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--- base:8bit_multiplication_16bit_product_fast_no_tables [2020-04-09 21:34] – ftc
+++ base:8bit_multiplication_16bit_product_fast_no_tables [2023-03-15 03:25] (current) – djmips
@@ Line 7: / Line 7: @@
 ; by djmips
 ;
-; inputs are mul1 and mul2 and should be zero page.
+; inputs are mul1 and X.  mul1 and mul2 should be zp locations
 ; A should be zero entering but if you want it will factor in as 1/2 A added to the result.
 ;
@@ Line 16: / Line 16: @@
 ; total cycles best case = 97
 ; avg = 105
-; inner loop credits supercat
+; inner loop credits Damon Slye CALL APPLE, JUNE 1983, P45-48.
 MUL:
-     dec mul2	; decrement mul2 because we will be adding with carry set for speed (an extra one)
+     cpx #$00
+     beq zro
+     dex          ; decrement mul2 because we will be adding with carry set for speed (an extra one)
+     stx mul2
      ror mul1
      bcc b1
@@ Line 53: / Line 56: @@
 b8:  ror
      ror mul1
-     inc mul2
+     inx          ; Optional - this preserves X across the call - could also do inc mul2 or leave out
      rts
+zro: stx mul1
+     txa
+     rts
 </code>