Study Guide #4

Question 1

What are the three types of instructions in our theoretical computer?

Answer 1

Memory reference, register reference, and I/O

Question 2

opcode 000-110

Answer 2

memory reference

Question 3

opcode 111, I = 0

Answer 3

register reference

Question 4

opcode 111, I = 1

Answer 4

I/O

Question 5

PC

Answer 5

program counter

Question 6

AR

Answer 6

address register

Question 7

IR

Answer 7

instruction register

Question 8

TR

Answer 8

temporary register

Question 9

OUTR

Answer 9

output register

Question 10

INPR

Answer 10

input register

Question 11

DR

Answer 11

data register

Question 12

AC

Answer 12

accumulator

Question 13

Direct addressing

Answer 13

I = 0; address field contains effective address (1 memory read)

Question 14

Indirect addressing

Answer 14

I = 1; address field contains address of effective address (2 memory reads)

Question 15

Instructions for branching to a subroutine

Answer 15

BSA: Branch and save return address

Question 16

Three machine instructions that are used for program control

Answer 16

BSA, BUN, ISZ

Question 17

Are recursive subroutines supported on our theoretical computer?

Answer 17

No because the return address would be overwritten every time

Question 18

What happens when an interrupt cycle occurs

Answer 18

PC set to 0
return address is stored in memory address zero
BUN goes to I/O that handles interrupt
* can occur at any instruction

Question 19

How does the computer return from the interrupt routine?

Answer 19

BUN (branch unconditional) to blank word before subroutine
ION: turns interrupt flag back on

Question 20

Can a second interrupt occur during the interrupt routine?

Answer 20

No the first one has to finish

Question 21

Can a subroutine be interrupted?

Answer 21

Yes as long as you are not in the I/O program

Question 22

What is the purpose of the timing signals and how are they generated?

Answer 22

Allow sequencing of events
generated by the control unit (4-bit counter into decoder)

Question 23

Describe all flags associated with the I/O process

Answer 23

Registers: OUTR and INPR
Flags: FGI and FGO

Question 24

What two flags are associated with I/O and explain what they control

Answer 24

FGI and FGO control input and output

Question 25

Program-controlled I/O

Answer 25

loops continuously until FGI or FGO = 1
LESS EFFICIENT

Question 26

Interrupt-driven I/O

Answer 26

allows CPU to do other things, then interrupts it when FGI or FGO = 1

Question 27

How do we create a loop in the machine language in the theoretical computer?

Answer 27

ISZ (increment and skip if zero)

Question 28

Describe the two decoders in the theoretical computer

Answer 28

Both in control unit
4x16: timing pulses
3x8: chooses opcode

Question 29

Name and describe two ways that the control signals can be generated in a computer. Which one does our theoretical computer use?

Answer 29

Hardware: actually build the circuit (theoretical computer)
microprogram control: store 0s and 1s for control signals in memory (modern computers)

Question 30

What takes place during timing pulse T3 on our theoretical computer?

Answer 30

Indirect addressing: indirect cycle
otherwise skip

Question 31

T/F: execution always begins at timing pulse T4

Answer 31

True

Question 32

Give similarities and differences between a subroutine call and the interrupt cycle

Answer 32

• Subroutine called through BSA and is done as an instruction in the program
• Interrupts can occur anywhere since its hardware driven

Question 33

What two instructions are required at the end of an interrupt routine and explain

Answer 33

ION: turn interrupt flag back on to allows interrupts again
BUN: go back to the saves return address at address 0

Question 34

What are the inputs to the control unit of our basic computer?

Answer 34

Increment, Clear, Clock

Question 35

What are the two flags that are associated with the interrupt cycle? what is the purpose of each

Answer 35

FGI: input flag (1: keyboard -> INPR, 0: INPR -> AC)
FGO: output flag (1: AC -> OUTR, 0: OUTR -> screen/printer)

Question 36

What was the purpose of Lab 8

Answer 36

to make a circuit that connect to all 8 LEDs and turns one on at a time from left to right

Question 37

What chips were used in Lab 8

Answer 37

Personally: AND, JK Flip-flips, Decoder, Inverters