update cycle count for imul16

mandel.s

@@ -92,8 +92,8 @@ next:
 positive:
 .endmacro
 
-; min 454 cycles
-; max 756 cycles
+; min 470 cycles
+; max 780 cycles
 .proc imul16
     arg1 = FR0   ; 16-bit arg (clobbered)
     arg2 = FR1   ; 16-bit arg (clobbered)
@@ -112,10 +112,10 @@ positive:
 
     ; unrolled loop for maximum speed, at the cost
     ; of a larger routine
-    ; 424 to 672 cycles
+    ; 440 to 696 cycles
     .repeat 16, bitnum
-        ; first half: 22 to 40 cycles
-        ; second half: 29 to 47 cycles
+        ; bitnum < 8: 25 or 41 cycles
+        ; bitnum >= 8: 30 or 46 cycles
         bitmul16 arg1, arg2, result, bitnum
     .endrepeat
 

readme.md

@@ -19,7 +19,7 @@ Enjoy! I'll probably work on this off and on for the next few weeks until I've g
 
 The 16-bit signed integer multiplication seems to be working, though I need to double-check it some more. It takes two 16-bit inputs and emits one 32-bit output in the zero page, using the Atari OS ROM's floating point registers as workspaces. Inputs are clobbered.
 
-The main loop is a basic add-and-shift, using 16-bit adds which requires flipping the sign of negative inputs (otherwise you'd have to add all those sign-extension bits). Runs in circa 450-750 cycles depending on input (I'll run the exact numbers again later).
+The main loop is a basic add-and-shift, using 16-bit adds which requires flipping the sign of negative inputs (otherwise you'd have to add all those sign-extension bits). Runs in 480-780 cycles depending on input.
 
 The mandelbrot loop is partly sketched out but I have future updates to make on that.