROM, while the console CPU still uses its own RAM to run the game (similar to disk-based systems), the game data on the ROM can be loaded into this RAM more quickly because it is directly attached to the motherboard. This makes the ROM cartridge effectively a fast, directly connected storage device.
However, despite its speed, the limitation of a ROM cartridge is its storage capacity—it can hold only as much data as the chip can accommodate. In contrast, a disk can store much more data. This larger storage capacity allows not just for bigger and more complex games but also for the possibility of expanding the game through additional data downloaded to the console’s hard drive. Such expansions or patches represent additional data from the console’s storage that, combined with the original disk data, are synthesized in memory at runtime for enhanced or updated gameplay experiences.
(Note however that modern cartridge-based consoles like the Nintendo Switch can also combine ROM-data with expanded data from the console HDD, sourced from the internet.)
Both disks and ROMs are forms of static storage in the context of a game console. While disks can technically be overwritten with new data, in practice, manufacturers implement various locks and protections to prevent this. Any changes or additions to the game, such as patches or expansions, are not written back to the original disk. Instead, these updates are stored on the console’s hard drive.
Therefore, if you download an expansion for a game on your Xbox, the original disk remains unchanged as when you first purchased it. Consequently, if you were to take that disk and load it on another Xbox, it would not include any of the expansions or updates. The game would function in its original form, as the additional content and updates reside solely on the hard drive of the first console where they were downloaded.
