Week 8

Question 1

Draw out truth tables for And, Or or Xor

Answer 1

See 8.1 at 5 min

Question 2

What is a bitwise boolean operation?

Answer 2

A bitwise boolean operation is applied separately to every bit of a binary codeword.

Question 3

What do shift left (shiftl) and shift right (shiftr) do?

Answer 3

If we have Ra, Rb, Rc: shiftl shifts Rb left by Rc places and stores in Ra. We add zeroes to the end of Ra for the spaces that we’ve shifted and on the other side, the bits are shifted out and dropped off.

Similarly for shiftr, except for the other side.

Question 4

What kind of number will MUL, DIV, CMPGT, CMPLT work with?

Answer 4

Two’s complement

Question 5

What kind of number will ADD, SUB work with?

Answer 5

Unsigned or two’s complement

Question 6

What kind of number will CMPEQ work with?

Answer 6

Any codewords, including non-numeric ones

Question 7

What happens in R15 when we DIV?

Answer 7

The remainder will go into R15. It will override what is already in R15.

Question 8

What is an instruction cycle?

Answer 8

An instruction cycle is what it takes to completely process an instruction (fetch it, then execute it!)

Question 9

What is the memory cycle?

Answer 9

The time involved in reading or writing a single word from/to a single memory location. Memory cycle will take a fixed amount of time.

Question 10

How many memory cycles does an RRR instruction use?

Answer 10

1 (to fetch the operation word)

Question 11

How many memory cycles does a JUMP take?

Answer 11

To fetch it, it has two words of machine code.

So two memory cycles

Question 12

What does the control unit maintain?

Answer 12

Information about the current and next instruction.

Question 13

What two registers will you find always in a CPU?

Answer 13

1) instruction register: contains the operation word of the instruction being executed
2) Program counter: contains the address of the next instruction to be executed.

Question 14

What are the two machine specific control registers in Sigma 16?

Answer 14

1) The Address Register: contains the effective address specified in an RX or X instruction. It is used to hold initially any base address that’s contained in an indexing mode.
2) The Data Register holds temporary data.

Question 15

Does the number of memory cycles required to fetch the whole instruction affect the time?

Answer 15

Yes, the more memory cycles, the slower the instruction takes to execute/run.
Memory cycles are much slower than internal operations.

Question 16

What questions do we need to ask when examining the machine code instructions on a new CPU?

Answer 16

1) how many memory cycles are needed to fetch the whole instruction?
2) How many cycles are needed to fetch or store data?
3) How many independent memory locations can the instruction access?

Question 17

Why can’t we have multiple types of instructions in a CPU?

Answer 17

Many complex instructions mean a long operation word and a complex control unit.
A complex control unit with many instruction types is slower than a simple one with few types

Question 18

What is RISC?

Answer 18

A reduced instruction set computer.

Question 19

What is CISC?

Answer 19

Conventional machines with complex instructions.

Question 20

What is a virtual machine?

Answer 20

it’s a software model of a machine that may or may not exist as hardware.

Question 21

What is a hypervisor used for?

Answer 21

It’s a software that is often run on the host to create and manage new guests that can be complete copies of real machines.

Question 22

Why are virtual machines useful?

Answer 22

They allow a certain amount of protection. It allows us to do certain experiments that might be dangerous to do on our own comp.
Confine sensitive activities.

Question 23

What happens if a VM crashes?

Answer 23

The host will usually be fine and so will the hypervisor.

Question 24

What is a subroutine?

Answer 24

The low level structure (assembly level structure) that support HLL functions. Rather than repeating instructions in memory, we can split the repeated instruction into a subroutine and say JUMP when the subroutine should be called and then once this is run, it returns back to the instruction.

Question 25

What is the return address?

Answer 25

After the subroutine is called, we must return to the return address.

Question 26

How do we know where to jump back to after running a subroutine?

Answer 26

Use Jump and Link (JAL). JAL stores the return address in Rf, if we have:
JAL Rf, work[R0]

Question 27

Can you have subroutines inside subroutines?

Answer 27

Yes, these are called nested subroutine calls

Question 28

Where should we store many nested subroutine calls?

Answer 28

In memory

Question 29

What is a stack?

Answer 29

Stack is setup and maintained by a programmer.
Unlike an array, a stack can grow and shrink (dynamic).
Occupies consecutive memory locations.
The first address is the stack bottom, the last one is the stack top (grows upwards in memory).

Question 30

Where are the address of the stack bottom and stack top originally set to?

Answer 30

The bottom address. At this point the stack is empty. After a subroutine call, store return address on stack and increment the stack pointer (usually points to one above the stack top)

Question 31

what kind of data structure is a stack (fifo, lifo)?

Answer 31

LIFO (last in first out)

Question 32

What is storing an item at the stack top called?

Answer 32

a PUSH operation

Question 33

What is retrieving an item from the stack top called?

Answer 33

POP or PULL operation. The item popped is always the last one pushed

Question 34

How does the stack work in Sigma 16?

Answer 34

• it’s programmer maintained.
• special push and pop instructions
• the stack grows upwards in Sigma 16, but in many CPUs it grows downwards.

Question 35

What is a stack overflow?

Answer 35

When the stack size reaches its maximum