Question

Towards the end of the chapter (p. 160-161), examples of deriving control signals are given. Table 5-6 lists out the "actions" to fetch/decode/execute each CPU instruction.

List out the control signals given when a CPU instruction is being executed. For example, when each of the instructions X, Y, and Z is being executed, the control unit gives a set of signals at different time T:
X: at T4: A0 A1 C0
at T5, A1 B0
(function/inst X and at T4: signals A0 A1 C0 are given
function/inst X and at T5, signals A1 B0 are given)

Y: at T3, signals B1 C0
at T4, signals A1 B1 C1

Z: at T4: signals A0 C1
at T5, signals A1 B0 C0

Then, each signal is given when:
A0 = X T4 + Z T4 = (X+Z) T4
A1 = X T4 + X T5 + Y T4 + Z T5 = (X+Y) T4 + (X+Z) T5
B0 = X T5 + Z T5 = (X+Z) T5
B1 = Y T3 + Y T4 = Y (T3+T4)
C0 = X T4 + Y T3 + Z T5
C1 = Y T4 + Z T4 (Y+Z) T4

Q1. For the Fetch, Decode, and Indirect steps (T0 through T3) list out signals like the above. First list out signals used in Fetch, Decode, then Indirect. Then for each signal, list its timing definition.
(There should be 8 different signals appeared 12 times.)
(10 pts)

Q2. For signals used to execute 7 different memory-reference
instructions, sort them out the same way as above. First
by instruction name, then by signal name.
Order the listing alphabetically. (10 pts)

Q3. What is interrupt-handling and why is it useful for operation of I/O devices? (2 pts)

Answer 1

Solution:

1. To fetch and decode an instruction.

• Listing out the signals first

• Then for every signal, we list the timing definition and ordering the list alphabetically

2.  To execute 7 memory reference instructions and sort them by signal name

• To execute 7 memory reference instructions and sort list alphabetically

3. Interrupt handling

1. An Interrupt handling is also termed as an interrupt service routine (ISR),

2. It is a special code piece associated with an explicit interrupt condition.

3. Interrupt handlers (IH) are initialized by software interrupts, software exceptions, or hardware interrupts. 4. They are employed to implement transitions between the protected modes of operations like system calls

or to implement device drivers.

Use of interrupt handling

1. Give better performance when there is a busy polling.

2. Especially used in the Input/output of the computer organization,

3. Interrupt service routines can channel the operating system security when the critical privileged kernel

code executes only through the interrupts.

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