比较Motorolla 68k中的两个地址寄存器

Question

I have a simple program, where I'm trying to understand how to compare two address registers, and branch if A1 is less than or equal to A2

As such:

    ORG    $8000
START:                  ; first instruction of program

    CMP A2,A1
    BLE LOOP ;if A1 is less then or equal to A2, BRANCH

LOOP: 

* Put variables and constants here

    END    START        ; last line of source

Is this the correct way of doing it? What if I used the macro CMPA here instead, would it make a difference?

Answer 1

You have to use cmpa to compare with an addressregister on 68K. You don't have a choice.

Also it's best to include the size suffix .L to make sure the full 32 bits are compared as opposed to a sign-extended version of the low 16 bits.

CMPA.L A2,A1
BLE.S  LOOP

Answer 2

I'd use cmp.L for sure, since many 68k assemblers default to .w when no size is set and then you'd end up with CPU sign-extending lower 16 bit contents of addresses and then comparing them as 32-bit quantities (68k does so only for address arithmetics).

In contrary, most assemblers (at least known to me) will automatically select cmp a when destination of cmp is an address register.

Also, I see no reason in signed comparison of addresses with ble . Unsigned comparisons are more appropriate here, for example:

     lea   Start,a0
     lea   Start+size,a1
loop:clr.l (a0)+
     cmp.l a0,a1
     bhi.s loop