Embedded Software Engineering
A microcontroller is made up of three simple things
Instructions
Registers
Memory
Instructions
Instructions are the things that a microcontroller knows how to do.
Instructions operate on numbers that are stored in registers and memory.
Register
Registers are very fast storage that holds the numbers that instructions operate on. One way to think of it is that a register is a scratchpad for instructions to use.
A microcontroller has a small number registers, typically 8-32.
Memory
Memory is also storage for number but memory is much more plentiful and slower than registers.
Where does a microcontroller get instructions from?
When a microcontroller powers up it looks in a defined memory location for the first instruction to run. It runs the first instruction, and then looks at the next memory location for the second instruction, runs it and so on.
We have some instructions in memory, along with some data. How does the microcontroller know where to find the instructions?
It uses a special register called a Program Counter(PC) to keep track of where in memory it's looking for instructions.
When a microcontroller powers up or is reset, the PC is automatically set to some known starting address so it always starts at the same place.
A microcontroller does its job by following three simple steps
Read the instruction at the memory address in the PC register
Run the instruction
Update the PC register to point to the next instruction in memory
These three steps are repeated forever until the power is turned off.
Status Register
the status register is a special register. It doesn't hold regular values like R0-R3 or even PC. Instead the status register has a few different bits that instructions can be use to check the status of previous operation.
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