Scalars - 2022.2 English

Vitis Unified Software Platform Documentation: Application Acceleration Development (UG1393)

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2022.2 English

Scalar arguments are used to pass control type information to the kernels. Scalar arguments cannot be read back from the host. For each argument that is specified, a corresponding register is created to facilitate passing the argument from software to hardware. See the following figure.

Figure 1. RTL Kernel Wizard Scalars Page
Number of scalar kernel input arguments
Specifies the number of scalar input arguments to pass to the kernel. For each number specified, a table row is generated that allows customization of the argument name and argument type. There is no required minimum number of scalars and the maximum allowed by the wizard is 64.

The following is the scalar input argument definition:

Argument name
The argument name is used in the generated Verilog control register module as an output signal. Each argument is assigned an ID value. This ID value is used to access the argument from the host software. The ID value assignments can be found on the summary page of this wizard. To ensure maximum compatibility, the argument name follows the same identifier rules as the kernel name.
Argument type
Specifies the data type, and hence bit-width, of the argument. This affects the register width in the generated RTL kernel module. The data types available are limited to the ones specified by the OpenCL C Specification Version 2.0 in "6.1.1 Built-in Scalar Data Types" section. The specification provides the associated bit-widths for each data type. The RTL wizard reserves 64 bits for all scalars in the register map regardless of their argument type. If the argument type is 32 bits or less, the RTL Wizard sets the upper 32 bits (of the 64 bits allocated) as a reserved address location. Data types that represent a bit width greater than 32 bits require two write operations to the control registers.