There are two basic modes of operation of 8255, They are:
1. I/O mode. 2. BSR mode.
In I/O mode, the 8255 ports work as programmable I/O ports, while In BSR mode only port C (PC0-PC7) can be used to set or reset its individual port bits. Under the IO mode of operation, further there are three modes of operation of 8255, So as to support different types of applications, viz. mode 0, mode 1 and mode 2.
1) Mode 0 - Basic I/O mode
2) Mode 1 - Strobed I/O mode
3) Mode 2 - Strobed bi-directional I/O