Fido Pascal Conference PASCAL
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......

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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