1.1 Nuttx build configuration and mode
Nuttx has three different build configurations
Ø FLAT build: This build is where all code resides in the public address space.
1) The application, kernel and board logic are in a flat address environment;
2) All address spaces have the same attributes;
Ø PROTECTED build: This build uses a Memory Protection Unit (MPU) to separate memory into a privileged mode for the OS, and a non-privileged mode for all applications.
1) The application works in user mode;
2) The kernel program works in kernel mode;
3) Memory is divided into kernel and application heap;
Ø KERNEL build: This build uses the Memory Management Unit (MMU) to place the OS in a private address space and place the task (or process) in its own virtual address space.
1) The application works in user mode;
2) The kernel program works in kernel mode;
3) Memory is divided into kernel and application heap;
4) Further isolation between user processes, each application works in its own space;
For PROTECTED builds vs. KERNEL builds, the application resides outside the OS address space, and in neither of these builds does the application have access to any internal resources of the OS.