Signed Multiply-to-Divide

[dedndave]

i wanted to convert mouse wheel steps, which are multiples of 120 (WHEEL_DELTA), into unary steps
whenever i think "divide by a constant", the first thing that comes to mind is multiply-to-divide
never tried it with signed values, before   :8)

this seems to work pretty well

Code Select Expand

;EAX = signed wheel distance * WHEEL_DELTA

         imul    eax,139810
         sar     eax,24
         adc     eax,0

;EAX = signed wheel distance

i wrote a little test program - here are the results...

Code Select Expand

-128 -127 -126 -125 -124 -123 -122 -121 -120 -119 -118 -117 -116 -115 -114 -113
-112 -111 -110 -109 -108 -107 -106 -105 -104 -103 -102 -101 -100 -99 -98 -97 -96
-95 -94 -93 -92 -91 -90 -89 -88 -87 -86 -85 -84 -83 -82 -81 -80 -79 -78 -77 -76
-75 -74 -73 -72 -71 -70 -69 -68 -67 -66 -65 -64 -63 -62 -61 -60 -59 -58 -57 -56
-55 -54 -53 -52 -51 -50 -49 -48 -47 -46 -45 -44 -43 -42 -41 -40 -39 -38 -37 -36
-35 -34 -33 -32 -31 -30 -29 -28 -27 -26 -25 -24 -23 -22 -21 -20 -19 -18 -17 -16
-15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12
13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 3
9 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92
93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 11
4 115 116 117 118 119 120 121 122 123 124 125 126 127

now, all we need is for qWord to make a new version of MagicNumber for signed mul2div
and a new set of macros
that will make him happy - to be working on mul2div, again   :bg

[qWord]

now, all we need is for qWord to make a new version of MagicNumber for signed mul2div

It is allready implemented in the program: there is a checkbox named 'signed'  :8)

for 120 it shows:

Code Select Expand

  mov ecx,X
mov eax,088888889h
imul ecx
add edx,ecx
sar edx,06h
shr ecx,31
add edx,ecx
; quotient now in edx

EDIT: wrong text copied

[dedndave]

ah - things are back to normal - you're a step ahead of me   :bg