Input-Output

Memory-Mapped I/O

Memory-mapped I/O uses the same address space to address both main memory and I/O devices.

• Reads/writes to those addresses are interpreted as commands to that device
• When the processor issues the address and data, the memory system ignores it since that region is mapped to I/O. The address encodes the device identity and type of transmission.

A memory-mapped file is a segment of virtual memory that has been assigned a direct byte-for-byte correlation with some portion of a file.

• source: https://federico-busato.github.io/Modern-CPP-Programming/htmls/23.Optimization_II.html

Benefits:

• Orders of magnitude faster than system calls
• Input can be “cached” in RAM memory (page/file cache)
• A file requires disk access only when a new page boundary is crossed
• Memory-mapping may bypass the page/swap file completely
• Load and store raw data (no parsing/conversion)

I'm confused, what is the alternative?

The alternative is the standard IO operations, where you read() or write() from IO, which is a System Call.