Setting Clearing and Copying bits of data

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Clearing bits

AND xx will clear the bits in A that are also clear in xx, for example:

      A            xx      after AND xx
  %abcdefgh    %01010101    %0b0d0f0h

Setting bits

ORA xx will set the bits in A that are also set in xx, for example:

      A            xx      after ORA xx
  %abcdefgh    %01010101    %a1c1e1g1

Toggling bits

EOR xx will toggle the bits in A that are set in xx, for example:

      A            xx      after EOR xx
  %abcdefgh    %01010101    %aBcDeFgH

Clear complementary bits

To clear the bits in A that are set in xx, use both ORA and EOR, for example:

      A            xx      after ORA xx  after EOR xx
  %abcdefgh    %01010101    %a1c1e1g1    %a0c0e0g0

Copying bits

You can copy a number of bits to a memory location without changing the other bits using EOR and AND. For example, to copy the top four bits of A into a memory location without changing the bottom four bits, use the following:

               A=12345678  dst=abcdefgh
   EOR dst       ********      abcdefgh
   AND #&F0      ****0000      abcdefgh
   EOR dst       1234efgh      abcdefgh
   STA dst       1234efgh      1234efgh

This is much more efficient than the usual code:

   PHA
   LDA dst:AND #&0F:STA dst
   PLA
   AND #&F0:ORA dst:STA dst

Swapping data

Data can be swapped between two different memory locations without using a temporary location by EORing the data in both directions:

   LDA val1:EOR val2:STA val1
            EOR val2:STA val2
            EOR val1:STA val1

This is in contrast to using a temporary location or register such as with:

   LDA val1:STA tmp
   LDA val2:STA val1
   LDA tmp:STA val2

Jgharston 18:58, 30 August 2007 (BST) Jgharston (talk) 07:10, 30 June 2018 (CEST)