Re: CCITT Group 4 compression

Dave,

I see that yours is a more accurate translation.

Very creative use of the XLATEWT instruction.

Steve Richter

----- Original Message -----
From: "Dave McKenzie" <davemck@zbiggroup.com>
To: <mi400@midrange.com>
Sent: Saturday, December 15, 2001 5:55 PM
Subject: Re: [MI400] CCITT Group 4 compression


> bitmap is an array of chars.
> cp is a pointer to a char.
>
> bitmap[*cp++] means: the char from bitmap at index *cp++.  (C indexes are
> 0-relative, so bitmap[0] is the first char.)
>
> *cp means: the char pointed to by cp.
> The ++ after cp means: increment cp after using it to address a char.
>
> So bitmap[*cp++] means: Extract the char from bitmap at the index equal to
> the integer value of the char pointed to by cp.  Then increment cp by 1.
>
> X << BitsAvail means: left shift X by BitsAvail bits (where BitsAvail is
an
> integer).
>
> BitAcc |= X means: "or" X into BitAcc.  It's shorthand for BitAcc = BitAcc
| X
> where "|" is the "or" operator.
>
> Here's MI that attempts to do the same thing:
>
>          DCL SPCPTR CP        AUTO;
>          DCL DD     C         CHAR(1) BAS(CP);
>          DCL DD     BITMAP    CHAR(256) AUTO;
>          DCL DD     CHAR4     CHAR(4) AUTO;
>          DCL DD     BITSAVAIL UNSGND BIN(2) AUTO BDRY(2);
>          DCL DD     UBIN4     UNSGND BIN(4) AUTO BDRY(4);
>          DCL DD     BITACC    UNSGND BIN(4) AUTO BDRY(4);
>
>          XOR(S)     CHAR4, CHAR4;
>          XLATEWT    CHAR4(4:1), C, BITMAP;
>          ADDSPP     CP, CP, 1;
>          CPYBTLLS   UBIN4, CHAR4, BITSAVAIL;
>          OR(S)      BITACC, UBIN4;
>
> --Dave
>
>
> On Saturday 15 December 2001 07:25, Harry Williarms wrote:
> > Ok,
> > The code I am stuck on is
> > BitAcc |= bitmap[*cp++]<<BitsAvail;
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