a flash chip or a sdram in this specific case would have the same purpose.
Really? What's the purpose of the flash chip / sdram in this specific case then?
The purpose is storing constants used in the computational tasks and save logic space for other uses.
Use of Block RAM (BRAM) for storage of constants [8]. Reconfigurable hardware devices
such as FPGAs often have on-board memories which can be pre-loaded.
Storing the Kt constants in these memories frees up space in the device
which can then be used to implement extra logic. The free space also leads to
improved routing and, thus, a general speed-up in circuit operation.
From "Optimisation of the SHA-2 Family of Hash Functions on FPGAs"
Robert P. McEvoy, Francis M. Crowe, Colin C. Murphy and William P. Marnane
Department of Electrical & Electronic Engineering,
University College Cork, Ireland
{robertmce, francisc, cmurphy, liam}@rennes.ucc.ie