What a microcontroller is

An embedded system is a computer built into a product to do one job: a washing machine controller, a bike light, a thermostat, a drone. It has no screen or keyboard of its own. It starts as soon as power arrives and runs the same program for years.

The computer inside is usually a microcontroller, often written MCU. One chip holds the processor core, the memory and the peripherals. Peripherals are the parts that connect to the outside world: pins, timers, serial ports and analog inputs. Popular families for learning are the ARM Cortex-M chips (STM32, nRF52, RP2040), the AVR chips on the original Arduino boards such as the Uno, and the ESP32.

You need to know 2 kinds of memory from the start. Flash stores your program and your constants, and keeps them when the power is off. RAM holds the variables your program changes while it runs. At power up, RAM holds nothing useful until the startup code fills it in. Flash is larger and RAM is faster, but both are small: a beginner board may have 256 KB of flash and 32 KB of RAM, hundreds of thousands of times less than a laptop.

On Cortex-M chips, ARM fixes the main memory regions, and you will see them in every debugger: the low addresses are for code, RAM starts at 0x20000000 and the peripherals start at 0x40000000. The chip maker decides where the flash sits inside the code region: 0x08000000 on STM32, 0x10000000 on the RP2040 and 0x00000000 on nRF52. The first digits of a pointer value tell you what kind of memory it points to.

When the chip starts, a short piece of startup code copies the initial values of your initialised global variables from flash into RAM. It sets the globals that have no initial value to zero, and then calls main(). The rest of RAM is left as it is. The local variables of a function live on the stack, a region of RAM that grows and shrinks as functions are called and return. Your main() then runs a loop that never ends. There is no operating system to return to.