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Quiz based on Digital Principles and Computer Organization

1) Base of hexadecimal number system? Answer : 16 2) Universal gate in digital logic? Answer : NAND 3) Memory type that is non-volatile? Answer : ROM 4) Basic building block of digital circuits? Answer : Gate 5) Device used for data storage in sequential circuits? Answer : Flip-flop 6) Architecture with shared memory for instructions and data? Answer : von Neumann 7) The smallest unit of data in computing? Answer : Bit 8) Unit that performs arithmetic operations in a CPU? Answer : ALU 9) Memory faster than main memory but smaller in size? Answer : Cache 10) System cycle that includes fetch, decode, and execute? Answer : Instruction 11) Type of circuit where output depends on present input only? Answer : Combinational 12) The binary equivalent of decimal 10? Answer : 1010 13) Memory used for high-speed temporary storage in a CPU? Answer : Register 14) Method of representing negative numbers in binary? Answer : Two's complement 15) Gate that inverts its input signal? Answer : NOT 16)...

Pointers to Pointers

Pointer to Pointers
In C, you can also use pointers that point to pointers. The pointers in turn point to data or even to other pointers. To declare pointers to pointers, just add an asterisk * for each level of reference.
          Fig: Pointer to Pointer
For example, consider the following code:
int x=10;
int *px, **ppx;
px = &x;
ppx = &px;
Let us assume, the memory locations of these variables are as shown in Fig.above
Now if we write,
printf("\n %d", **ppx);
Then, it would print 10, the value of x.

Drawbacks of Pointers
Although pointers are very useful in C, they are not free from limitations. If used incorrectly, pointers can lead to bugs that are difficult to unearth. For example, if you use a pointer to read a memory location but that pointer is pointing to an incorrect location, then you may end up reading 
a wrong value. An erroneous input always leads to an erroneous output. Thus however efficient your program code may be, the output will always be disastrous. Same is the case when writing a value to a particular memory location.
Let us try to find some common errors when using pointers.
int x, *px;
x=10;
*px = 20;
Error: Un-initialized pointer. px is pointing to an unknown memory location. Hence it willoverwrite that location’s contents and store 20 in it.
int x, *px;
x=10;
px = x;
Error: It should be px = &x;
int x=10, y=20, *px, *py;
px = &x, py = &y;
if(px<py) 
printf("\n x is less than y");
else
printf("\n y is less than x");
Error: It should be if(*px< *py)

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