CODEWITHSUNDEEP
assemblyavropcode
user3286125
user3286125

Reputation: 33

RJMP OPcode Help, Avr assembler

I have a question for solving the value of K in the opcode for rjmp. The opcode is 1100 kkkk kkkk kkkk. Where Pc<- PC +k + 1. When I solve for this I get k = -1 and then i know the 12 bits for K in the opcode is 0xFFF. I then assumed my answer is CFFF, but it is is incorrect. The answer is CFFD. What am I doing incorrectly? Heres the code I was given.

foo: NOP
  NOP
  RJMP foo

Upvotes: 1

Views: 2283

Answers (1)

Jester
Jester

Reputation: 58792

You want to jump back 1 word, and the formula gives k + 1 = -1 so k = -2 which is0xFFE on 12 bits. Plugging in the opcode gives 0xCFFE. You can check by using an assembler:

00000000 <foo>:
   0:   00 00           nop
   2:   fe cf           rjmp    .-4             ; 0x0 <foo>

Other way to look at it is that the offset is measured from the next instruction, because the cpu already incremented PC when the offset is added. That's where the +1 comes from.

Upvotes: 2

Related Questions