Electrical – Do I need to use the ‘add’ function in mips to store into a variable or can I store into a variable with load word

$s and $t are register specifiers. Your interpretation MEM[RegisterData(s) + offset] is basically correct. $s means, essentially, "get the data out of register number s in the register file." So if s in the instruction is the bit pattern 10001, then $s means: read the value out of register 17, add the sign-extended offset bits (the 16 bits represented by i in the encoding), and use that as the address to access the memory. Similarly for the $t in the lw instruction. It means write the data back into the register numbered t.

Your question about sign extension: look up two's complement. The 16 bit two's complement number 0000 1111 1111 1111 is decimal +4095. The 16 bit two's complement number 1000 1111 1111 1111 is decimal -28673. The 32 bit two's complement number 0xffff8fff is also -28673. The 32 bit two's complement number 0x80008fff is decimal -2147446785, which is clearly not what you want.

These segments are merely metadata - they have no meaning to the AVR CPU or to your code. They are instructions for the linker.

When you compile a C program with avr-gcc, it adds what is called a 'stub'. This code contains the interrupt vectors and reset vector. When the CPU starts up, it does not immediately execute your main method, instead it starts executing in the reset vector. The reset vector points to the startup code in the assember stub. This code initializes the stack pointer and the data segments. After this is finished, it jumps into your main method. If you write in pure assembly on bare metal, none of this is done for you unless you do it yourself.

The different segments are used primarily by the compiler, assembler, and linker to put the pieces of your program together intelligently and make sure each static variable has its own piece of address space. When the pieces are put together, address pointers and offsets are substituted into the program code in appropriate locations. After the program is compiled and linked, the segments no longer really exist in the output bin/hex file. Actually, none of the data segments end up in the hex file at all, only a copy of the initialization data for the initialized segment in a location that has been substituted into the startup code. For example, there is no difference from the microcontroller's standpoint between initialized and uninitialized data. However, the startup code would like all of the initialized data to be in one contiguous block so it can loop over it in one loop. Also, the startup code needs to know where the segment is located and how big it is. The linker combines the segments from the individual object files into one big segment, then it substitutes the addresses into the startup code after it has figured out where to put everything.

Also, there is a rather important difference between an AVR microcontroller and a standard PC: in the AVR, instructions and data have completely separate address spaces. This means that you need to know which address space you're referring to when you perform a memory access. The assembler may be able to help you with this if it is intelligent enough.