assemblyeasy68k

Mortorola 68k : How String ASCII is stored in address register a0

I am very new to Assembly programming in 68k. I am using Easy68k.

I have a program:

    ORG    $1000
START:                

* Put program code here

    lea MSG,a1      ; loads MSG into address register a1
    move.b #14,d0   ; 14 gets coverted to hex E
    trap #15

    move.b #9,d1    ; 9 decimal gets converted t hex 9
    SIMHALT             ; halt simulator

* Put variables and constants here

MSG dc.b    'It is a long established fact that a reader will be distracted by the readable content of a page when looking at its layout',0 

    END    START        ; last line of source

As far is I know, the address register a1 can store long-word length items. But you see the string

"It is a long established fact that a reader will be distracted by the readable content of a page when looking at its layout"

convert it to its ASCII format, you will see how the hex for each word would exceed the long-word length.

So How is this string stored? I want to understand how assembly is storing and displaying the string to the screen.

Can someone please explain?

Solution

  • The comment "loads MSG into address register a1" is somewhat misleading. What you're loading into a1 is not the contents at MSG; you're loading the address of MSG. A label (e.g. MSG) is just a name for some location in your program that makes it convenient to refer to that location.

    The address has a fixed size which fits into the register and is independent of the data stored at that address.