Optron #24 22 февраля 1999

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Likbez - Assembler view from afar: Logical operations, operations AND, OR, XOR, NOT; mnemonics. Results of teams.

       Assembler: a view from afar

{} Infarh


  Continued. Home - a | | 20, 21


           Logical operations


  Sorting out the last time with arithmetic operations, go to 
step logical. Unlike BASIC, in Assembler are equally important.


  Those who are not familiar with the algebra of logic
I will say that its main difference from ordinary arithmetic is 
that it actions are performed not over the whole number,

and over his bits - ie Transfer of Discharge
the discharge is not performed.

  Z80 processor is able to perform the following logical 
operations: 


             AND

             OR

             XOR

             NOT


  But not looking at the tables mnemonics
NOT - it is written as "CPL".

  A look into the appointment of each logical operation will 
help us a truth table. 



               AND operation



                   AND


                A B Y


                0 0 0

                0 1 0

                1 0 0

                1 1 1



  The symbols A and B denote the operands, and Y
- Is the result.

  In English, "AND" means "I".
As a chip that performs this
function that can lead 555LI1.

  The essence of the operation "and" (logical multiplication) 
can be derived from the following example. 

  Denote that you should read my article by Y = 1, and unread - 
through the Y = 0. To get the result you need

perform two operations:

  - Upload your Speccy "Optron" | 24
(If yes, then A = 1, and if not, then A = 0);

  - Found in "Optocouplers" Field LIKBEZ (respectively, B = 1 
or B = 0). 

  So, to get as a result of a logical "1", both operands as
must be equal to logic "1".

  Here is the equation of this operation:


                 Y = A * B



               OR operation


  "OR" is translated as "OR". This operation is also called 
"logical addition". Again we turn to the example. 

  To read my texts in
"Optocouplers" | 24 (ie, to obtain Y = 1), you
to select headings LIKBEZ (A = 1)
or LITSTRANICHKA (B = 1). Clearly, for
a positive result, one must be equal to either A or B, or
both together.

  That is - a truth table for this operation:



                   OR


                A B Y


                0 0 0

                0 1 1

                1 0 1

                1 1 1



  But - its equation:


                Y = A + B


  A sample of microcircuitry - 555LL1.



               XOR operation


  This - EXCLUSIVE OR.

  Again, cite an example.

  If you want, read my article BOTH play "Black Crow", I
I say:

  "No, darling, after two hares chase - you will not catch! Y = 
0!" 

  Respectively - and the truth table:



                   XOR


                A B Y


                0 0 0

                0 1 1

                1 0 1

                1 1 0



  In ordinary arithmetic, this operation is known as "the sum 
modulo 2: 


                1 + 1 = 0


  Similarly, in the decimal system with "summation modulo 10:


                9 + 1 = 0


  (In "addition modulo" transfer "is in the mind").



               NOT operation


  "NOT" means "not." This operation - not
simple, but very simple. It is
only on one operand and the changes it
meaning the opposite:



                   NOT


                  A Y


                  0 1

                  1 0



  For lovers of equations:

                      _

                  Y = A


  For "zhelezyachnikov: 555LN1.



          We turn to the mnemonic


  The impact of arithmetic operations on
a flag register, remind, does not cause problems ... But, then 
she and logic to brains of the processor to soar! So now,

considering options for a mnemonic, we indicate the influence 
of logical operations on flags: 



      Mnemonic Register F Bits

                       (Flags)


    Operator
     pa-Operand C Z P / V S N H

     tion


     AND r 0 x P x 0 1

     AND (HL)

     AND n

     AND (ii + n)


     OR r 0 x P x 0 0

     OR (HL)

     OR n

     OR (ii + n)


     XOR r 0 x P x 0 0

     XOR (HL)

     XOR n

     XOR (ii + n)


     CPL. . . . January 1



  Legend:

  Operations are always performed on the operand specified in 
the table, and the contents of the battery; 

  "." - Flag is not changed as a result of the operation;

  "X" - flag is set depending
on the result of the operation;

  Other designations - see | 21.



  The results of the commands (examples)


  When you run


                  AND E

have an operation "and" over the contents of
Register "A" and "E". Let's say they have
the following values:


               A = 10011010

               E = 01111001


  In this case we have, according to the truth table for AND, 
we get: 


             1 0 0 1 1 0 1 0

                   AND

             0 1 1 1 1 0 0 1


             0 0 0 1 1 0 0 0


  The result "00011000" is stored in the battery. "E" does not 
change its value. 

  Similarly, when executing commands


                    OR

and


                   XOR

with the same values ​​of the operands, we obtain:


             1 0 0 1 1 0 1 0

                    OR

             0 1 1 1 1 0 0 1


             1 1 1 1 1 0 1 1



             1 0 0 1 1 0 1 0

                   XOR

             0 1 1 1 1 0 0 1


             1 1 1 0 0 0 1 1


  Team "CPL" (Operation "NOT", are not forgotten?) Is on one 
operand - contents of register "A". That's what will

after its execution:


             1 0 0 1 1 0 1 0

         CPL

             0 1 1 0 0 1 0 1


  Since logical operations - everything. To be continued.