Falk Joensson's
Learning Programming with Eas
(Easy Application Script Tutorial)

How the Computer Works:10. The Boot Sequence

Besides the memory types ROM (read-only) and RAM (random access = read and write) there also exists EEPROM, which stands for "electrically erasable and programmable ROM", and of course its modern version, flash. While EEPROMs need to be deleted completely, which takes some seconds up to even minutes, before you can re-program (write to) them, flash can be deleted and overwritten quickly in small sub-blocks. The program memory of a microcontroller typically is an EEPROM, programmed from a computer through a data connection, as an I/O device of the programmer's PC. When the microcontroller is turned on, its sole program starts to run right from the program memory address "0" on. This is what happens in every pocket calculator, washing machine, tv remote control, and so on, all the many devices all around us today that use microcontrollers. The computer, however, uses EEPROM or flash only for its lowest program memory address range, while the vast majority of its address space is located in RAM. The (EEP)ROM program, the firmware, runs the boot sequence which tests for the presence of essential I/O devices such as a keyboard and a monitor, implements a boot manager user interface where the user can change fundamental settings of the computer after pressing hotkeys during the early boot stage, and then loads the main program (usually the operating system or an operating-system installer) from the storage device set in the boot manager. After the loading (copying from the storage device to the program memory RAM), the program pointer is set to the start address of the loaded main program, and so it is finally run.
10. The Boot Sequence
T Two Things You Need
D Donate
C Contact
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