RTCSA 2006 Work in Progress Section

User mode program usually requires the assistance from hardware in order to execute a mode change before using system service. This also allows user mode program to gain complete control of hardware. In this paper, new instructions are proposed; the OS may then take advantage of these instructions to more effectively use the system service, while reducing the kernel size and adding to the flexibility in design. Introduction System call acts as a communication interface between the Operation System (OS) and User Mode Program (UMP). To guarantee that all hardware resources are under the control of OS, the processor must have 2 or more execution modes (i.e. kernel mode and user mode). In addition, mode changes (e.g. user mode => kernel mode) must be executed such that system security is not harmed in any way. Currently, most OS’s and CPUs use software interrupts to complete mode changes. Since all system calls have the same entry point, kernels must use certain types of dispatch mechanisms (e.g. table lookup) to allow the mapping between system calls and Kernel Service Routines (KSR). Such mechanisms make the pre-processing mechanisms of system calls fairly complex. Thus, in this paper we propose a new method called Fast System Call (FSC) to increase the efficiency of system calls. Take as an example Linux’s system calls operating on a Pentium processor. Figure 1 shows that 5 steps are required to complete a system call: 1. UMP generates a software interrupt (i.e. int 0x80); 2. processor loads the content at location 0x80 in the Interrupt Descript Table (IDT) into the Program Counter (PC); 3. when the new PC is loaded, the OS will call out to different KSRs based on different value of AX registers; 4. returning from the called KSR; 5. returning from kernel.