This example shows
some simple arithmetic operations and type
conversions. The following shows the xlSimpleArith.m file,
which specifies the xlSimpleArith M-function.
function [z1, z2, z3, z4] = xlSimpleArith(a, b)
% xlSimpleArith demonstrates some of the arithmetic operations
% supported by the Xilinx MCode block. The function uses xfix()
% to create Xilinx fixed-point numbers with appropriate
% container types.%
% You must use a xfix() to specify type, number of bits, and
% binary point position to convert floating point values to
% Xilinx fixed-point constants or variables.
% By default, the xfix call uses xlTruncate
% and xlWrap for quantization and overflow modes.
% const1 is Ufix_8_3
const1 = xfix({xlUnsigned, 8, 3}, 1.53);
% const2 is Fix_10_4
const2 = xfix({xlSigned, 10, 4, xlRound, xlWrap}, 5.687);
z1 = a + const1;
z2 = -b - const2;
z3 = z1 - z2;
% convert z3 to Fix_12_8 with saturation for overflow
z3 = xfix({xlSigned, 12, 8, xlTruncate, xlSaturate}, z3);
% z4 is true if both inputs are positive
z4 = a>const1 & b>-1;
This M-function uses addition and subtraction operators. The MCode block calculates these operations in full precision, which means the output precision is sufficient to carry out the operation without losing information.
One thing worth discussing is the
xfix function call. The function requires two arguments: the
first for fixed-point data type precision and the second indicating the value. The precision
is specified in a cell array.
The first element of the precision cell array is the type value. It can be one
of three different types: xlUnsigned, xlSigned, or xlBoolean.
The second element is the number of bits of the fixed-point number.
The third is the binary point position. If the element is xlBoolean, there is no need to specify the number of bits and
binary point position. The number of bits and binary point position must be specified in
pair.
The fourth element is the quantization mode. The quantization mode can be one
of xlTruncate, xlRound, or
xlRoundBanker.
The fifth element is the overflow mode. The overflow mode can be one of
xlWrap, xlSaturate, or
xlThrowOverflow.
You must specify quantization mode and overflow mode as a pair. If the
quantization-overflow mode pair is not specified, the xfix
function uses xlTruncate and xlWrap for signed and unsigned numbers. The second argument of the xfix function can be either a double or an AMD fixed-point number. If a constant is an integer number, you do not need to
use the xfix function. The Mcode block converts it to the
appropriate fixed-point number automatically.
After setting the dialog box
parameter MATLAB function to xlSimpleArith, the block shows two input ports a and b, and four output ports z1,
z2, z3, and z4.
You can test M-functions using
AMD data types and functions in the MATLAB Command Window. For example, if you type: [z1, z2, z3, z4] = xlSimpleArith(2, 3) in the MATLAB Command Window, you'll get the following lines:
UFix(9, 3): 3.500000
Fix(12, 4): -8.687500
Fix(12, 8): 7.996094
Bool: true
xfix call.