Packing data...

**Nick Austin** · Nov 13 '05, 05:42 PM

Re: Packing data...

On Mon, 25 Aug 2003 13:02:56 GMT, "FMorales" <altf2o@comcast .net>
wrote:
[color=blue]
>Hello all, quick hopefully easy question. Here goes.
>
>I've started on the Art Of Assembly tutorial(s)
>(http://webster.cs.ucr.edu/Page_asm/0_ArtOfAsm.html)
>and got to the sample projects and kinda got stuck. I'm needing to use a
>language of my choosing (which
>i easily chose C) to do this project where it wants me to pack data into a
>16 bit variable. Now i am
>familiar with C to a pretty decent extent, just not bit manipulation. So
>what i'm needing help on is getting
>this data packed into an unsigned short int, data type.
>
>The example it gives is : 0100 0001 0101 1000 or 4158h
>0100 being a 4 for the month,
>00010 being 2 for the day, and
>1011000 being 88 for the year.[/color]

Bitfield packing is done with the left-shift operator and the
bitwise inclusive or operator:

unsigned short month = 4;
unsigned short day = 2;
unsigned short year = 88;
unsigned short dmy;

dmy = month << 12 | day << 7 | year;

However it's a good idea to add range checks and/or masking so
that one field cannot overflow into the adjacent field.
[color=blue]
>These will come into my function as 3 seperate variables.I'm using 2 single
>characters for the month & day, then an array for the year.(they need to
>enter
>in say 1988 for the above example, and i just grab out the 88. Not a
>problem).[/color]

I'm glad it's "not a problem". You can combine your solution to
this bit with the packing operation from above.

Nick.

**Pieter Droogendijk** · Nov 13 '05, 05:42 PM

Re: Packing data...

On Mon, 25 Aug 2003 13:02:56 GMT
"FMorales" <altf2o@comcast .net> wrote:[color=blue]
> So what i'm needing help on is getting this data packed into an unsigned short
> int, data type.[/color]

bitfields or shifts.
[color=blue]
> The example it gives is : 0100 0001 0101 1000 or 4158h
> 0100 being a 4 for the month,
> 00010 being 2 for the day, and
> 1011000 being 88 for the year.
>
> So i would basically get something along the lines of, myFunc('4', '2',
> "1988");[/color]

what about december 31 2003? myFunc ('12', '31', "03")?
[color=blue]
> And i just need help getting those values in their correct "spot" packed
> into my
> 16 bit data type. (I can get 1 of them, but after the first one i keep
> messing
> up the way it's packed in by trying to pack the next value in.).[/color]

You can pack it into a data type, but it's printed value will change when you
run it on a big-endian system.

union {
unsigned short int si;
struct {
unsigned year:7;
unsigned day:5;
unsigned month:4;
} bf;
} cram;
main() {
cram.bf.month = 4;
cram.bf.day = 2;
cram.bf.year = 88;
printf ("%X\n", cram.si);
}

or if you like less readable but much shorter code:

main () {
unsigned short int sint = 0;
sint = 4<<12 | 2<<7 | 88;
printf ("%X\n", sint);
}

I'd pick the first option. It's understandable, and reading the variables is
easier.

--
char*x(c,k,s)ch ar*k,*s;{if(!k) return*s-36?x(0,0,s+1):s ;if(s)if(*s)c=1 0+(c?(x(
c,k,0),x(c,k+=* s-c,s+1),*k):(x(* s,k,s+1),0));el se c=10;printf(&x( ~0,0,k)[c-~-
c+"1"[~c<-c]],c);}main(){x(0 ,"^[kXc6]dn_eaoh$%c","-34*1'.+(,03#;+, )/'///*");}

Packing data...

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