Hello what does this code: $02 mean in assembly language

Question

Hello I am having a problem to understand a code. it is the following:

ldi R17, $02
out PORTD, R17

I don't know what $02 means. Can you please help me? Thank you in advance

Answer 1

The LDI opcode is Load Immediate this places the number 2 into register R17. The following line then copies that value from R17 into the register PORTD (the actual address of that register is dependent on the exact Atmel chip that you are running is is defined in the header file). These two lines would be generated from a line of c code like PORTD=2;

The reason it takes two lines is because the port registers are higher in memory space and immediate values cannot be loaded directly into them. So, the the value is loaded into a register first, then copied to the desired final location.