I am getting a little bit deeper in exploring the STM32's MCU and I went bare metal. I am using an STM32F103C8T6 BluePill.
Just to make sure I'm fully understanding what is going on, what is the nature of the addresses of the MCU? I mean, for my board, the RCC boundary address is 0x4002 1000 - 0x4002 13FF and by creating a custom named pointer to this address and type casing this "0x40021000" to be treated as an actual address by the compiler, is this an address of an actual register?
I mean what I understood is that the MCU has 32bit registers, those registers have addresses, those addresses are the addresses of the registers and each 32bit register in the MCU has a unique address, a physical one! And therefore to manipulate a specific register we create a pointer to the address of that register and then dereferencing the pointer and change the register it is pointing to.
For example, the RCC address starts at "0x40021000" and this address is of the register 0x0000XX83 (where X is undefined as per the reference manual), Am I right? Do those registers actually have addresses in the MCU so they can be accessed and manipulated? If so, are those registers bits are just transistors where when setting a bit in that register to 1 the transistor turns on?
Also if what I understood was correct, does that mean that the Reset Values are the actual register values which have addresses (as in registers are analogous to variables e.g x,y, etc.. and there addresses are those in the reference manual as in RCC address = 0x40021000 and dereferencing this address gives us 0x0000XX83 ??
I know I might have confused you but this is the best I can explain! Thank you in advance.
Addresses are addresses on the bus*. When the compiler generates an instruction for the processor to read an address (LDR) it asks the bus to fetch what is at that address. Similarly a write instruction (STR) tells the bus what the address is and what data it wants to put there.
At the other end of the bus could be anything, but in most cases it will be either memory or a memory-mapped peripheral register. This could have any number of bits, but the most you can access in one go is 32. This is because the width of the bus is 32-bits. The datasheet lists the address ranges used by each peripheral, and the reference manual says what purpose of each peripheral register is.
In your example the RCC is a peripheral, its address range is 0x40021000-0x400213FF. The first register in the RCC is RCC_CR, which has address offset 0x00, making its address equal to 0x40021000. 0x0000XX83 is the value of the RCC_CR register at boot.
There are another set of registers which instructions can access directly. These are the processor core registers. Because they are not accessed through the bus they do not have an address. Most of the processor core registers have 32-bits and that is why the STM32 is called a 32-bit microcontroller.
(*in a Harvard architecture part like the Cortex-M3 used in STM32F1 there are actually several buses sharing an address space but as a beginner I would suggest you can overlook that.)
In terms of transistors, each bit in a register is usually made of more than 1 transistor. Often 6 transistors are wired together to make a flip-flop which represents a single bit.