Transcript Quiz 1 answers

1.
a)
b)
Given the importance of registers, what is the rate
of increase in the number of registers in a chip
over time?
Very fast: They increase as fast as Moore’s law
Very slow: They only increase as the instruction
set changes
Ans: (b)
• Moore’s law projects the doubling of transistors every
couple of years and has nothing to do with registers.
• The number of registers will change only if instruction set
architecture is changed. Because machine code has
only 5 bits to represent each register, so it would require
a substantial change (a new ISA)
2. Define the R-format instruction:
a)
b)
c)
d)
e)
What are the various fields?
What is the basic lay-out?
What does each field represent?
Are all of the bits used / needed for each field?
Given an example of an R-format instruction in MIPS
assembler and the corresponding machine code
Fields
•
•
•
•
•
Example:
Op-code: operation code
add r1, r2, r0
Rs, Rt: source registers
000000 00010 00000 00001 00000 100000
Rd: destination register
SA: shift amount
Funct-code: function specifier
3. Define the I-format instruction:
a)
b)
c)
d)
e)
What are the various fields?
What is the basic lay-out?
What does each field represent?
Why this instruction format needed as opposed to the Rformat?
Give an example of an I-format instruction in MIPS assembler
and the corresponding machine code
Fields
•
•
•
•
Op-code: operation code
Rs: source register
Rt: destination register
Immediate: 2’s complement
constant
Example:
addi r1, r2, 1
001000 00010 00001 0000000000000001
4. What is the range of addresses for conditional branches in the
MIPS ISA?
PC  4  4  2  address  PC  4  4  (2  1)
15
15
5. Answer the following questions as true or false:
a)
The beq instruction always modifies the program counter
register
b) The add instruction does not modify the PC
c)
The jal instruction always modifies the PC
d) The instruction beq r1,r2,1 will advance the PC by one byte if
[r1]=[r2]
e)
The instruction beq r1,r2,1 will advance the PC by one word if
[r1]=[r2]
Ans:
•
T. PC = PC+4 even if r1 <> r2
•
F. PC = PC+4
•
T. Unconditional branch
•
F. PC = PC+4 + 4 x 1
•
F. PC = PC+4 + 4 x 1
6. Find the shortest sequence of core MIPS instruction to
determine the absolute value of a two’s complement integer.
Please the result into r3.
There are several possible answers to this question.
However, in general, all involve the following two steps:
1.
2.
Convert the number to a positive value if it is
negative, which can be done by multiplication, xor,
nor, etc…
Move the value to r3
7. Write out the truth table, the logic equation and draw the gates
for the Sum bit of a 1-bit adder
Cin
Sum
Sum bit is 1 if an odd
number of the three inputs 1
→XOR the three inputs
a
Cout
b
half adder