Differences

This shows you the differences between two versions of the page.

--- base:advanced_optimizing [2022-01-14 12:26] – [Incrementing related pointers] bitbreaker
+++ base:advanced_optimizing [2022-02-08 13:56] – [Shifting] bitbreaker
@@ Line 718: / Line 718: @@
 The advantage is, that you can move bits also across registers and are not restricted to the accumulator only.
+When shifting, we handle 9 bits, as the bit falling out at one edge of the byte will be stored in the carry, and the carry will be shifted in. This will introduce a gap of one bit, when we wrap around bits:
+<code>
+        lda #%11111111
+        clc
+        rol
+        rol
+        ;-> A = %11111101
+        ;              ^
+        ;             gap :-(
+</code>
+To avoid that behaviour there's several ways around it:
+<code>
+        lda #%11111111
+        asl
+        adc #0
+        ...
+        lda #%11111111
+        anc #$ff
+        rol
+        ...
+        lda #%11111111
+        cmp #$80
+        rol
+</code>
+This way bit 7 is copied to carry first and then shifted in on the right end again.
 ====== Jumpcode ======