I have a coldfire sequence from IDAPro that looks like this:


var_8 = -8
var_2 = -2
arg_0 = 8
arg_6 = $E

link a6, #-4;  
move.l a2, -(sp)
movea.l arg_0(a6),a2
tst.w arg_6(a6)
bne.s loc_29c00
bset #5, $1e(a2)
bra.s loc_29c14


In the documentation here, I'm having some issues with understanding what exactly the documentation is saying.

LINK Ay,#<displacement> W SP – 4→SP; //subtract SP by 4 and set into SP Ay → (SP); //set address pointed to in Ay and set into value of SP SP→Ay, SP + dn → SP //set SP into Ay, add the offset specified by <displacement> and set into SP

Are ALL of these steps executed for the LINK instruction, or are these the possibilities based on some processor states that I haven't found out about yet? (This appears to be setting up the stack for a function call.)

The next question I have about the sequence is the second instruction, move.l a2, -(sp)

This appears to be moving a longword from a2 into the stack pointer, HOWEVER the negation looks odd to me. It appears to be "move address in a2 into SP and then multiply its value by -1" but I could also see the parenthesis being used as specifying an order of operations to be applied to the address.

  • 1
    Your second question is explained in (uh) about any 68K documentation: it's just a pre-decrement. If you are familiar with x86 code: it's a push, but to any A-register, and 'you' need to decrement the address yourself, or it'll just store the new value. – Jongware Feb 17 '15 at 21:07
  • @Jongware, I'm using the ColdFire User Manual, and the only reference to "predecrement" in the entire 700+ pages was a description of the algorithm used by the MASK instruction. I had to dig up a different "programmer's reference" to get more detail. – avgvstvs Feb 17 '15 at 21:20
  • Gave me the answer to LINK as well. – avgvstvs Feb 17 '15 at 21:21

Answer to the LINK question was provided on page 119 here.

Pre/Post decrement operations are described on page 44.

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