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--- playground:reversing_bits_in_a_byte [2020-02-12 16:42] – created icepic
+++ playground:reversing_bits_in_a_byte [2020-03-12 13:05] (current) – tlr
@@ Line 19: / Line 19: @@
 using X and Y for temporary storage and only short 2-cycle instructions in the loop.
 Of course a table lookup will be faster if you do this a lot in your code.
+--
+this would be equally fast, at 100cycl +rts (enter with the value in .A, result in .A, .X will be 0)
+<code>
+        ldx #8
+loop    asl
+        ror $2
+        dex
+        bne loop
+        lda $2
+        rts
+</code>
+or unrolled (56 cycl +rts):
+<code>
+        asl
+        ror $2
+        asl
+        ror $2
+        asl
+        ror $2
+        asl
+        ror $2
+        asl
+        ror $2
+        asl
+        ror $2
+        asl
+        ror $2
+        asl
+        lda $2
+        ror
+        rts
+</code>
+--
+Optimized version, at 84cycl +rts (enter with the value in .A, result in .A)
+<code>
+	sta	$02	; 3
+	lda	#1	; 2
+lp:
+	ror	$02	; 5
+	rol		; 2
+	bcc	lp	; 3(2)
+			; =84
+	rts
+</code>
+or slightly unrolled:
+<code>
+	sta	$02	; 3
+	lda	#1	; 2
+lp:
+	ror	$02	; 5
+	rol		; 2
+	ror	$02	; 5
+	rol		; 2
+	bcc	lp	; 3(2)
+			; =72
+	rts
+</code>