Re: Is there an easy way to scan for bits? -- MI400

Harry,

1. Step 1 would be to copy the next 12 bits in your source string, right
adjusted, to 2 char fld.

Use CPYBTL to do this.    CPYBTL  NextBitsRightAdjusted, From, StartBitOfs,
12.

Only problem is that StartBitOfs cannot be a variable. So you need 12 CPYBTL
instructions, one for each variable offset location.

This results in the next 12 bits in your source string, right adjusted, in a
2 char fld.


Step 2.   Use CPYBTLLS to shift everybody over 4 bits, so that your 12 bits
now start in bit 0 of a 2 char fld:

CPYBTLLS NextBits, NextBitsRightAdjusted, 4


Step 3.  Isolate the 1st 2 bits ( the 2 in from 2 to 12 bits ) right
adjusted in a 1 char idx fld.

CPYBTL  Idx, NextBits, 0, 2

Use this Idx into the 2Bit decode table. ( 4 entries ). Each entry is 2
bytes wide. A resulting table entry value of > 999 means "not a valid bit
pattern".  Any other value is your decode result.


Step 4. Repeat step 3, this time for 3 bits. Use the 3Bit decode table ( 8
entries ).


Step 5.  Repeat the decode scheme, each time for the next nbr of bits, using
a decode table with the max nbr of entries needed.


What are you using this for Harry ?  I have seen something similar in fax
compression schemes.

Regards,

Steve Richter







----- Original Message -----
From: "Harry Williams" <planesmart@teleteam.com>
To: <MI400@midrange.com>
Sent: Thursday, December 20, 2001 8:02 AM
Subject: [MI400] Is there an easy way to scan for bits?


> I need to change a data  into variable length bits, then translate these
> bits to a number.  As an example. I start reading the bits, and if I
> read  00110101 then I translate it to the number 0.  If I read 000111 I
> translate it to 1,  0111 translates to 2, 1000 goes to 3, 10011 is 8,
> ..... 0100111 translates to 18.   There is about 100 of these codes.  My
> concern is speed, since I have to translate up to 30k of these 100
> codes, if I take too many steps doing this, or if I am not efficient
> then it can easily eat up the CPU.  The table needs to be from 2 bits to
> 12 bits.  I currently see no easy way of doing this, efficiently.   But
> I know there are a lot smarter people than me on this list.  Any
> suggestions?
>
> Example.  00110101000111011110001011000000000001 would translate to
> 0,1,2,3,4,999.
>
> Next,
> I need to take the translated number and turn on bits.  If I get
> 1,2,3,4, then I would create
> 1111111111.  Which the 1 produces 1 bit, 2 would produce 11, 3 would
> produce 111, and 4 would produce 1111.  With these number I could turn
> bits on or off.  1,-2,3,-4 would be 1001110000.
>
>
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