Shift Arithmetic - Shift bits or binary point of

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Shift bits or binary point of signal

Libraries:
Simulink / Logic and Bit Operations
HDL Coder / Logic and Bit Operations

Description

The Shift Arithmetic block can shift the bits or the binary point of an input signal, or both.

For example, shifting the binary point on an input of data type sfix(8) by two places to the right and left gives these decimal values.

This block performs arithmetic bit shifts on signed numbers. Therefore, the block recycles the most significant bit for each bit shift. Shifting the bits on an input of data typesfix(8) by two places to the right and left gives these decimal values.

Examples

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This example shows the effect of binary point shifts.

model='ex_shift_arithmetic_block_binary_point_shifts'; open_system(model)

This example shows a comparison of the behavior of right bit shifts using the dialog box versus the block input port.

model='ex_shift_arithmetic_block_right_bit_shifts'; open_system(model)

Ports

Input

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Number to be operated on, specified as a scalar, vector, or array.

Data Types: single | double | half | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | fixed point

Number of bits to shift, specified as a scalar.

Data Types: single | double | half | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | fixed point

Output

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Result of operation, returned as a scalar, vector, or array.

Data Types: single | double | half | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | fixed point

Parameters

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Specify whether to enter the bits to shift on the dialog box or to inherit the values from an input port.

Tunable: No

Programmatic Use

Specify the direction in which to shift bits: left, right, or bidirectional.

Tunable: No

Programmatic Use

Specify a scalar, vector, or array of bit shift values.

Tunable: No

Dependencies

To enable this parameter, set Bits to shift: Source to Dialog.

Programmatic Use

Specify an integer number of places to shift the binary point of the input signal. A positive value indicates a right shift, while a negative value indicates a left shift.

Tunable: No

Programmatic Use

Specify whether to produce a warning or error during simulation when the block contains an out-of-range shift value. Options include:

• None — The software takes no action.
• Warning — The software displays a warning and continues the simulation.
• Error — The software terminates the simulation and displays an error.

For more information, see Simulation and Accelerator Mode Results for Out-of-Range Bit Shift Values.

Programmatic Use

Select this check box to include conditional statements in the generated code that protect against out-of-range bit shift values. This check box is available when Bits to shift: Source isInput port.

For more information, see Code Generation for Out-of-Range Bit Shift Values.

Programmatic Use

Block Characteristics

More About

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The Shift Arithmetic block icon shows the block behavior using these variables:

• Q — Stored integer value
• V — Real world value
• E — Exponent
• Qy — Stored integer value of output
• Qu — Stored integer value of input
• Vy — Value of output
• Vu — Value of input
• Ey — Exponent of output
• Eu — Exponent of input

The Behavior of Right Bit Shifts example compares the behavior of right bit shifts using the dialog box versus the block input port.

The key block parameter settings of the Constant blocks are:

The key block parameter settings of the Shift Arithmetic blocks are:

The top Shift Arithmetic block takes an input of 124, which corresponds to 01111100 in binary format. Shifting the number of bits two places to the right produces 00011111 in binary format. Therefore, the block outputs 31.

The bottom Shift Arithmetic block performs the same operation as the top block. However, the bottom block receives the bit shift value through an input port instead of the dialog box. By supplying this value as an input signal, you can change the number of bits to shift during simulation.

The Effect of Binary Point Shifts example shows the effect of binary point shifts.

The key block parameter settings of the Constant blocks are:

The key block parameter settings of the Shift Arithmetic blocks are:

The top Shift Arithmetic block takes an input of 124, which corresponds to 01111100 in binary format. Shifting the binary point three places to the right produces 01111100000 in binary format. Therefore, the top block outputs 995.

The bottom Shift Arithmetic block also takes an input of 124. Shifting the binary point three places to the left produces 01111.100 in binary format. Therefore, the bottom block outputs 15.5.

Algorithms

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Suppose that WL is the input word length. The shaded regions in the following diagram show out-of-range bit shift values for left and right shifts.

Similarly, the shaded regions in the following diagram show out-of-range bit shift values for bidirectional shifts.

The diagnostic for out-of-range bit shifts responds as follows, depending on the mode of operation:

Suppose that U is the input, WL is the input word length, and Y is the output. The output for an out-of-range bit shift value for left shifts is as follows:

Similarly, the output for an out-of-range bit shift value for right shifts is as follows:

For bidirectional shifts, the output for an out-of-range bit shift value is as follows:

For generated code, the method for handling out-of-range bit shifts depends on the setting of Check for out-of-range 'Bits to shift' in generated code.

For right shifts on signed negative inputs, most C compilers use an arithmetic shift instead of a logical shift. Generated code for the Shift Arithmetic block depends on this compiler behavior.

Extended Capabilities

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HDL Coder™ provides additional configuration options that affect HDL implementation and synthesized logic.

You can generate HDL code when Bits to shift: Source isDialog or Input port.

HDL Architecture

The generated VHDL code uses the shift_right function andsll operator.

The generated Verilog code uses the >>> and<<< shift operators.

HDL Block Properties

Complex Data Support

This block supports code generation for complex signals.

Restrictions

When Bits to shift: Source is Input port, binary point shifting is not supported.

Version History

Introduced before R2006a