Questions
Verilog Question 1Q: What is the difference between a Verilog task and a Verilog function? (Answer)
Verilog Question 2Q: Given the following Verilog code, what value of "a" is displayed?
always @(clk) begin a = 0; a <= 1; $display(a); end(Answer)
Verilog Question 3Q: Given the following snipet of Verilog code, draw out the waveforms for "clk" and "a".
always @(clk) begin a = 0; #5 a = 1; end(Answer)
Verilog Question 4Q: What is the difference between the following two lines of Verilog code?
#5 a = b; a = #5 b;(Answer)
Verilog Question 5Q: Write the Verilog code to provide a divide-by-3 clock from the standard clock. (Answer)
Verilog Question 6Q: What is the difference between:
c = foo ? a : b; and if (foo) c = a; else c = b;(Answer)
Verilog Question 7Q: Using the given, draw the waveforms for the following versions of a (each version is separate, i.e. not in the same run):
reg clk; reg a; always #10 clk = ~clk; (1) always @(clk) a = # 5 clk; (2) always @(clk) a = #10 clk; (3) always @(clk) a = #15 clk; Now, change a to wire, and draw for: (4) assign #5 a = clk; (5) assign #10 a = clk; (6) assign #15 a = clk;(Answer)
Vera Question 1Q: What is the difference between a Vera task and a Verilog task? (Answer)
Vera Question 2Q: What is the difference between running the following snipet of code on Verilog vs Vera?
fork { task_one(); #10; task_one(); } task task_one() { cnt = 0; for (i = 0; i < 50; i++) { cnt++; } }(Answer)
Programming Question 1Q: Given $a = "5,-3,7,0,-5,12"; Write a program to find the lowest number in the string. (Answer)
Programming Question 2Q: Write the code to sort an array of integers. (Answer)
Programming Question 3Q: Write the code to find the factorial of an integer. Use a recursive subroutine. (Answer)
Programming Question 4Q: In C, explain the difference between the & operator and the * operator. (Answer)
Programming Question 5Q: Write a function to determine whether a string is a palindrome (same forward as reverse, such as "radar" or "mom"). (Answer)
Programming Question 6Q: Write a function to output a diamond shape according to the given (odd) input. Examples: Input is 5 Input is 7 * * *** *** ***** ***** *** ******* * ***** *** * (Answer)
General Question 1Q: Given the following FIFO and rules, how deep does the FIFO need to be to prevent underflowing or overflowing?
(Answer)
RULES: 1) frequency(clk_A) = frequency(clk_B) / 4 2) period(en_B) = period(clk_A) * 100 3) duty_cycle(en_B) = 25%
General Question 2Q: Draw the state diagram to output a "1" for one cycle if the sequence "0110" shows up (the leading 0s cannot be used in more than one sequence). (Answer)
General Question 3Q: Explain the differences between "Direct Mapped", "Fully Associative", and "Set Associative" caches. (Answer)
General Question 4Q: Design a four-input NAND gate using only two-input NAND gates. (Answer)
General Question 5Q: Draw the state diagram for a circuit that outputs a "1" if the aggregate serial binary input is divisible by 5. For instance, if the input stream is 1, 0, 1, we output a "1" (since 101 is 5). If we then get a "0", the aggregate total is 10, so we output another "1" (and so on). (Answer)
Answers
Q: What is the difference between a Verilog task and a Verilog function?
Verilog Answer 1
A:
The following rules distinguish tasks from functions:A function shall execute in one simulation time unit; a task can contain time-controlling statements. A function cannot enable a task; a task can enable other tasks or functions. A function shall have at least one input type argument and shall not have an output or inout type argument; a task can have zero or more arguments of any type. A function shall return a single value; a task shall not return a value. (Back)
Verilog Answer 2A:Q: Given the following Verilog code, what value of "a" is displayed?
always @(clk) begin a = 0; a <= 1; $display(a); end
This is a tricky one! Verilog scheduling semantics basically imply a four-level deep queue for the current simulation time: 1: Active Events (blocking statements) 2: Inactive Events (#0 delays, etc) 3: Non-Blocking Assign Updates (non-blocking statements) 4: Monitor Events ($display, $monitor, etc). Since the "a = 0" is an active event, it is scheduled into the 1st "queue". The "a <= 1" is a non-blocking event, so it's placed into the 3rd queue. Finally, the display statement is placed into the 4th queue. Only events in the active queue are completed this sim cycle, so the "a = 0" happens, and then the display shows a = 0. If we were to look at the value of a in the next sim cycle, it would show 1. (Back)
Verilog Answer 3A:Q: Given the following snipet of Verilog code, draw out the waveforms for clk and a
always @(clk) begin a = 0; #5 a = 1; end
10 30 50 70 90 110 130 ___ ___ ___ ___ ___ ___ ___ clk ___| |___| |___| |___| |___| |___| |___| |___ a ___________________________________________________________ This obviously is not what we wanted, so to get closer, you could use "always @ (posedge clk)" instead, and you'd get 10 30 50 70 90 110 130 ___ ___ ___ ___ ___ ___ ___ clk ___| |___| |___| |___| |___| |___| |___| |___ ___ ___ a _______________________| |___________________| |_______ (Back)
Verilog Answer 4A:Q: What is the difference between the following two lines of Verilog code?
#5 a = b; a = #5 b;
#5 a = b; Wait five time units before doing the action for "a = b;". The value assigned to a will be the value of b 5 time units hence. a = #5 b; The value of b is calculated and stored in an internal temp register. After five time units, assign this stored value to a. (Back)
Verilog Answer 6A:Q: What is the difference between:
c = foo ? a : b; and if (foo) c = a; else c = b;
The ? merges answers if the condition is "x", so for instance if foo = 1'bx, a = 'b10, and b = 'b11, you'd get c = 'b1x. On the other hand, if treats Xs or Zs as FALSE, so you'd always get c = b. (Back)
Verilog Answer 7A:Q: Using the given, draw the waveforms for the following versions of a (each version is separate, i.e. not in the same run):
reg clk; reg a; always #10 clk = ~clk; (1) always @(clk) a = #5 clk; (2) always @(clk) a = #10 clk; (3) always @(clk) a = #15 clk; Now, change a to wire, and draw for: (4) assign #5 a = clk; (5) assign #10 a = clk; (6) assign #15 a = clk;
10 30 50 70 90 110 130 ___ ___ ___ ___ ___ ___ ___ clk ___| |___| |___| |___| |___| |___| |___| |___ ___ ___ ___ ___ ___ ___ ___ (1)a ____| |___| |___| |___| |___| |___| |___| |_ ___ ___ ___ ___ ___ ___ ___ (2)a ______| |___| |___| |___| |___| |___| |___| (3)a __________________________________________________________ Since the #delay cancels future events when it activates, any delay over the actual 1/2 period time of the clk flatlines... With changing a to a wire and using assign, we just accomplish the same thing... 10 30 50 70 90 110 130 ___ ___ ___ ___ ___ ___ ___ clk ___| |___| |___| |___| |___| |___| |___| |___ ___ ___ ___ ___ ___ ___ ___ (4)a ____| |___| |___| |___| |___| |___| |___| |_ ___ ___ ___ ___ ___ ___ ___ (5)a ______| |___| |___| |___| |___| |___| |___| (6)a __________________________________________________________ (Back)
Vera Answer 1A:Q: What is the difference between a Vera task and a Verilog task?
(Back)
Vera Answer 2A:Q: What is the difference between running the following snipet of code on Verilog vs Vera?
fork { task_one(); #10; task_one(); } task task_one() { cnt = 0; for (i = 0; i < 50; i++) { cnt++; } }
(Back)
Programming Answer 1A:Q: Given $a = "5,-3,7,0,-5,12"; Write a program to find the lowest number in the string.
// BEGIN PERL SNIPET $a = "5,-5,-1,0,12,-3"; (@temp) = split (/,/, $a); $lowest = $temp[0]; for ($i=0; $i<6; $i++) { if ($temp[$i] < $lowest) { $lowest = $temp[$i]; } } print "Lowest value found was: $lowest\n"; // END PERL SNIPET NOTE: You could also replace the for loop with this: foreach $value (@temp) { if ($value < $lowest) { $lowest = $value; } } (Back)
Programming Answer 2A:Q: Write the code to sort an array of integers.
/* BEGIN C SNIPET */ void bubblesort (int x[], int lim) { int i, j, temp; for (i = 0; i < lim; i++) { for (j = 0; j < lim-1-i; j++) { if (x[j] > x[j+1]) { temp = x[j]; x[j] = x[j+1]; x[j+1] = temp; } /* end if */ } /* end for j */ } /* end for i */ } /* end bubblesort */ /* END C SNIPET */ Some optimizations that can be made are that a single-element array does not need to be sorted; therefore, the "for i" loop only needs to go from 0 to lim-1. Next, if at some point during the iterations, we go through the entire array WITHOUT performing a swap, the complete array has been sorted, and we do not need to continue. We can watch for this by adding a variable to keep track of whether we have performed a swap on this iteration. (Back)
Programming Answer 3A:Q: Write the code for finding the factorial of a passed integer. Use a recursive subroutine.
// BEGIN PERL SNIPET sub factorial { my $y = shift; if ( $y > 1 ) { return $y * &factorial( $y - 1 ); } else { return 1; } } // END PERL SNIPET (Back)
Programming Answer 4A:Q: In C, explain the difference between the & operator and the * operator.
& is the address operator, and it creates pointer values. * is the indirection operator, and it dereferences pointers to access the object pointed to. Example: In the following example, the pointer ip is assigned the address of variable i (&i). After that assignment, the expression *ip refers to the same object denoted by i: int i, j, *ip; ip = &i; i = 22; j = *ip; /* j now has the value 22 */ *ip = 17; /* i now has the value 17 */ (Back)
Programming Answer 5A:Q: Write a function to determine whether a string is a palindrome (same forward as reverse, such as "radar" or "mom").
/* BEGIN C SNIPET */ #include <string.h> void is_palindrome ( char *in_str ) { char *tmp_str; int i, length; length = strlen ( *in_str ); for ( i = 0; i < length; i++ ) { *tmp_str[length-i-1] = *in_str[i]; } if ( 0 == strcmp ( *tmp_str, *in_str ) ) printf ("String is a palindrome"); else printf ("String is not a palindrome"); } /* END C SNIPET */ (Back)
Programming Answer 6A:Q: Write a function to output a diamond shape according to the given (odd) input. Examples: Input is 5 Input is 7 * * *** *** ***** ***** *** ******* * ***** *** *
### BEGIN PERL SNIPET ### for ($i = 1; $i <= (($input * 2) - 1); $i += 2) { if ($i <= $input) { $stars = $i; $spaces = ($input - $stars) / 2; while ($spaces--) { print " "; } while ($stars--) { print "*"; } } else { $spaces = ($i - $input) / 2; $stars = $input - ($spaces * 2); while ($spaces--) { print " "; } while ($stars--) { print "*"; } } print "\n"; } ### END PERL SNIPET ### (Back)
General Answer 1A:Q: Given the following FIFO and rules, how deep does the FIFO need to be to prevent underflowing or overflowing?
Assume clk_B = 100MHz (10ns) From (1), clk_A = 25MHz (40ns) From (2), period(en_B) = 40ns * 100 = 4000ns, but we only output for 1000ns, due to (3), so 3000ns of the enable we are doing no output work. Therefore, FIFO size = 3000ns/40ns = 75 entries. (Back)
General Answer 2A:Q: Draw the state diagram to output a "1" for one cycle if the sequence "0110" shows up (the leading 0s cannot be used in more than one sequence).
(Back)
General Answer 3A:Q: Explain the differences between "Direct Mapped", "Fully Associative", and "Set Associative" caches.
If each block has only one place it can appear in the cache, the cache is said to be direct mapped. The mapping is usually (block-frame address) modulo (number of blocks in cache). If a block can be placed anywhere in the cache, the cache is said to be fully associative. If a block can be placed in a restricted set of places in the cache, the cache is said to be set associative. A set is a group of two or more blocks in the cache. A block is first mapped onto a set, and then the block can be placed anywhere within the set. The set is usually chosen by bit selection; that is, (block-frame address) modulo (number of sets in cache). If there are n blocks in a set, the cache placement is called n-way set associative. (Back)
General Answer 4A:Q: Design a four-input NAND gate using only two-input NAND gates.
Basically, you can tie the inputs of a NAND gate together to get an inverter, so...(Back)
General Answer 5A:Q: Draw the state diagram for a circuit that outputs a "1" if the aggregate serial binary input is divisible by 5. For instance, if the input stream is 1, 0, 1, we output a "1" (since 101 is 5). If we then get a "0", the aggregate total is 10, so we output another "1" (and so on).
We don't need to keep track of the entire string of numbers - if something is divisible by 5, it doesn't matter if it's 250 or 0, so we can just reset to 0. So we really only need to keep track of "0" through "4".(Back)