Counting, incrementing, decrementing
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This will count up from any value and end at zero:

    CMP #1         :\ Carry set if A>0
    ADC #0         :\ If A>0, A=A+1; if A=0, A=A+0

This code will count down from any value and ends at &FF:

    CMP #&FF       :\ Carry set if A=&FF
    SBC #0         :\ If A<&FF, A=A-1; if A=&FF, A=A-0


Comparing various values
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    ORA #0         :\ Sets EQ if A=0, the usual test for zero

    CMP #0         :\ Sets EQ if A=0, also sets CS

    CMP #1         :\ Sets CC if A=0, sets CS if A<>0

Sign extension
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    AND #&80       :\ A become &00 or &80
    BPL P%+4       :\ Skip past keeping &00
    LDA #&FF       :\ Change &80 to &FF

Extends bit 7 of the A register to a full byte so an 8-bit byte can be sign-
extended to a 16-bit word, ie &80-&FF, &00-&7F becomes &FF80-&FFFF, &0000-&007F.