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Msg  : 286 of 360                                                               
From : Ruurd Pels                          2:282/317.19         28 May 93  16:31 
To   : David Todd                          1:259/423.0                           
Subj : Masking......                                                          
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Howdy David!

On 21 May 93, David Todd wrote the following message to All:

DT>  Can anybody tell me how to bitmask? Well here what I'm, doing. I am
DT> setting the Map Mask Register to plane 0, and writing all the data needed
DT> for plane 0 to the screen with a Movsb, samething with plane 1, and so on.
DT> Now I want to mask out any pixels that are 0 on the palette. How would I go
DT> about doing that? Do I have to do a compare for each pixel to make sure
DT> it's not a 0, before I write it or what? Well thanks to anybody who can
DT> help me...

================= S T A R T   O F   L E C T U R E ========================

A lecture on Boolean Algebra. Just look at bit 3, counting from 0
from the left.

==============================AND=========================================

Before   :          01101110            bit 3 is ON
Mask     :          00001000
                    ---------AND
Result   :          00001000            equals mask if bit 3 is ON
                    ========

So for ANDing bits, the truth table for each bit is:


      AND  |   0    |    1
     ------+--------+--------
       0   |   0    |    0
     ------+--------+--------
       1   |   0    |    1

Read this table as:

     0    AND  0    equals    0
     0    AND  1    equals    0
     1    AND  0    equals    0
     1    AND  1    equals    1

Use: To check if a bit is set.

============================OR============================================


Before   :          01100110       bit 3 is OFF
Mask     :          00001000
                    ---------OR
Result   :          01101110       bit 3 is ON
                    ========

So you switched the bit ON, regardless of its state, without
affecting the other bits. For each bit, the truth table for each bit is:


      OR   |   0    |    1
     ------+--------+--------
       0   |   0    |    1
     ------+--------+--------
       1   |   1    |    1

Use: To switch a bit on regardless of its prior state.
====================================XOR===================================

Before   :          01101110      bit 3 is on
Mask     :          00001000
                    ---------XOR
Result   :          01100110      bit 3 is off
Mask     :          00001000
                    ---------XOR
Result   :          01101110      bit 3 is on again
                    ========

So for XORing bits, the truth table for each bit is:


      XOR  |   0    |    1
     ------+--------+--------
       0   |   0    |    1
     ------+--------+--------
       1   |   1    |    0

Use: To toggle a bit regardless of its state
===============================NOT========================================

The effect of NOTing is inverting each bit in a variable.

Truth table:

     NOT(1) --> 0
     NOT(0) --> 1

So if you NOT the mask, you get:

     NOT(00001000) --> 11110111

So if you want to switch a bit off, regardless of state, you do this:


Before   :                         01101110       bit 3 is ON}
Mask     :    NOT(00001000) -->    11110111
                                   ---------AND
Result                             01100110       bit 3 is OFF
                                   ========

===============================NOT========================================

Keep in mind that all operators are binary, except for NOT, which is unary.

=================== E N D   O F   L E C T U R E ==========================

By substituting a different value for the mask, you may manipulate more than
one bit  at the same time.

Grtz, RFP ;-)

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