CORDIC for dummies
The CORDIC algorithm is still mysterious to many. Here is a presentation using only simple arithmetic.
In the late 50’s Convair designed for the USAF, the B58 Hustler supersonic jet bomber.
In 56, a project was started to replace the analog computer driven navigation system with a digital computer.
Jack Volder, at that time, senior engineer in the Convair’s aero electronics department discovered an algorithm to compute transcendental functions needed for realtime navigation problems.
He started is research by the « massaging of the basic angle addition equations » that he found in his « 1946 edition of the Handbook of Chemistry and Physics » :
Let’s take now a very simple example. The angle is 0.984736 radians and we are
looking for the tangent.
We proceed by successive subtractions until overdraft. We can subtract only one time “arc tan” 1 and two times “arc tan” 0.1 :
0.984736 


0.785398163500000 
1 
0.785398163500000 
0.199337836500000 


0.586060327000000 
KO 
0.785398163500000 



0.199337836500000 
1 

0.099669184004000 
2 
0.099668652496000 
0.000000531508000 

0.099668652496000 
0.099668120988000 
KO 
0.099668652496000 
Now we start back with the residual vector (X = 1, Y = 0.000000531508000) and we rotate counter clock wise (CCW) two times through angle “arc tan” 0.1 and one time through angle “arc tan” 1 :





CCW rotations 
X' 
Y' 
X 
Y 
X' = X  Y 10j 




Y' = Y + X 10j 









1 
0.999999947 
0.100000532 
1 
0.000000531508000 
2 
0.989999894 
0.200000526 
0.999999947 
0.100000531508000 
1 
0.789999368 
1.19000042 
0.989999894 
0.200000526 







Y/X= 
1.50633085144 

Important: there is no multiplication in the process, only shift right and subtraction operations
Look at the first CCW rotation, to calculate X’, two shift right (10^{2} ) on Y and a subtraction are enough (the « digits » are shifted right in this implementation ; but in the Volder’s original Cordic the bits are shifted).
X 
1,000000000000000 


Y 
0,000000531508000 

0,000000005315080 




X’ = 
0,999999994684920 
_______________________________________________________________
(1)
The addition formula is given by Abul Wafa in the 10th century:
http://en.wikipedia.org/wiki/Abul_Wáfa
See also the Briggs’ work:
http://www.jacqueslaporte.org/Briggs%20and%20the%20HP35.htm
(2) Implementing Cordic, Volder worked with powers of 2, because of the binary
ALU. HP introduced the first decimal Cordic in the HP 9100 (03/1968).
http://en.wikipedia.org/wiki/Rotation_%28mathematics%29
(3)
http://en.wikipedia.org/wiki/Imaginary_number
http://en.wikipedia.org/wiki/Complex_number
NB: The underlying structure explaining that Cordic can be presented in terms of rotations or complex number is the orthogonal group SO(2,R).
http://en.wikipedia.org/wiki/Orthogonal_group
and
http://en.wikipedia.org/wiki/Rotation_group.
See also for a unified “digit by digit” presentation of Volder’s Cordic and
Meggitt’s pseudo division and pseudo multiplication method (and the links to
download Volder’s and Meggitt’s work):
http://www.jacqueslaporte.org/digit_by_digit.htm