set nth bit

[Danjovic]

This function will set a bit within an array of elements, from msbit to lsbit.

 

Examples:
; array = 00 00 00 00 00 00 00 ...
;

;     a = 0
; after run...
; array = 80 00 00 00 00 00 00 ...    
;
;     again, now with a = 1
; after run...
; array = c0 00 00 00 00 00 00 ...    
;
;     again, now with a = 8
; after run...
; array = c0 80 00 00 00 00 00 ...    
;
;     again, now with a = 7
; after run...
; array = c1 80 00 00 00 00 00 ...    
;  

 

Here's the code:

define _a $d7 ; address of first element of array
lda #8  ; nth bit of array elements

setnthbit:
      ldx #0
a1:   sec        ; divide by 8, x = quocient, a = reminder-8
      sbc #8
      bcc a2
      inx
      jmp a1
a2:   clc     ; rotate the mask 'a' times   
      adc #9  ; 8 + 1 due to compensate first dey
      tay     ; y is the counter used to rotate the mask
      lda #$80 ; mask = 1000.0000
a3:   dey     ; mask rotate loop
      beq a4
      lsr    
      jmp a3
a4:   ora _a,x  ; set the nth bit
      sta _a,x   
      rts
      

 

Edited March 30, 2022 by Danjovic
corrected instruction (bcc a2 instead of bmi a2)