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base:using_a_byte_as_bitcounter_and_value_container_at_the_same_time [2015-04-17 04:34]
127.0.0.1 external edit
base:using_a_byte_as_bitcounter_and_value_container_at_the_same_time [2015-07-20 11:31] (current)
lft
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 +====== Using a byte as bitcounter and value container simultaneously ======
  
 +This code was posted by Enthusi on the CSDb forum. Inventor uknown. The idea is to use the byte (in $bd in this case) as a "​counter"​ counting eight times at the same time as it is filled with each of the bits of a byte value. This way you don't need to use extra registers to keep track of the eight iterations.
 +
 +The getbit routine is not included here since this article is intended to demonstrate a general concept, regardless of how the actual getbit routine is designed in each actual case. For a more elaborate example, see [[Decoding bitstreams]].
 +
 +<​code>​
 +getbyte:
 + lda #$01   ;This bit will be rotated to carry when a complete byte is read.
 + sta $bd
 +
 +loop:
 + jsr getbit ;This subroutine reads one bit from the datastream and stores it in the carry flag.
 + rol $bd
 + bcc loop   ;As long as the bit doesn'​t roll off the byte (after 8 loops), do the loop
 + lda $bd
 + rts
 +</​code>​
 +
 +From IRC:
 +Krillye: franticHT: that "​trick"​ is pretty common with serial transfer routines as well [in addition to tape loading routines], and some maths routines
base/using_a_byte_as_bitcounter_and_value_container_at_the_same_time.txt ยท Last modified: 2015-07-20 11:31 by lft