Inside a Micro-controller

A chip to be considered a cpu, should have at least a 1 control unit (cu) , 1 arithmetic logic unit (alu) and some registers,

• CU: is the brain of the cpu it controls every aspect of the cpu
• ALU: the component that is responsible for doing bit wise arithmetic in binary

The alu has register bank (aka a bunch of registers), the cortex-M has 16 registers and 5 special registers. The 16 register are divided into 13 (r0 to r12) general purpose registers, a stack pointer (sp) which can be ‘the main sp’ or ‘program sp’ depending on the mode of execution. The stack pointer register is a specialized CPU register used to keep track of the top of the stack

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By default the any firmware you write starts executing in Handler mode (the privileged mode) that uses the msp. The bit 1 of the Control register indicates if the msp or psp is in use.

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Link Register (LR) contains the return address after a function is done

Program counter (PC), also known as the instruction pointer or instruction address register, is a special register within a computer's central processing unit (CPU) that stores the memory address of the next instruction to be executed.

The stack

Thumb Instruction Set

Thumb is a compressed instruction set architecture used in ARM processors that provides:

• 16-bit instruction encoding (compared to ARM's 32-bit instructions)
• Higher code density (typically 30-40% smaller code size)
• Reduced memory bandwidth requirements

Key Characteristics:

• Originally had reduced functionality compared to full ARM instruction set
• Many Thumb instructions map to equivalent ARM instructions
• Processors switch between Thumb and ARM states when needed