The Australian National University Mid Semester Examination – April 2019
Comp2300 & Comp6300 Computer Organisation & Program Execution
Study period: Time allowed: Total marks: Permitted materials:
15 minutes
1.5 hours (after study period) 50
None
Questions are not equally weighted – sizes of answer boxes do not nec- essarily relate to the number of marks given for this question.
All your answers must be written in the boxes provided in this booklet. You will be provided with scrap paper for working, but only those answers written in this booklet will be marked. Do not remove this booklet from the examination room. There is additional space at the end of the booklet in case the boxes provided are insufficient. Label any answer you write at the end of the booklet with the number of the question it refers to.
Greater marks will be awarded for answers that are simple, short and concrete than for answers of a sketchy and rambling nature. Marks will be lost for giving information that is irrelevant to a question.
Student number:
The following are for use by the examiners
Total mark
Q1 mark
Q2 mark
Q3 mark
1. [16 marks] Digital Logic / Basic CPU architecture
(a) [8 marks] The following circuit diagrams show a correctly working, 3-bit ripple-carry adder.
(i) [3 marks] Use this adder to calculate 2+3 by labelling all inputs and outputs cor- rectly with ‘1’ or ‘0’.
A0 B0 S0 A1 B1 S1 A2 B2 S2
C0 C1 C2
XOR AND
XOR XOR
XOR XOR
AND
AND
OR
AND
AND
OR
(ii) [1 mark] What is the meaning of C2 in the circuit diagram above and what does it say about your calculation 2+3?
(iii) [3 marks] Use the following adder to calculate 2-3 by labelling all inputs and out- puts correctly with ‘1’ or ‘0’.
A0 B0 S0 A1 B1 S1 A2 B2 S2
C0 C1 C2
XOR AND
XOR XOR
XOR XOR
AND
AND
OR
AND
AND
OR
(iv) [1 mark] What is the meaning of C2 in the circuit diagram above and what does it say about your calculation 2-3?
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(b) [3 marks] The function q^a, b, ch is defined by the following truth-table. Use minterms or maxterms to determine and simplify the output q.
a
b
c
Output q
min- or max terms
Simplified min- or maxterms
F
F
F
F
F
F
T
F
F
T
F
F
F
T
T
F
T
F
F
T
T
F
T
T
T
T
F
T
T
T
T
T
(c) [3 marks] In the following circuit diagram all four inputs A, B, C, D are set to ‘1’.
‘1’ A ‘1’ B
‘1’ C ‘1’ D
You need to change all four inputs to ‘0’ (“power down the circuit”). In which order do you need to change those inputs to ‘0’ (“cut the wires”), such that Q never changes to ‘1’ (“bomb goes off”)? If there are multiple sequences, describe them all.You cannot change the value of multiple inputs at the same time.
XOR
AND Q OR
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(d) [2 marks] Which if the following statements are correct? Tick all correct statements – marks will be subtracted for wrongly ticked statements, so do not just tick all of them.
☐ If you add two whole numbers with machine instructions, the CPU knows whether they are natural numbers (unsigned numbers) or integers (signed num- bers).
☐ If you add two whole numbers with machine instructions, you need to tell the CPU whether they are natural numbers (unsigned numbers) or integers (signed numbers).
☐ If you add two whole numbers with machine instructions, the CPU does not care whether they are natural numbers (unsigned numbers) or integers (signed num- bers) and will provide the result for either case.
☐ If you remove subtraction from your machine instruction set, you can theoretically still express the same number of algorithms.
☐ If you remove conditional branching from your machine instruction set, you can theoretically still express the same number of algorithms.
☐ If you remove function calls from your machine instruction set, you can theoreti- cally still express the same number of algorithms.
☐ The next CPU instruction is held in the PC register.
☐ The address of the next CPU instruction is held in the PC register.
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2. [14 marks] Machine instructions
(a) [4 marks] Consider the following ARM assembly code:
mov r0, 0xFFFF lsl r1, r0, #16 add r1, r0 addsr1, 0x01
What will be the values of the following status flags after those instructions have been executed?
(b) [6 marks] Compile the following code snippet:
a = 0;
while (a < 100) {
a = a + 5; }
into ARM assembly code and keep the value of a in r0.You are not losing marks for minor syntax errors.
Negative (N) = Zero (Z) = Carry (C) = Overflow (V) =
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(c) [4 marks] Consider the following ARM code:
ldr r0, =array ldr r4, =limiter
loop:
ldr r1, [r0], #4 cmp r0, r4
beq done
ldr r2, [r0]
cmp r1, r2 bls loop
mov r0, #0 b end
done:
mov r0, #1 end:
b end array:
; r0 is incremented by #4 after load
; eq stands for “equal”
; ls stands for “lower or same”
.word 0, 2, 4, 6, 8, 19, 21, 21, 27, 28, 30 limiter:
.word 0
(i) [1 mark] What do you expect the value of r0 to be after this program reached end?
(ii) [3 marks] In a single English sentence: what does the value of r0 represent in the end?
r0 =
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3. [20 marks] Functions
(a) [5 marks] Draw a diagram depicting a complete stack frame with all possible items which you might find there. If it makes your diagram more comprehensible, draw a few stack frames around it as well.
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(b) [5 marks]Your local variables inside a function look like this (in two different syntacti- cal styles for you, expressing in both cases that you have a single Boolean and 10 word- sized Numbers):
C-style:
typedef enum {false, true} bool;
bool flag;
unsigned int field[10];
Algol-style:
Flag : Boolean;
Field : array [1..10] of Natural;
(i) [2 marks] Write ARM assembly code which allocates memory space for those vari- ables and explain your answer. You are not losing marks for minor syntax errors.
(ii) [3 marks] When does this allocation code need to be executed and how and when do you de-allocate those local variables again, once they are no longer needed? Give precise answers.
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(c) [5 marks] Write an ARM assembly function which returns the sum of two numbers (both are parameters of your function).You are not losing marks for minor syntax er- rors.
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(d) [5 marks] Write an ARM assembly function which returns the sum of an array of num- bers (which is a parameter of your function). If you need to make assumptions about the structure or definition of your array, then explain those assumptions. You are not losing marks for minor syntax errors.
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continuation of answer to question part
continuation of answer to question part
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continuation of answer to question part
continuation of answer to question part
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