2017 AQA A Level Computer Science Paper 2 Unofficial Markscheme

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Tuffyandtab
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Tell me the answers and I will edit this markscheme. Also suggest some grade boundary predictions.
The actual unofficial markscheme
Spoiler:
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1.1 Fetch stage -
Program counter passes info to MAR. Using address bus to to relay address in main memory. Main memory returns data and passes to MBR via data bus, At the same time the program counter increments by one.

1.2 Why not MBR - if the instruction was decoded in MBR then if there is a direct address operand, the MAR would get the address of the datum and bring it back into the MBR which would overwrite the contents of the MBR and hence the current instruction.

1.3 Harvard - Harvard, parallel aquasition of data and instruction as memories are independent. Harvard memories can also be different IE: instructions are read only and data is read/write. Simultaneous access to data and instruction memories.

2.1 KAITLEN
2.2 Vernam - I said only use key once and make sure key is not just the same letter repeated more than once
2.3 Symmetric (same key) asymmetric (different keys)

3.1 No. of bits - 3
3.2 1500 bps
3.3 Which line - B
3.4 Serial communication - signal doesn't degrade over long distances, cheaper to manufacture since it only requires one wire.

4.1 AND with A and B and OR C and D followed by ANDing these 2 results
4.2 (A.B).(C+D)
4.3 Boolean algebra - B + C?
4.4 D-type Flip-flop - stores current state of input
4.5 Clock signal and this is when the flip-flop outputs its current state

5.1 Direct addressing (operand is memory location) immediate (operand is datum)
5.2 10 40 50 and 1 were the numbers, don't remember what specific registers they went into (and memory locations)
5.3 -
Using low level language is appropriate for:
  • Allows for a single codebase to be used across multiple architectures (more portable).

6.1 10
6.2 [1,9,25], [1,5], 18
6.3 Higher-order function - Takes a function as an argument and/or returns a function as a result

7 - consequences, wifi - WPA/WPA2, SSID broadcast disabled, white list - legal issues e.g. data protection act.
Camera - (how it works in capturing data)
Machine learning algorithms to blur out faces and car registration
Don't transmit data till the car is back at a company hub.
I said introduce Code of Conduct, more thorough testing of systems.

8.1 1200 kilobytes
8.2 Nyquists theorem, should have been >= 29000Hz
8.3 MIDI
MIDI is an alternative method for storing and communicating sound digitally that does not use sound waves; instead, information about each musical note is stored. This information includes:
  • Notation and duration; note-on and note-off.
  • Instrument.
  • Velocity.
  • Volume.
  • Timbre.
  • Pedal effects.
  • MIDI channel number.
  • Aftertouch.
  • Pitch bend.
    Advantages
  • Can easily synthesise musical notation from the file.
  • Can be played on different instruments.
  • Easy to transpose to a different key/pitch.
  • Produces relatively small files.
  • Easy to manipulate the data.
  • Allows for easy interface with electronic musical instruments.
  • No data is lost about each musical note.

9.1 something.something.something.x x != 0 or 255
9.2 Logical (conceptually how it works) and physical topologies (how it is actually laid out). A physical star topology can work logically as a bus topology.
9.3 peer to peer (equal status), client server (one or more nominated as servers, majority are clients that request services from the servers)

Databases
10.1 - Composite primary key with CarRegNo and Date
10.2 - Relations were to be in 3NF, no non-key dependencies, eliminate data redundancy and inconsistencies.
10.3 - ER diagram - Some possibilities: part to job is many to many; PartUsedForJob to part is many to one; car to job is one to many.
10.4 - UPDATE, SET, WHERE
10.5 -
INSERT INTO PartUsedForJob
VALUES ([jobID], [partID], 2)
10.6 -
SELECT [parameters in question]
FROM PartUsedForJob, Part
WHERE PartID = 106 ' number given in question; may not have been 106
AND Part.PartID = PartUsedForJob.PartID
ORDER BY PartID
10.7 - Nasty! any ideas?
new link table for the models and parts?

Floating Point
11.1 - C?
11.2 - B?
11.3 - -5
11.4 - exponent was 6 which is 0110
11.5 - 0.05
11.6 - 0.36%









Mark allocations:
Spoiler:
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1.1 - 4
1.2 - 2
1.3 - 2
2.1 - 1
2.2 - 2
2.3 - 1
3.1 - 1
3.2 - 1
3.3 - 1
3.4 - 2
4.1 - 3
4.2 - 2
4.3 - 4
4.4 - 1
4.5 - 2
5.1 - 2
5.2 - 4
5.3 - 1
5.4 - 4
6.1 - 1
6.2 - 3
7 - 12
8.1 - 2
8.2 - 2
8.3 - 4
9.1 - 2
9.2 - 2
9.3 - 4
10.1 - 1
10.2 - 2
10.3 - 2
10.4 - 3
10.5 - 2
10.6 - 5
10.7 - 3
11.1 - 1
11.2 - 1
11.3 - 2
11.4 - 3
11.5 - 1
11.6 - 1



Previous grade boundaries (make some predictions)
Spoiler:
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AS 2016 - 100/150 for an A

COMP3 Grade boundaries 2016:
Max.100 A*87 A78 B69 C60 D52 E44
And 2015:
A*87 A79 B71 C64 D57 E50
And 2014:
A*87 A79 B71 C63 D55 E47
June 2013:
A*83 A75 B67 C59 D51 E43
June 2010 (when the old spec was the new spec, so like ours):
A*79 A69 B59 C49 D40 E31
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MarshJam
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DISCLAIMER -> Idk if these answers are correct :P (in no particular order)

1) Program counter passes info to MAR. Using address bus to to relay address in main memory. Main memory returns data and passes to MBR via data bus, At the same time the program counter increments by one.

2) (hella unsure on this one) MBR can't split into opcode and operand. MBR is also used if more data needs to be fetched.

3) Harvard, paralell aquasition of data and instruction as memories are independent. Harvard memories can also be different IE: instructions are read only and data is read/write.

Vernam stuff:
Requirements -> One time pad needs to be randomly generated (ie from white noise)
-> One time pad needs to be of equal or greater length than the plain text.

Boolean stuff:
(A . B) . (C + D) (respective gates as well for the diagram)

Bigbad meme of boolLogic question: B + C

Assembly stuff:
10 40 50 and 1 were the numbers, don't remember what specific registers they went into (and memory locations)
What does it do? No idea, said something like compares logical shifted value in one memory location with another memory location, writes 1 if equal or 0 if unequal.

Long answer
Did anyone else put stuff like this:
Camera - (how it works in capturing data)
WiFi network security (hide ssid, use WPA2 over WPA, strong passwords, MAC address whitelisting)

(Social stuff)
People might not like faces being recorded - due to beliefs -> might take offence

(Legal)
Recording home residency registration for a living person might be in breach of the Data protection act

(Solve by)
Machine learning algorithms to blur out faces and car registration
Don't transmit data till the car is back at a company hub.

->Apparently I forgot a bit to do with data protection)


Anything like that??

MIDI stuff.
MIDI -> sound built up from instructions
MIDI can easily modify pitch or timbre.
Sampled sound is subject to discrepancies due to approximation under ADC where as MIDI is not.

HLL and assembly:
HLL -> Nearer english, easier to understand
-> something about it helping develop algorithms
Assembly -> Composition of pseudonyms to make development easier ... something bad rofl
-> Closer to machine code, more development freedom

Functional stuff:

What is a highlevel func - > rip me
Return from that headtailtail junk = 10
1,9,25
something else
then 18

Number stuff:
(You were given a calculator, god help you if you had a hard time with that #feelsbadman #noDisrespect)

Might be wrong in some places (or even all of it) let me know what you think.
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ShatnersBassoon
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Last question: Absolute error 0.05; relative 0.36%.

Sound: 3 bits per sample; 1500 bits per second; 1200 kilobytes?; lozenge B (straight line). How does MIDI work (event messages) and what are its advantages (smaller files; easy to manipulate)?

Boolean algebra: draw a circuit; A.B.(C+D); simplify the expression: B+C.
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username2796236
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How many marks was the ceaser cipher question ?
Where u decoded the encrypted message
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Tuffyandtab
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(Original post by sideshowbobya)
How many marks was the ceaser cipher question ?
Where u decoded the encrypted message
one mark I believe
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ShatnersBassoon
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(Original post by MarshJam)
Vernam stuff:
Requirements -> One time pad needs to be randomly generated (ie from white noise)
-> One time pad needs to be of equal or greater length than the plain text.
You could also say one time pad can only be used once (duh but still a valid point) and must be transmitted securely / known only to the intended recipient of the message.

For the SQL, something like:
[can't remember if PartUsedForJob is the right name for the relation but that's what I'm using]

INSERT INTO PartUsedForJob
VALUES ([jobID], [partID], 2)

and

SELECT [parameters in question]
FROM PartUsedForJob, Part
WHERE PartID = 106 ' number given in question; may not have been 106
AND Part.PartID = PartUsedForJob.PartID
ORDER BY PartID

Label on the diagram three relationships. Some possibilities: part to job is many to many; PartUsedForJob to part is many to one; car to job is one to many.
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tdbridger
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First functional programming question was head(tail(tail() of a list.
The three functional programming questions following were map, filter and fold the numbers in a list. (I guessed these as had not seen them before, presumably map is use the function square on all the elements, filter is filter according to condition (<10) and fold is add all the elements to create one element)
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MarshJam
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(Original post by ShatnersBassoon)
You could also say one time pad can only be used once (duh but still a valid point) and must be transmitted securely / known only to the intended recipient of the message.

For the SQL, something like:
[can't remember if PartUsedForJob is the right name for the relation but that's what I'm using]

INSERT INTO PartUsedForJob
VALUES ([jobID], [partID], 2)

and

SELECT [parameters in question]
FROM PartUsedForJob, Part
WHERE PartID = 106 ' number given in question; may not have been 106
AND Part.PartID = PartUsedForJob.PartID
ORDER BY PartID

Label on the diagram three relationships. Some possibilities: part to job is many to many; PartUsedForJob to part is many to one; car to job is one to many.
Agree with the selection statement, but I'm pretty sure it was an UPDATE questionIE:

UPDATE (Something)
SET (something)
WHERE (something)
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Tuffyandtab
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(Original post by MarshJam)
Agree with the selection statement, but I'm pretty sure it was an UPDATE questionIE:

UPDATE (Something)
SET (something)
WHERE (something)
One of them was UPDATE and one was INSERT INTO
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cookiemunch12
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(Original post by ShatnersBassoon)
You could also say one time pad can only be used once (duh but still a valid point) and must be transmitted securely / known only to the intended recipient of the message.

For the SQL, something like:
[can't remember if PartUsedForJob is the right name for the relation but that's what I'm using]

INSERT INTO PartUsedForJob
VALUES ([jobID], [partID], 2)

and

SELECT [parameters in question]
FROM PartUsedForJob, Part
WHERE PartID = 106 ' number given in question; may not have been 106
AND Part.PartID = PartUsedForJob.PartID
ORDER BY PartID

Label on the diagram three relationships. Some possibilities: part to job is many to many; PartUsedForJob to part is many to one; car to job is one to many.
Can we use UPDATE instead of insert?
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ShatnersBassoon
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(Original post by MarshJam)
What is a highlevel func - > rip me
Return from that headtailtail junk = 10
1,9,25
something else
then 18
The something else is [1, 5] from the filter <10 function.

EDIT:

(Original post by MarshJam)
Assembly stuff:
10 40 50 and 1 were the numbers, don't remember what specific registers they went into (and memory locations)
What does it do? No idea, said something like compares logical shifted value in one memory location with another memory location, writes 1 if equal or 0 if unequal.
I believe it performed a test to see if [value in 100] * 5 = [value in 101], storing the result in 102. The value stored in 102 was 1 because 100 contained 10 and 101 contained 50.
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ShatnersBassoon
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(Original post by cookiemunch12)
Can we use UPDATE instead of insert?
That was a separate question, but I can't remember any of the details.
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mahmzo
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(Original post by MarshJam)
Agree with the selection statement, but I'm pretty sure it was an UPDATE questionIE:

UPDATE (Something)
SET (something)
WHERE (something)
Yeah I thought so too want it update?

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MarshJam
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(Original post by mahmzo)
Yeah I thought so too want it update?

Posted from TSR Mobile
I believe so.

One was to update job duration

One was to update the parts used for the stock. Did he have to reduce the quantity of the total stock for that question???
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cookiemunch12
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(Original post by MarshJam)
I believe so.

One was to update job duration

One was to update the parts used for the stock. Did he have to reduce the quantity of the total stock for that question???
Thats what I did, pretty sure the part should be in the table then we - 2 for the quantity.
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ShatnersBassoon
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(Original post by MarshJam)
One was to update the parts used for the stock. Did he have to reduce the quantity of the total stock for that question???
This is where I used an INSERT INTO, to add a record into PartsUsedForJob (or whatever it was called). But that's a very good point about reducing the quantity in stock - I didn't think to do so, but that would have needed an UPDATE. I'm sure the question was worth about 3 marks though, but 2 different SQL commands should be worth more than that, surely?
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MarshJam
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(Original post by cookiemunch12)
Thats what I did, pretty sure the part should be in the table then we - 2 for the quantity.
Thing is I don't recall them even asking for that
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jacob_pro
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Did anyone get -625 for the floating point q11.3 ?

Edit:

Nope I know what went wrong now; i times by 10^3, instead of 2^3

It should be -5
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MarshJam
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(Original post by ShatnersBassoon)
This is where I used an INSERT INTO, to add a record into PartsUsedForJob (or whatever it was called). But that's a very good point about reducing the quantity in stock - I didn't think to do so, but that would have needed an UPDATE. I'm sure the question was worth about 3 marks though, but 2 different SQL commands should be worth more than that, surely?
I don't think they wanted us to do that. #BlessUP.


Anyone have any ideas about grade boundaries?
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Tuffyandtab
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(Original post by jacob_pro)
Did anyone get -625 for the floating point q11.3 ?
I can't remember, but I don't remember that.
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