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Fixed-Point Data Types inSystemC

Part ofHW/SW Codesign of

Embedded Systems Course (CE 40-226)

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Topics Fast Fixed-Point Types Dynamic Fixed-Point Types

Context concept and its usage Defined Operators on Fixed-Point Types Complementary Notes on Fixed-Point Class

Some other member functions of Fixed-Point class

Type casting Arrays of Fixed-Point Data

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Fixed-Point Data Types inSystemC

Fast Fixed-Point Data Types

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Fast Fixed-Point Data Types Arbitrary Precision vs. Simulation Speed Achieving Faster Speed

Use double as underlying data type Mantissa limited to 53 bits Range limited to that of double

Fast Fixed-Point Types sc_fixed_fast, sc_ufixed_fast sc_fix_fast, sc_ufix_fast

Exactly the same declaration format and usage as before

All fixed-point data types, can be mixed freely

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Fast Fixed-Point Data Types:Simple Example#include “systemc.h”

float adder(float a, float b)

{sc_fixed_fast<4,2,SC_RND, SC_WRAP> Inputa = a;sc_fixed_fast<6,3, SC_RND, SC_WRAP> Inputb = b;sc_fixed_fast<7,4,SC_RND, SC_WRAP> Output;

Output = (Inputa + Inputb);return (Output);

}

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Fixed-Point Data Types inSystemC

Dynamic Fixed-Point Data Types

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Dynamic Fixed-Point Data Types Parameters passing

sc_fxtype_params Type data members

wl: word length iwl: integer word length q_mode: quantization mode o_mode: overflow mode n_bits: saturated bits some others

Reading/writing individual data members .wl() .iwl() .q_mode() .o_mode()

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Dynamic Fixed-Point Data Types (cont’d) Example

sc_fxtype_params my_fx_type(8,4,SC_RND,SC_SAT);

sc_fix my_fx(my_fx_type);

my_fx = 10.2;

my_fx_type.wl(15);

cout<<my_fx_type.wl();

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Context Concept Specifies default values for (dynamic) fixed-point

variables. Built-in defaults: <32,32,SC_TRN,SC_WRAP> Declaration syntax

sc_fxtype_context obj_name(<sc_fxtype_params>) Scope

C/C++ scopes for variables/objects Context is activated when declared

Declaration without activating is supported sc_fxtype_context

obj_name(<sc_fxtype_params>, SC_LATER) Selective enabling/disabling of Contexts is supported

.begin() and .end() member functions

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Context Concept:Simple Example

sc_fxtype_params myparams(SC_RND, SC_SAT);

sc_fxtype_context mycontext(myparams);

sc_fix_fast adder(sc_fix_fast a, sc_fix_fast b)

{

sc_fix_fast Output(a.wl() +1, a.iwl() +1);

Output = a + b;

return(Output);

}

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Context Concept:Complex Example

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sc_fxtype_params param1(12,3);

sc_fxtype_params param2(32,3,SC_RND,SC_SAT);

sc_fxtype_params param3(16,16,SC_TRN,SC_SAT_ZERO);

.............

sc_fxtype_context c_1(param1,SC_LATER);

sc_fxtype_context c_2(param2);

sc_fxtype_context c_3(param3, SC_LATER);

sc_fix a;

c_1.begin();

sc_fix b;

c_3.begin();

sc_fix c;

sc_fixed<13,5> zz;

c_3.end();

sc_fix d;

c_1.end();

sc_fix e;

c_2.end();

sc_fix f;

c_2active

c_1active

c_3active

c_1active

c_2active

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Fixed-Point Data Types inSystemC

Fixed-Point Operators

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Operators on Fixed-Point Data Types

Bitwise: & | ^ ~ Arithmetic: * / + - ++ --

<< >> Equality: == != Relational: < <= > >= Assignment: = &= |= ^= +=

-= *= /= <<= >>=

Bit Selection: [] Part Selection .range(,)

Bit selection return type: like sc_bit Part selection return type: like sc_bv

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Operators on Fixed-Point Data Types Alignment is done before each binary

operation

&

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Fixed-Point Data Types inSystemC

Complementary Notes onFixed-Point Classes

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Useful State Information Status member functions

is_neg() is_zero() overflow_flag() quantization_flag()

sc_fixed<10,2> my_var;

if (my_var.is_zero())

...

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Conversion To String Conversion member function

to_string(number_representation, format) Available number representations

SC_DEC (is the default) SC_BIN SC_BIN_US SC_BIN_SM SC_OCT SC_OCT_US SC_OCT_SM SC_HEX SC_HEX_US SC_HEX_SM SC_CSD

Available formats SC_F Fixed Notation (is the default) SC_E Scientific Notation

sc_fixed<10,5> my_var;cout << my_var.to_string(SC_CSD);

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Type Casting Done during initialization and/or assignment (if

required) Type casting operations

1. Quantization2. Overflow handling(sign-extension and/or zero filling is done wherever

necessary) Can be turned ON or OFF, using

Current Context Parameter during fix-point data declaration

SC_ON, SC_OFFsc_ufixed<16,16> d(SC_OFF);

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Type Casting (cont’d) Turning Casting off will turn off fixed-

point handling of the operand It will be treated as a large float Bit-accurate behavior of the operand will not

be available

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Arrays of Fixed-Point Data Static fixed-point data types

sc_fixed<10,5> a[8]; Only CAST Switch is determined by current

Context The same for all Static fixed-point types

Dynamic fixed-point data types sc_fix a[8]; Default constructor is called. Default values are taken from current

context

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Example:17 Tap FIR Filter

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sc_fixed<32,3,SC_RND, SC_WRAP> fir_fx(sc_fixed<4,2,SC_RND, SC_WRAP> Input)

{const int NumberOfCoefficients = 17;

static sc_fixed<4,2,SC_RND, SC_WRAP> state[16];static sc_fixed<32,0,SC_RND, SC_WRAP> coeff[17] = {

1.05162989348173e-02, …};

sc_fixed< 4,2,SC_RND, SC_WRAP> * pstate;

sc_fixed<32,0,SC_RND, SC_WRAP> * pcoeff;

sc_fixed<32,3,SC_RND, SC_WRAP> sum;

int i;

/* FIR filter output */

pcoeff = &coeff[0]; pstate = &state[0];

sum = ((*pcoeff++ ) * (Input));

for (i = 0; i<16; i++)

sum = (sum + ((*pcoeff++ ) * (*pstate++ )));

/* shift state */

pstate = &state[15]; pcoeff = (pstate - 1);

for (i = 0; i < 15; i++)

*pstate-- = *pcoeff-- ;

*pstate = Input;

return(Sum);

}

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What we learned today Fix-point data types

Faster versions Dynamic versions Default behaviors Operators Arrays Other complementary notes

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Complementary Notes:Assignments Today is due date for all LATE

assignments. From now on, 5% penalty is re-

enabled