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Computer System Structures System Operation Computer System Operation o Boot Process o BIOS, UEFI o Loaders, Init o Interrupt Driven OS Textbook Silberschatz, Chapter 2 Booting The procedure of starting a computer by loading the kernel is known as booting the system. On most computer systems, a small piece of code known as the bootstrap program or bootstrap loader locates the kernel loads it into main memory and starts its execution. Some computer systems, such as PCs, use a two-step process in which a simple bootstrap loader fetches a more complex boot program from disk which in turn loads the kernel. After Reset CPU receives a reset event - powered up or rebooted The instruction address register is loaded with a predefined memory location, and execution starts there. At that location is the initial bootstrap program. This program is in the form of read-only memory (ROM), because the RAM is in an unknown state at system startup. ROM is convenient because it needs no initialization (keeps the content with power off computer) and cannot easily be infected by a computer virus. Bootstrap Program - Requirements. Firmware The bootstrap program should be simple. It should not be lost when the power is off. So it should reside somewhere in ROM. It should know how to load the OS. It should ensure that the hardware is OK. The bootstrap program usually is the part of the larger entity called firmware – something between the hardware and software Firmware is the combination of persistent memory and program code and data stored in it It’s slow, expensive The Booting Process, BIOS, UEFI, loaders, Init The system’s component Resides on … The Action Power on. Start the BIOS. HDD Initial Loader UEFI BIOS OS Loader Firmware(BIO S, UEFI, Uboot, bootstrap, ..) ROM EEPROM Initialize the system hardware. Initial Boot Loader OS Loader HDD Master Boot Record Recognize HDD partitions. Choose OS. Call IBL Call OS loader. HDD Partition’s boot Kernel sector Load the OS All the time (kernel). continuously run in the system waiting Run kernel’s initial events. process. 1st partition Linux 2nd partition Windows Init (initial process) Once more Interrupt driven OS HW Signals Interrupts (Traps) Hardware Interrupts System Calls Events Software Interrupts Init Serving Interrupts Interrupt Vector Table in Low Memory location indexed by device numbers Serving HW Interrupts Serving SW Interrupts Saving the current state in the stack Serve Interrupt Restore the saved state from the stack Modern operating systems are interrupt driven. If there are no processes to execute, no I/O devices to service, and no users to whom to respond, an operating system will sit quietly, waiting for something to happen. The occurrence of an event is usually signaled by an interrupt from either the hardware or the software.