When dealing with monolithic operating systems such as Gaia where are system calls initiated from to achieve a required system level function?

In a monolithic operating system like Check Point Gaia, which is based on a Linux kernel, processes are separated into two distinct privilege levels: user mode and kernel mode. Applications run in the unprivileged user mode. When an application requires a service from the operating system, such as accessing hardware or managing files, it cannot perform the action directly. Instead, it must request the service from the kernel by making a system call. This system call is initiated from user mode, causing a trap that transitions the CPU's execution state to the privileged kernel mode, where the kernel can safely perform the requested function.

A. Kernel Mode: This is where the system call is executed by the operating system's kernel, not where it is initiated. The initiation is the request from the user-level application.
B. Slow Path: This is a specific term within Check Point's firewall architecture (SecureXL) for a packet-processing path. It is not a general operating system mode for initiating system calls.
C. Medium Path: Similar to the Slow Path, this is a specific packet-processing path in Check Point's SecureXL technology and is unrelated to the fundamental mechanism of system call initiation.

1. Silberschatz, A., Galvin, P. B., & Gagne, G. (2018). Operating System Concepts (10th ed.).

Section 2.3, "System Calls": This section explains the mechanism. A figure (Figure 2.5 in the 9th edition, concept remains the same) explicitly illustrates that a user application in "user mode" makes a system call, which then traps to the kernel to execute in "kernel mode." The process of making the call originates from the user space.

2. Check Point Certified Security Administrator (CCSA) R81.10 Study Guide.

Chapter 2, "Check Point Architecture and OS," page 37: "Gaia is a unified security Operating System (OS)... It is a 64-bit OS based on Linux..." This confirms Gaia's architecture is based on a standard monolithic kernel (Linux), which adheres to the user mode/kernel mode separation and system call principles described in general OS literature.

3. MIT OpenCourseWare. (2018). 6.033 Computer System Engineering, Spring 2018.

Lecture 3: The Kernel Abstraction, Section 3.2 "Protection": The lecture notes detail that to protect the system, user processes run in an unprivileged mode. To perform a privileged operation, a user process must execute a special "trap" instruction (a system call). This instruction causes the hardware to switch to privileged (kernel) mode and begin executing code at a pre-defined entry point in the kernel. This confirms the initiation is from the user process.