Section 3: Software, Synchro and Device Drivers

Question 1

What are the three abstraction layers of software?

Answer 1

From most abstract to least abstract: Main Program, Functional Layer, Hardware Abstraction Layer.

Question 2

What is the purpose of the main program

Answer 2

Completely abstract code. May make use of functional layer APIs

Question 3

What is the purpose of the functional layer?

Answer 3

The functional layer maps api actions into hardware calls

Question 4

What is the purpoe of the hardware abstraction layer

Answer 4

The hardware abstraction layer comminucates with IO devices via ports, handles synchronization using polling and other low level operations

Question 5

What are CPU and device Latency?

Answer 5

CPU latency: The time between the reciept of a service request and the beginning of processing. Device Latency: The amount of time between a request being sent and a response being recieved. CPU Latency < Device Latency

Question 6

What is a real time system?

Answer 6

A system that garuntees worst case latency

Question 7

What is Latency

Answer 7

The time between arrival of request and completion of service. Could consider avg or worst case

Question 8

What is throughput

Answer 8

Measure of #of items processed per unit time

Question 9

What are the five types of sychronization mechanisms

Answer 9

Blind Polling, Ocassional Polling, Periodic Polling, Tight Polling, Interrupt Handling

Question 10

What is Blind Cycle Syncrho

Answer 10

Grab data at software’s convenience regardless of device status

Question 11

What is Ocassional Polling

Answer 11

Device status is checked at a variable frequency (at designer’s convenience)

Question 12

What is periodic polling

Answer 12

Device status is checked at a set frequency

Question 13

What is Tight Polling

Answer 13

The software does no other meaningful work other than checking if the device is ready (empty loop)

Question 14

What is Interrupt Handling

Answer 14

Device requests service from cpu by sending an interrupt signal

Question 15

What is CPU oriented sychronization? Which mechanisms follow this?

Answer 15

Device waits for cpu (blind cycle, ocassional polling, periodic polling)

Question 16

What is Device oriented synchronization

Answer 16

Device demands service from cpu (reduce latency). Examples: tight polling and interrupt handling

Question 17

What are the steps to do a read via tight polling

Answer 17

wait for ready (spin loop)
read data
clear data ready bit
process data

Question 18

What are the steps to do a write via tight polling

Answer 18

wait for device ready
clear ready signal
output data

Question 19

What are the 11 steps of handling an interrupt

Answer 19

1. Device notifies CPU
2. CPU completes current instruction
3. CPU suspends execution
4. Disable interrupts
5. Save registers
6. Acknowledge device (optional)
7. Select ISR
8. Run ISR
9. Restore registers
10. Re-enable interrupts
11. Continue Main program execution

Question 20

What are the pros and cons of having a single vs multiple interrupt line

Answer 20

Pros: Less wires
Cons: Slower

Question 21

What are the two methods to selecting

Answer 21

Non Vectored: All interrupt requests are routed to a genral isr that polls each device to check which device fired the interrupt. The appropriate isr is then selected.
Vectored: Each request is associated with a vector. Vectors have priorities and associated ISRs at that vector address

Question 22

What are two rules for ISR programming

Answer 22

Execute as fast as possible, no blocking IO.

Question 23

What are 7 general steps for an ISR

Answer 23

1. Save registers modified by the ISR
2. Acknowledge Device
3. Re enable interrupts
4. Test for valid interrupt
5. Run logic
   6 Resture registers (may need to disable interrupts)
6. return

Question 24

How are interrupts initialized

Answer 24

1. Disable all interrupts
2. Enable Device Interrupts
3. Set interrupt mask
4. Initialize interrupt vectors with ISR adresses
5. Enable interrupts

Question 25

What is a device driver

Answer 25

A driver is the software associated with a particular device. It includes data structures, initialization IO and ISRs