The module "multiply_karatsuba"
a recursive VHDL implementation of the Karatsuba multiplication algorithm

The recursive VHDL module "multiply_karatsuba" (see symbol) calculates the signed product of a multiplicand and a multiplier with the Karatsuba algorithm.

The Karatsuba algorithm is based on dividing the factors (multiplicand and multiplier) into an upper half and a lower half.
The multiplication is then realized by products calculated only with this halfs.
This dividing does not work with negative numbers in 2's complement, so the factors are first converted to unsigned.
Their sign is remembered and later on used to convert the always positive product back into 2's complement.
The Karatsuba algorithm expects the factors to have the same number of bits.
So the factor with less bits is extended to have the same number of bits as the other factor.
As the number of bits is halfed at each stage of the Karatsuba algorithm, the bit number
of the factors is extended to a power of 2.

The module "multiply_karatsuba" was developed with HDL-SCHEM-Editor.


Port name Direction Description
res_i input asynchronous reset input, 1-active
clk_i input clock input
start_i input This input expects an 1-active impulse of 1 clock cycle width in order to start the calculation.
multiplicand_i(g_multiplicand_width-1:0) input Signed multiplicand (g_multiplicand_width is a generic)
multiplier_i(g_multiplier_width-1:0) input Signed multiplier (g_multiplier_width is a generic)
ready_o output 1-active impulse of 1 clock cycle width, when the calculation is ready (in the next clock cycle after start_i=1).
product_o(g_multiplicand_width+g_multiplier_width-1:0) output Signed product. Valid at ready_o=1.


Generic name Minimum Value Maximum Value Description
g_multiplicand_width 2 none Number of bits of the multiplicand
The first bit represents the sign as the operands have to be coded in 2's complement.
g_multiplier_width 2 none Number of bits of the multiplier
The first bit represents the sign as the operands have to be coded in 2's complement.

The module "multiply_karatsuba" is a hierarchical module, which uses only the submodule multiply_karatsuba_step.
This submodule multiply_karatsuba_step has only instances of itsself, so there are several submodules used, but they are all from tpye multiply_karatsuba_step.

Submodule name Functionality

This submodule implements the Karatsuba algorithm.

The incoming unsigned operands must have a identical number of bits.
This number of bits must be a power of 2.

The incoming operands are divided into 2 halfs with identical bit width.
These half sized operands are connected to other multiply_karatsuba_step modules located inside multiply_karatsuba_step.
This leads to recursive hardware.

There are no limitations for the generics g_multiplicand_width and g_multiplier_width (except that they must be bigger than 1).
These generics are most of the time determined by the environment, where the module multiply is used.

symbol symbol symbol

Source code for HDL-SCHEM-Editor and HDL-FSM-Editor for module "multiply_karatsuba" and its testbench (Number of downloads = 107 ).
With these files the schematics and the state-diagram of module "multiply_karatsuba" can be loaded into HDL-SCHEM-Editor or HDL-FSM-Editor and can be easily read and modified:

All module VHDL-files of the module "multiply_karatsuba" (Number of downloads = 116 ).
These files were generated by HDL-SCHEM-Editor and HDL-FSM-Editor:

All testbench VHDL-files of the testbench of the module "multiply_karatsuba" (Number of downloads = 39 ).
These files were generated by HDL-SCHEM-Editor and HDL-FSM-Editor:

Relocation hints:

You should extract all archives into a folder named "multiply_karatsuba".
Then you must replace "M:/gesicherte Daten/Programmieren/VHDL/multiply_karatsuba" in all "hdl_editor_designs/*.hse" source files by your path to this directory.
Now you can navigate through the design by HDL-SCHEM-Editor and generate HDL by HDL-SCHEM-Editor for all modules .
Of course there is only need for generating HDL, if you change something at the modules, because you can find the HDL in and
If you want to simulate or modify the modules by HDL-SCHEM-Editor you also must adapt the information in the Control-tab of the toplevel you want to work on.
There you must define a "Compile through hierarchy command", an "Edit command", the path to your HDL-FSM-Editor and a "Working directory".

Change log:

Version 1.0 (07.04.2024):

If you detect any bugs or have any questions,
please send a mail to "".