Internals of Scan

This document describes the structure and execution of dynamic evaluation module,implemented as scanCol function.

Scan is a function to transfer data form external DDR/HBM port to an internal HLS stream. As we known, a stream based data interface can be easily processed in FPGA. So it provides a bridge to across from external memory to FPGA. There are 3 versions of scanCol in xf_database now:

Version1: defined in L1/include/hw/xf_database/scan_col.hpp.

In this head file, 6 kind of scanCol are provided to cope with 1-6 column number in DDR/HBM. Each column has its own DDR/HBM port as input, and the data is scanned into 1-6 stream as output. We also provide an example to scan one-column’s data into multiple-channel’s output stream, and this logic is very easy to be re-designed for other dedicated purposes.

Version2: defined in L1/include/hw/xf_database/scan_col_2.hpp.

Unlike version1, a data structure is designed in this version, and it provides row number of a column inside the memory header. So there is no need for Nrow in API of version2. It is suggested to use this version for cases in which the row number of a column is unknown or not decided before calling kernels.

Version3: defined in L1/include/hw/xf_database/scan_cmp_str_col.hpp.

This version provides an internal logic scan and compare string. The output is a boolean value to indicate whether the input string is equal to the constant string. It is more cost efficiency to process the boolean result than directly using the original string on FPGA.