General Purpose Interfacing Device

https://www.tutorialspoint.com/microprocessor/microprocessor_intel_8255a_programmable_peripheral_interface.htm

https://www.tutorialspoint.com/microprocessor/microprocessor_8257_dma_controller.htm

http://www.slideshare.net/coolsdhanesh/8251-8251-usart

https://www.tutorialspoint.com/microprocessor/microprocessor_intel_8253_programmable_interval_timer.htm

https://www.tutorialspoint.com/microprocessor/microprocessor_quick_guide.htm

http://www.slideshare.net/VijayKumar486/8259-programmable-interrupt-controller-by-vijay-33632077