Chapter 4: Processor Fundamentals

Question 1

How are interrupts handled

Answer 1

 An interrupt flag is raised in the (interrupt) register

 At the end of the current FE cycle // at the start of the next FE cycle

 The system checks the interrupt register for higher priority interrupts
than current process

 If true, it stores the current contents of the registers on the stack

 The appropriate interrupt service routine (ISR) for the key press is
called

 The input data from the keyboard is processed

 The contents of the registers are restored from the stack

 … and control is passed back to previous process

Question 2

What are the special registers

Answer 2

CIR

IX

MAR

MDR

PC

Sr

Question 3

What does the CIR do

Answer 3

Stores the current instruction being decoded and executed

Question 4

What does the MAR do

Answer 4

Stores the address of the memory location being read from or written to

Question 5

What does the PC do

Answer 5

Stores the address where the next instruction to be read can be found

Question 6

What does the MDR do

Answer 6

Stores data that has just been read from memory or which is about to be written to memory

Question 7

What is the FE Cycle

Answer 7

MAR <— [PC]
PC <— [PC] +1
MDR <—[[MAR]]
CIR <—[MDR]