Elaborate the usage of following registers in 8259 Interrupt Request Register (IRR) In-Service Register (ISR) Priority Resolver

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Sai Chetan

Asked: April 3, 20222022-04-03T22:23:00+05:30 2022-04-03T22:23:00+05:30In: Microprocessor

Q1) Elaborate the usage of following registers in 8259

Q2)

Write down the mode 0 control words of 8255 for the following two cases:

(a) Port A = Input port, Port B = not used, Port CU = Input port and Port CL = Output port.

(b) Port A = Output port, Port B = Input port, Port C = Output port

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I'M ADMIN · Accepted Answer · 2022-04-04T01:02:40+05:30

Q1) Elaborate the usage of following registers in 8259

Interrupt Request Register (IRR):The IRR is used to store all the interrupt levels which are requesting service.

In-Service Register (ISR): ISR is used to store all the interrupt levels which are being serviced.

Priority Resolver: It examines all the three registers and set the priority of interrupts and according to the priority of the interrupts, interrupt with highest priority is set in ISR register. Also, it reset the interrupt level which is already been serviced in IRR.

Interrupt Mask Register (IMR): The IMR stores the bits which mask the interrupt lines to be masked. It stores the interrupt level which have to be masked by storing the masking bits of the interrupt level.

Control Logic Section: The function of this block is to accept OUTput commands from the CPU. It contains the Initialization Command Word (ICW) registers and Operation Command Word (OCW) registers which store the various control formats for device operation. This function block also allows the status of the 8259A to be transferred onto the Data Bus.

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