Special function registers are in the upper area of addressable memory, from address 0x80 to 0xFF. This area of memory cannot be used for data or program storage, but is instead a series of memory-mapped ports and registers. All port input and output can therefore be performed by memory move operations on specified addresses in the SFR region. Also, different status registers are mapped into the SFR, for use in checking the status of the 8051, and changing some operational parameters of the 8051. Some SFR bits may be set directly using SETB/LDB instructions on the SFR's address, whereas others may require usage of specific instructions. The
Intel 80196 class microcontroller has 24 SFRs, each 1 byte in size; standard
Intel 8051 chips have 21 SFRs. ==External links==