Double dabble
In computer science, the double dabble algorithm is used to convert binary numbers into binarycoded decimal (BCD) notation.^{[1]}^{[2]} It is also known as the shiftandadd3 algorithm, and can be implemented using a small number of gates in computer hardware, but at the expense of high latency.^{[3]}
Contents
Algorithm
The algorithm operates as follows:
Suppose the original number to be converted is stored in a register that is n bits wide. Reserve a scratch space wide enough to hold both the original number and its BCD representation; n + 4×ceil(n/3) bits will be enough. It takes a maximum of 4 bits in binary to store each decimal digit.
Then partition the scratch space into BCD digits (on the left) and the original register (on the right). For example, if the original number to be converted is eight bits wide, the scratch space would be partitioned as follows:
100s Tens Ones Original 0010 0100 0011 11110011
The diagram above shows the binary representation of 243_{10} in the original register, and the BCD representation of 243 on the left.
The scratch space is initialized to all zeros, and then the value to be converted is copied into the "original register" space on the right.
0000 0000 0000 11110011
The algorithm then iterates n times. On each iteration, the entire scratch space is leftshifted one bit. However, before the leftshift is done, any BCD digit which is greater than 4 is incremented by 3. The increment ensures that a value of 5, incremented and leftshifted, becomes 16, thus correctly "carrying" into the next BCD digit.
Essentially, the algorithm operates by doubling the BCD value on the left each iteration and adding either one or zero according to the original bit pattern. Shifting left accomplishes both tasks simultaneously. If any digit is five or above, three is added to ensure the value "carries" in base 10.
The doubledabble algorithm, performed on the value 243_{10}, looks like this:
0000 0000 0000 11110011 Initialization 0000 0000 0001 11100110 Shift 0000 0000 0011 11001100 Shift 0000 0000 0111 10011000 Shift 0000 0000 1010 10011000 Add 3 to ONES, since it was 7 0000 0001 0101 00110000 Shift 0000 0001 1000 00110000 Add 3 to ONES, since it was 5 0000 0011 0000 01100000 Shift 0000 0110 0000 11000000 Shift 0000 1001 0000 11000000 Add 3 to TENS, since it was 6 0001 0010 0001 10000000 Shift 0010 0100 0011 00000000 Shift 2 4 3 BCD
Now eight shifts have been performed, so the algorithm terminates. The BCD digits to the left of the "original register" space display the BCD encoding of the original value 243.
Another example for the double dabble algorithm – value 65244_{10}.
10^{4} 10^{3} 10^{2} 10^{1} 10^{0} Original binary 0000 0000 0000 0000 0000 1111111011011100 Initialization 0000 0000 0000 0000 0001 1111110110111000 Shift left (1st) 0000 0000 0000 0000 0011 1111101101110000 Shift left (2nd) 0000 0000 0000 0000 0111 1111011011100000 Shift left (3rd) 0000 0000 0000 0000 1010 1111011011100000 Add 3 to 10^{0}, since it was 7 0000 0000 0000 0001 0101 1110110111000000 Shift left (4th) 0000 0000 0000 0001 1000 1110110111000000 Add 3 to 10^{0}, since it was 5 0000 0000 0000 0011 0001 1101101110000000 Shift left (5th) 0000 0000 0000 0110 0011 1011011100000000 Shift left (6th) 0000 0000 0000 1001 0011 1011011100000000 Add 3 to 10^{1}, since it was 6 0000 0000 0001 0010 0111 0110111000000000 Shift left (7th) 0000 0000 0001 0010 1010 0110111000000000 Add 3 to 10^{0}, since it was 7 0000 0000 0010 0101 0100 1101110000000000 Shift left (8th) 0000 0000 0010 1000 0100 1101110000000000 Add 3 to 10^{1}, since it was 5 0000 0000 0101 0000 1001 1011100000000000 Shift left (9th) 0000 0000 1000 0000 1001 1011100000000000 Add 3 to 10^{2}, since it was 5 0000 0000 1000 0000 1100 1011100000000000 Add 3 to 10^{0}, since it was 9 0000 0001 0000 0001 1001 0111000000000000 Shift left (10th) 0000 0001 0000 0001 1100 0111000000000000 Add 3 to 10^{0}, since it was 9 0000 0010 0000 0011 1000 1110000000000000 Shift left (11th) 0000 0010 0000 0011 1011 1110000000000000 Add 3 to 10^{0}, since it was 8 0000 0100 0000 0111 0111 1100000000000000 Shift left (12th) 0000 0100 0000 1010 0111 1100000000000000 Add 3 to 10^{1}, since it was 7 0000 0100 0000 1010 1010 1100000000000000 Add 3 to 10^{0}, since it was 7 0000 1000 0001 0101 0101 1000000000000000 Shift left (13th) 0000 1011 0001 0101 0101 1000000000000000 Add 3 to 10^{3}, since it was 8 0000 1011 0001 1000 0101 1000000000000000 Add 3 to 10^{1}, since it was 5 0000 1011 0001 1000 1000 1000000000000000 Add 3 to 10^{0}, since it was 5 0001 0110 0011 0001 0001 0000000000000000 Shift left (14th) 0001 1001 0011 0001 0001 0000000000000000 Add 3 to 10^{3}, since it was 6 0011 0010 0110 0010 0010 0000000000000000 Shift left (15th) 0011 0010 1001 0010 0010 0000000000000000 Add 3 to 10^{2}, since it was 6 0110 0101 0010 0100 0100 0000000000000000 Shift left (16th) 6 5 2 4 4 BCD
Sixteen shifts have been performed, so the algorithm terminates. The BCD digits is: 6*10^{4} + 5*10^{3} + 2*10^{2} + 4*10^{1} + 4*10^{0} = 65244.
C implementation
The double dabble algorithm might look like this when implemented in C. Notice that this implementation is designed to convert an "input register" of any width, by taking an array as its parameter and returning a dynamically allocated string. Also notice that this implementation does not store an explicit copy of the input register in its scratch space, as the description of the algorithm did; copying the input register into the scratch space was just a pedagogical device.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
/*
This function takes an array of n unsigned integers,
each holding a value in the range [0, 65535],
representing a number in the range [0, 2**(16n)1].
arr[0] is the most significant "digit".
This function returns a new array containing the given
number as a string of decimal digits.
For the sake of brevity, this example assumes that
calloc and realloc will never fail.
*/
void double_dabble(int n, const unsigned int *arr, char **result)
{
int nbits = 16*n; /* length of arr in bits */
int nscratch = nbits/3; /* length of scratch in bytes */
char *scratch = calloc(1 + nscratch, sizeof *scratch);
int i, j, k;
int smin = nscratch2; /* speed optimization */
for (i=0; i < n; ++i) {
for (j=0; j < 16; ++j) {
/* This bit will be shifted in on the right. */
int shifted_in = (arr[i] & (1 << (15j)))? 1: 0;
/* Add 3 everywhere that scratch[k] >= 5. */
for (k=smin; k < nscratch; ++k)
scratch[k] += (scratch[k] >= 5)? 3: 0;
/* Shift scratch to the left by one position. */
if (scratch[smin] >= 8)
smin = 1;
for (k=smin; k < nscratch1; ++k) {
scratch[k] <<= 1;
scratch[k] &= 0xF;
scratch[k] = (scratch[k+1] >= 8);
}
/* Shift in the new bit from arr. */
scratch[nscratch1] <<= 1;
scratch[nscratch1] &= 0xF;
scratch[nscratch1] = shifted_in;
}
}
/* Remove leading zeros from the scratch space. */
for (k=0; k < nscratch1; ++k)
if (scratch[k] != 0) break;
nscratch = k;
memmove(scratch, scratch+k, nscratch+1);
/* Convert the scratch space from BCD digits to ASCII. */
for (k=0; k < nscratch; ++k)
scratch[k] += '0';
/* Resize and return the resulting string. */
*result = realloc(scratch, nscratch+1);
return;
}
/*
This test driver should print the following decimal values:
246
16170604
1059756703745
*/
int main(void)
{
unsigned int arr[] = { 246, 48748, 1 };
char *text = NULL;
int i;
for (i=0; i < 3; ++i) {
double_dabble(i+1, arr, &text);
printf("%s\n", text);
free(text);
}
return 0;
}
VHDL implementation
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.numeric_std.all;
entity bin2bcd_12bit is
Port ( binIN : in STD_LOGIC_VECTOR (11 downto 0);
ones : out STD_LOGIC_VECTOR (3 downto 0);
tens : out STD_LOGIC_VECTOR (3 downto 0);
hundreds : out STD_LOGIC_VECTOR (3 downto 0);
thousands : out STD_LOGIC_VECTOR (3 downto 0)
);
end bin2bcd_12bit;
architecture Behavioral of bin2bcd_12bit is
begin
bcd1: process(binIN)
 temporary variable
variable temp : STD_LOGIC_VECTOR (11 downto 0);
 variable to store the output BCD number
 organized as follows
 thousands = bcd(15 downto 12)
 hundreds = bcd(11 downto 8)
 tens = bcd(7 downto 4)
 units = bcd(3 downto 0)
variable bcd : UNSIGNED (15 downto 0) := (others => '0');
 by
 https://en.wikipedia.org/wiki/Double_dabble
begin
 zero the bcd variable
bcd := (others => '0');
 read input into temp variable
temp(11 downto 0) := binIN;
 cycle 12 times as we have 12 input bits
 this could be optimized, we do not need to check and add 3 for the
 first 3 iterations as the number can never be >4
for i in 0 to 11 loop
if bcd(3 downto 0) > 4 then
bcd(3 downto 0) := bcd(3 downto 0) + 3;
end if;
if bcd(7 downto 4) > 4 then
bcd(7 downto 4) := bcd(7 downto 4) + 3;
end if;
if bcd(11 downto 8) > 4 then
bcd(11 downto 8) := bcd(11 downto 8) + 3;
end if;
 thousands can't be >4 for a 12bit input number
 so don't need to do anything to upper 4 bits of bcd
 shift bcd left by 1 bit, copy MSB of temp into LSB of bcd
bcd := bcd(14 downto 0) & temp(11);
 shift temp left by 1 bit
temp := temp(10 downto 0) & '0';
end loop;
 set outputs
ones <= STD_LOGIC_VECTOR(bcd(3 downto 0));
tens <= STD_LOGIC_VECTOR(bcd(7 downto 4));
hundreds <= STD_LOGIC_VECTOR(bcd(11 downto 8));
thousands <= STD_LOGIC_VECTOR(bcd(15 downto 12));
end process bcd1;
end Behavioral;
VHDL testbench
LIBRARY ieee;
USE ieee.std_logic_1164.ALL;
ENTITY bin2bcd_12bit_test_file IS
END bin2bcd_12bit_test_file;
ARCHITECTURE behavior OF bin2bcd_12bit_test_file IS
 Component Declaration for the Unit Under Test (UUT)
COMPONENT bin2bcd_12bit
PORT(
binIN : IN std_logic_vector(11 downto 0);
ones : OUT std_logic_vector(3 downto 0);
tens : OUT std_logic_vector(3 downto 0);
hundreds : OUT std_logic_vector(3 downto 0);
thousands : OUT std_logic_vector(3 downto 0)
);
END COMPONENT;
 WARNING: Please, notice that there is no need for a clock signal in the testbench, since the design is strictly
 combinational (or concurrent, in contrast to the C implementation which is sequential).
 This clock is here just for simulation; you can omit all clock references and process, and use "wait for ... ns"
 statements instead.
Inputs
signal binIN : std_logic_vector(11 downto 0) := (others => '0');
signal clk : std_logic := '0';  can be omitted
Outputs
signal ones : std_logic_vector(3 downto 0);
signal tenths : std_logic_vector(3 downto 0);
signal hunderths : std_logic_vector(3 downto 0);
signal thousands : std_logic_vector(3 downto 0);
 Clock period definitions
constant clk_period : time := 10 ns;  can be omitted
 Miscellaneous
signal full_number : std_logic_vector(15 downto 0);
BEGIN
 Instantiate the Unit Under Test (UUT)
uut: bin2bcd_12bit PORT MAP (
binIN => binIN,
ones => ones,
tens => tenths,
hundreds => hunderths,
thousands => thousands
);
 Clock process definitions  the whole process can be omitted
clk_process :process
begin
clk <= '0';
wait for clk_period/2;
clk <= '1';
wait for clk_period/2;
end process;
 Combine signals for full number
full_number <= thousands & hunderths & tenths & ones;
 Stimulus process
stim_proc: process
begin
 hold reset state for 100 ns.
wait for 100 ns;
wait for clk_period*10;
 insert stimulus here
 should return 4095
binIN <= X"FFF";
wait for clk_period*10; assert full_number = x"4095" severity error;  use "wait for ... ns;"
 should return 0
binIN <= X"000";
wait for clk_period*10; assert full_number = x"0000" severity error;
 should return 2748
binIN <= X"ABC";
wait for clk_period*10; assert full_number = x"2748" severity error;
wait;
end process;
END;
Optimized snippet Bin2BCD for SBA (VHDL)
^{[4]}
 /SBA: Program Details ===================================================
 Snippet: 16 bit Binary to BCD converter
 Author: Miguel A. RiscoCastillo
 Description: 16 bit to BCD converter using "Double Dabble" algorithm.
 Before call you must fill "bin_in" with the appropriate value, after called,
 the snippet put into the variable "bcd_out" the BCD result of the conversion.
 Put the snippet in the routines section of the user program.
 /SBA: End Program Details 
 /SBA: User Registers and Constants ======================================
variable bin_in : unsigned(15 downto 0);  16 bit input register
variable bcd_out : unsigned(19 downto 0);  20 bit output register
 /SBA: End User Registers and Constants 
 /SBA: User Program ======================================================
 /L:Bin2BCD
=> bcd_out := (others=>'0');
if bin_in=0 then SBARet; end if;  if zero then return
=> bcd_out(2 downto 0) := bin_in(15 downto 13);  shl 3
bin_in := bin_in(12 downto 0) & "000";
=> for j in 0 to 12 loop
for i in 0 to 3 loop  for nibble 0 to 3, last nibble do not need adjust
if bcd_out(3+4*i downto 4*i)>4 then  is nibble > 4?
bcd_out(3+4*i downto 4*i):=bcd_out(3+4*i downto 4*i)+3;  add 3 to nibble
end if;
end loop;
bcd_out := bcd_out(18 downto 0) & bin_in(15); shl
bin_in := bin_in(14 downto 0) & '0';
end loop;
SBARet;  return to main program
 /SBA: End User Program 
Historical
In the 1960s, the term double dabble was also used for a different mental algorithm, used by programmers to convert a binary number to decimal. It is performed by reading the binary number from left to right, doubling if the next bit is zero, and doubling and adding one if the next bit is one.^{[5]} In the example above, 11110011, the thought process would be: "one, three, seven, fifteen, thirty, sixty, one hundred twentyone, two hundred fortythree", the same result as that obtained above.
See also
 Lookup table – an alternate approach to perform conversion
References
 ^ Gao, Shuli; AlKhalili, D.; Chabini, N. (June 2012), "An improved BCD adder using 6LUT FPGAs", IEEE 10th International New Circuits and Systems Conference (NEWCAS 2012), pp. 13–16, doi:10.1109/NEWCAS.2012.6328944
 ^ "BinarytoBCD Converter: "DoubleDabble BinarytoBCD Conversion Algorithm"" (PDF). Archived from the original (PDF) on 20120131.
 ^ Véstias, Mario P.; Neto, Horatio C. (March 2010), "Parallel decimal multipliers using binary multipliers", VI Southern Programmable Logic Conference (SPL 2010), pp. 73–78, doi:10.1109/SPL.2010.5483001

^ "SBA". sba.accesus.com. Retrieved 20161231.
The Simple Bus Architecture
 ^ Godse, Deepali A.; Godse, Atul P. (2008). Digital Techniques. Pune, India: Technical Publications. p. 4. ISBN 9788184314014.
Further reading
 Falconer, Charles "Chuck" B. (20040416). "An Explanation of the DoubleDabble BinBCD Conversion Algorithm". Archived from the original on 20090325.