Machine code relative offsets

[a92651delgado]

I am trying to understand how assembly is translated into machine code. I understand how to get the machine coding formats but I'm not sure how to calculate 'rr' in all cases. For the example below 'bra' is 20 rr or 20 03, I know that the relative offset is 03 (to get past the two bytes required by the 'inc' instruction)to get to the 'iny' instruction.

But I'm not sure why the rr = F3 for the 'dbne' instruction, how does that relative offset get me to the Again label?


Dissassembled code for program:
[CODE]          org   $600  
                ldy   #Array             ;CD B6 15
                ldaa  #N                 ;86 07
                clr    7,y               ;69 47
Again           brclr  0,y %11110000, Not_gtr  ;0F 40 F0 02
                bra    Update            ;20 03
Not_gtr         inc    Count             ;72 B6 1C
Update          iny                      ;02
                dbne   a,Again           ;04 30 F3
                swi                     ;            

Array           fdb   8,7,2                          
N               equ   7
Count           rmb   1[/CODE]

[Edward Karpicz]

rr=F3 in dbne has most significant bit set, which means rr displacement is
negative, F3 = -0D. 13 bytes from Again label to location after dbne (swi
instruction).

The rest can be found in CPU12/(S)/(SX) reference manuals.

Edward


----- Original Message -----
From: "AlD" <al_delgado@...>
To: <68HC12@...>
Sent: Wednesday, October 21, 2009 11:07
Subject: [68HC12] Machine code relative offsets

>I am trying to understand how assembly is translated into machine code. I
>understand how to get the machine coding formats but I'm not sure how to
>calculate 'rr' in all cases. For the example below 'bra' is 20 rr or 20 03,
>I know that the relative offset is 03 (to get past the two bytes required
>by the 'inc' instruction)to get to the 'iny' instruction.
>
> But I'm not sure why the rr = F3 for the 'dbne' instruction, how does that
> relative offset get me to the Again label?
>
>
> Dissassembled code for program:
> [CODE]          org   $600
>                ldy   #Array             ;CD B6 15
>                ldaa  #N                 ;86 07
>                clr    7,y               ;69 47
> Again           brclr  0,y %11110000, Not_gtr  ;0F 40 F0 02
>                bra    Update            ;20 03
> Not_gtr         inc    Count             ;72 B6 1C
> Update          iny                      ;02
>                dbne   a,Again           ;04 30 F3
>                swi                     ;
>
> Array           fdb   8,7,2
> N               equ   7
> Count           rmb   1[/CODE]
>
>
>
> ------------------------------------
>
> Yahoo! Groups Links
>
>
>
>

[Redd, Emmett R-2]

The inc instruction is 3 bytes.  Yes, rr = F3 for the dbne instruction is correct.

A formula which might help:  PCnew = PCbranch + len(branch) + rr

Or learn to count forwards and backwards in hexadecimal with zero being the first byte of the instruction after the branch.

Emmett Redd Ph.D.   mailto:EmmettRedd@...
Professor                                 [phone]
Department of Physics, Astronomy, and Materials Science
Missouri State University             Fax [phone]
901 SOUTH NATIONAL                   Lab [phone]
SPRINGFIELD, MO  65897   USA    Dept [phone]

"In theory there is no difference between theory and practice. In practice there is." -- Yogi Berra or Jan van de Snepscheut

________________________________________
From: 68HC12@... [68HC12@...] On Behalf Of AlD [al_delgado@...]
Sent: Wednesday, October 21, 2009 3:07 AM
To: 68HC12@...
Subject: [68HC12] Machine code relative offsets

I am trying to understand how assembly is translated into machine code. I understand how to get the machine coding formats but I'm not sure how to calculate 'rr' in all cases. For the example below 'bra' is 20 rr or 20 03, I know that the relative offset is 03 (to get past the two bytes required by the 'inc' instruction)to get to the 'iny' instruction.

But I'm not sure why the rr = F3 for the 'dbne' instruction, how does that relative offset get me to the Again label?


Dissassembled code for program:
[CODE]          org   $600
                ldy   #Array             ;CD B6 15
                ldaa  #N                 ;86 07
                clr    7,y               ;69 47
Again           brclr  0,y %11110000, Not_gtr  ;0F 40 F0 02
                bra    Update            ;20 03
Not_gtr         inc    Count             ;72 B6 1C
Update          iny                      ;02
                dbne   a,Again           ;04 30 F3
                swi                     ;

Array           fdb   8,7,2
N               equ   7
Count           rmb   1[/CODE]



------------------------------------

Yahoo! Groups Links



    http://docs.yahoo.com/info/terms/

[a92651delgado]

Thanks, do I include the byte(s) for the instruction after the dbne, in this case for the swi instruction or stop with the 3 bytes of the dbne?

--- In 68HC12@..., "Edward Karpicz" <karpicz@...> wrote:

>
> rr=F3 in dbne has most significant bit set, which means rr displacement is
> negative, F3 = -0D. 13 bytes from Again label to location after dbne (swi
> instruction).
>
> The rest can be found in CPU12/(S)/(SX) reference manuals.
>
> Edward
>
>
> ----- Original Message -----
> From: "AlD" <al_delgado@...>
> To: <68HC12@...>
> Sent: Wednesday, October 21, 2009 11:07
> Subject: [68HC12] Machine code relative offsets
>
>
> >I am trying to understand how assembly is translated into machine code. I
> >understand how to get the machine coding formats but I'm not sure how to
> >calculate 'rr' in all cases. For the example below 'bra' is 20 rr or 20 03,
> >I know that the relative offset is 03 (to get past the two bytes required
> >by the 'inc' instruction)to get to the 'iny' instruction.
> >
> > But I'm not sure why the rr = F3 for the 'dbne' instruction, how does that
> > relative offset get me to the Again label?
> >
> >
> > Dissassembled code for program:
> > [CODE]          org   $600
> >                ldy   #Array             ;CD B6 15
> >                ldaa  #N                 ;86 07
> >                clr    7,y               ;69 47
> > Again           brclr  0,y %11110000, Not_gtr  ;0F 40 F0 02
> >                bra    Update            ;20 03
> > Not_gtr         inc    Count             ;72 B6 1C
> > Update          iny                      ;02
> >                dbne   a,Again           ;04 30 F3
> >                swi                     ;
> >
> > Array           fdb   8,7,2
> > N               equ   7
> > Count           rmb   1[/CODE]
> >
> >
> >
> > ------------------------------------
> >
> > Yahoo! Groups Links
> >
> >
> >
> >
>

[Edward Karpicz]

No, rr is relative to the address of instruction (or address of the first
byte of instruction) that follows immediately after dbne. If we add label
Next like this:

>> > Again           brclr  0,y %11110000, Not_gtr  ;0F 40 F0 02
>> >                bra    Update            ;20 03
>> > Not_gtr         inc    Count             ;72 B6 1C
>> > Update          iny                      ;02
>> >                dbne   a,Again           ;04 30 F3   <--- rr = $F3
>> > = -$0D
      Next
>> >                swi                     ;

, then rr will equal Next - Again. There are thirteen bytes between Again
and Next. rr is negative because target label is above the 'next' (Again is
at more lower address than Next).

Edward




----- Original Message -----
From: "AlD" <al_delgado@...>
To: <68HC12@...>
Sent: Wednesday, October 21, 2009 20:39
Subject: [68HC12] Re: Machine code relative offsets

> Thanks, do I include the byte(s) for the instruction after the dbne, in
> this case for the swi instruction or stop with the 3 bytes of the dbne?
>
> --- In 68HC12@..., "Edward Karpicz" <karpicz@...> wrote:
>>
>> rr=F3 in dbne has most significant bit set, which means rr displacement
>> is
>> negative, F3 = -0D. 13 bytes from Again label to location after dbne (swi
>> instruction).
>>
>> The rest can be found in CPU12/(S)/(SX) reference manuals.
>>
>> Edward
>>
>>
>> ----- Original Message -----
>> From: "AlD" <al_delgado@...>
>> To: <68HC12@...>
>> Sent: Wednesday, October 21, 2009 11:07
>> Subject: [68HC12] Machine code relative offsets
>>
>>
>> >I am trying to understand how assembly is translated into machine code.
>> >I
>> >understand how to get the machine coding formats but I'm not sure how to
>> >calculate 'rr' in all cases. For the example below 'bra' is 20 rr or 20
>> >03,
>> >I know that the relative offset is 03 (to get past the two bytes
>> >required
>> >by the 'inc' instruction)to get to the 'iny' instruction.
>> >
>> > But I'm not sure why the rr = F3 for the 'dbne' instruction, how does
>> > that
>> > relative offset get me to the Again label?
>> >
>> >
>> > Dissassembled code for program:
>> > [CODE]          org   $600
>> >                ldy   #Array             ;CD B6 15
>> >                ldaa  #N                 ;86 07
>> >                clr    7,y               ;69 47
>> > Again           brclr  0,y %11110000, Not_gtr  ;0F 40 F0 02
>> >                bra    Update            ;20 03
>> > Not_gtr         inc    Count             ;72 B6 1C
>> > Update          iny                      ;02
>> >                dbne   a,Again           ;04 30 F3
>> >                swi                     ;
>> >
>> > Array           fdb   8,7,2
>> > N               equ   7
>> > Count           rmb   1[/CODE]
>> >
>> >
>> >
>> > ------------------------------------
>> >
>> > Yahoo! Groups Links
>> >
>> >
>> >
>> >
>>
>
>
>
>
> ------------------------------------
>
> Yahoo! Groups Links
>
>
>
>