Typically, at least in your case, the microcontroller, AT89S52, is the master and the peripheral device, ADC0831, is the slave.
As both MICROWIRE and SPI allow for more than one peripheral device to be connected to the data input and output line at one time, the Slave Select (SS) or Chip Select (CS) line is simply a GPIO line which selects the particular peripheral device with which to communicate.
Serial Protocols Compared
Introduction to Serial Peripheral Interface
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