Memory addressing

[CmdrVimes]

Hi chaps,

Can someone explain the difference between:

LEA        mylabel,a1

and

MOVE.l    mylabel(PC),a1

please?

[hitchhikr]

If mylabel would look like this:

mylabel: dc.l $12345678

Then in the 1st case a1 will contain the memory address of mylabel and in the 2nd case it's content as 32 bit word ($12345678), 2nd case is usually used for pointers.

[CmdrVimes]

Originally Posted by hitchhikr

2nd case it's content as 32 bit word ($12345678), 2nd case is usually used for pointers.

So I would use the second one if I want to write something into it? e.g.

MOVE.l        mylabel(PC),a1
MOVE.w        #45,(a1)

Is that what you mean?

[Thorham]

Originally Posted by CmdrVimes

Is that what you mean?

They probably mean that the second form is used for retrieving a stored pointer, where the label points to the pointer storage space.

Basically, lea mylabel,a1 doesn't read from memory, and it just sets a1 to mylabel, while move.l mylabel(PC),a1 reads a 32 bit number from mylabel(pc) and stores it in a1.

[phx]

For example:

        move.l  #myLabel,a1
        lea     myLabel,a1

...have the same effect. You will load the address of myLabel into A1 (LEA = Load Effective Address).
So why does LEA exist at all? Because you can load any effective address, which the immediate form of MOVE can't do:

        lea     myLabel(pc),a1
        lea     128(a0),a1

        move.l  myLabel(pc),a1
        move.l  myLabel,a1

...have the same effect too. Both will read a longword starting at myLabel. The PC-relative form has the advantage that it is a bit faster and saves two bytes.

[Lonewolf10]

Ok, maybe I'm being stupid or too tired too think straight (I have been coding for about 7 hours straight), but there must be a better way to code this:

lea.l   square(pc),a0
lea.l   screen+3160(pc),a1
bsr     drawbob
lea.l   square(pc),a0
lea.l   screen+2840(pc),a1
bsr     drawbob
lea.l   square(pc),a0
lea.l   screen+2520(pc),a1
bsr     drawbob
...

It's a snippet of some code used to add some bobs to the title screen of my demo. I know there must be a better way to do it (a loop of some sort), but my attempts either result in crashes or simply not possible to do!!
Can anyone point me in the right direction, please?


Regards,
Lonewolf10

[StingRay]

Just create a table with the coordinates of the bob and read these coords in a loop. Eg. have a table like this:
COORDSTAB dc.w 3160, 2840, 2520
dc.w -1 ; end marker

and the create a loop where you read the coords from the table and call your "drawbob" routine.

[Lonewolf10]

Originally Posted by StingRay

Just create a table with the coordinates of the bob and read these coords in a loop. Eg. have a table like this:
COORDSTAB dc.w 3160, 2840, 2520
dc.w -1 ; end marker

and the create a loop where you read the coords from the table and call your "drawbob" routine.

That is what I was aiming for, but my attempts just kept on crashing. I have finally got it working. I did try using "move.l (a3,d0.w),a1" (with the 3 preceeding lines commented out), but that doesn't work with .w, and simply crashes with .l
Atleast I have my own working code, even if it is a little ugly!

    moveq    #25,d7            ;number of coords-1
title_sq:
    move.w    (a2)+,d0        ;next coord -> d0
    move.l    a3,d1            ;screen address -> d1
    add.w    d0,d1            ;add coord to screen address
    move.l    d1,a1            ;move new address -> a1
*    move.l    (a3,d0.w),a1        ;was .l (crashes), .w doesn't work
    bsr    drawbob            ;ummm....
    dbra    d7,title_sq

Regards,
Lonewolf10