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What happens when a CPU starts?

  • When a CPU receives power, it resets by receiving a pulse on its RESET (or RST) pin. This is because when the power supply is first powering up, even if it only takes a second or two, the CPU has already received “dirty” power, because the power supply was building up a steady stream of electricity while powering up. Digital logic chips like CPUs require precise voltages, and they get confused if they receive something outside their intended voltage range. This is why the CPU must be reset immediately after powering up. The reset pin must be activated for a certain number of clock cycles to fully reset the CPU.

  • After being reset, the CPU can get to work. The CPU gets some instructions from memory, in what is known as a ‘fetch’ cycle. Memory can be either RAM or ROM. RAM is like the CPUs workbench. ROM stored code that is read-only and controls the system itself. The CPU always fetches code from ROM first so that it knows what its job is. The CPU address memory (both RAM and ROM) through the address bus. The CPU has two buses, the address bus and the data bus. The memory responds to this request from the CPU on the address bus by sending the contents of the selected memory address over the data bus, back to the CPU.

  • Every CPU has a particular point in memory where it begins reading instructions after it has been reset. Some CPUs will jump to a set memory address and begin executing that code. Other CPUs have a ‘reset vector’, which means that it first checks a particular point in memory for a number which is the memory address to begin executing instructions at.

  • The CPU contains a register (internal cache memory, extremely fast) called the instruction pointer, which contains a number. The number in the instruction pointer (IP) is the memory address at which the next instruction is to be performed. The IP is incremented with each instruction, and in the event of a jump (JMP) instruction, which tells the CPU to jump to another location and start running the instructions there, the IP is set to the jump location and then CPU continues on its way from there. The CPU’s instructions are sometimes called “opcodes”. They are simply strings of binary 1s and 0s which together form an instruction. For example, on a standard Intel 80x86 CPU (such as a 486 or Pentium), the opcode 90h (or 10010000 binary) is a NOP (no operation) opcode. NOP is the simplest instruction in any CPU, and it simply means to do nothing and go on to the next instruction. If a cell in RAM or ROM contains this opcode and the CPU executes it, it will perform a NOP (in other words, it will do nothing) and then IP will be set to the next memory cell. (On some computer platforms, the instruction pointer is called the “program counter”, inexplicably abbreviated “PG”. However, on the PC (as in “IBM PC”) platform, the term “instruction pointer” is usually used, because that term is preferred by Intel with regard to its 80x86 CPU family.)

  • Regardless of where the CPU begins getting its instructions, the beginning point should always be somewhere in a ROM chip. The computer needs startup instructions to perform basic hardware checking and preparation (POST), and these are contained in a ROM chip on the motherboard called the BIOS. This is where any computer begins executing its code when it is turned on.

  • Once the BIOS code has been executed, what happens next depends entirely on what is in the BIOS, although normally the BIOS will begin looking for a disk drive of some kind and start executing the instructions there (which is usually an operating system). From that point onward, the OS takes over and usually runs a shell which the user then uses to operate the computer.