GATE CSE 2017 | Set 1 | Question: 11

Question

Consider the $C$  struct defined below:

struct data {
    int marks [100];
    char grade;
    int cnumber;
};
struct data student;

The base address of student is available in register $R1$. The field student.grade can be accessed efficiently using:

• A. Post-increment addressing mode, $(R1)+$
• B. Pre-decrement addressing mode, $-(R1)$
• C. Register direct addressing mode, $R1$
• D. Index addressing mode, $X(R1)$, where $X$ is an offset represented in $2's$ complement $16\text{-bit}$ representation

Comments

Structure is nothing bt the hetrogenous array only and for that implementation we use indexed adressing mode .

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• X = 400 (offset to grade)

• X(R1) accesses memory at R1 + 400

• Efficient and non-destructive to R1

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Reasoning of why option D is true 

The fields of a struct are stored sequentially in contiguous memory locations:

==> Base Address ($R1$): Points to the very beginning of the student structure.

==> Field marks[100]: Occupies the first section of the struct. Assuming a standard 4-byte int, marks occupies $100 \times 4 = 400$ bytes.

==> Field grade: Located immediately after the marks array at a fixed compile-time offset $X$:

$X = \text{Offset of } \texttt{grade} = \text{sizeof}(\texttt{marks}) = 400 \text{ bytes}$

To access a specific field within structure whose base address is in a register, the processor uses Displacement Addressing Mode

$\text{Effective Address (EA)} = [R1] + X$

==> Register ($R1$): Holds the base memory address of the struct instance.

==> Offset ($X$):  representing the relative distance from the base address to the field grade.

Note : 

16-bit 2's Complement Field: A 16-bit signed displacement field can represent offsets from $-32,768$ to $+32,767$. Since the offset $X = 400$ bytes easily falls within this range, it fits within a standard 16-bit instruction displacement field.

 

Answer 1

Answer is option (D).

Displacement Mode :-

Similar to index mode, except instead of a index register a base register will be used. Base register contains a pointer to a memory location. An integer (constant) is also referred to as a displacement. The address of the operand is obtained by adding the contents of the base register plus the constant. The difference between index mode and displacement mode is in the number of bits used to represent the constant. When the constant is represented a number of bits to access the memory, then we have index mode. Index mode is more appropriate for array accessing; displacement mode is more appropriate for structure (records) accessing.

Reference:

http://www.cs.iit.edu/~cs561/cs350/addressing/addsclm.html

Comments

Can we simply say as base address of student in register $R_{1}$

and it will be in an array

So index addressing mode will be more preferred.

what is meaning of " where X is an offset represented in 2's complement 16-bit representation. "

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x is used to fetch element  at certain position from base address.
as like A[10]=*(A+10)=element at(base address+displacement)

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pls provide answer

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@Niraj 

what does these lines mean?

"The difference between index mode and displacement mode is in the number of bits used to represent the constant. When the constant is represented a number of bits to access the memory, then we have index mode."

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Significance of offset is to determine attribute of that array element. Once you got the EA, (X + R1), offset field will help to determine the attribute of that element since arrays elements are saved in successive memory locations.

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In simple words, X (offset) is the position from the program counter, where the next operand should be fetched from.

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Suppose memory is byte addressable and an int takes 4 bytes space.
Assume that base address of student is $1000$ then $R1$ contains $1000$.
student.grade can be accessed by $X(R1)$
Here in our case $X = 100*4=400,$ so address of $student.grade = 400(1000) =400+1000= 1400.$
https://ideone.com/ZviClG

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@Ayush Upadhyaya He meant to say that in PC addressing mode, the address field is generally treated as a 2's complement number, whereas in the base register, the address field is an unsigned integer representation.

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first.  in options there's no displacement mode

2nd.  in BEST ANSWER last line implicitly days index is best for array accessing and displacement is best for structure accessing...

 

then how option d which is indexed addressing can be a answer..

index and displacement are totally different..

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AS WE ALL KNOW IT IS A LINKED LIST, HENCE I WOULD ALSO HAVE CHOSEN D AS THE OPTION.

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What if , register indirect addressing mode is given in the options???.. is it ok to move with this also???

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Subbu. the point of the question is to find out if the student knows which addressing mode to use if the base address is given. So technically register indirect can be used to access struct elements, provided that we know its address. But it is more eficcient if we access it using an offset to the base address given so i would say even then option D would have been more appropriate

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As @ajith j said ....Can somebody tell why the address field in base register mode is represented as unsigned integer

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@Subbu 
"What if , register indirect addressing mode is given in the options???.. is it ok to move with this also???"
still the index addressing mode would be the most efficient choice for accessing student.grade with a offset of 400

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In Base register adressing mode the unsigned used because here we have to consider a base address and we add the content of register to find the effective address. So, just simply add the content of register to jump/branch in previous or next inst. then it does not require to subtract the value so it is unsigned.

But for PC Related AM we have to calculate the PC again and again to find the EA. But in this case to reach previous inst. we have to subtract so it used unsigned offset.

Answer 2

struct data {
    int marks[100];  // 100 * 4 bytes = 400 bytes (assuming 4-byte ints)
    char grade;      // 1 byte
    int cnumber;     // 4 bytes (assuming int is 4 bytes)
};
struct data student;

Aur maan lo register R1 me student ka base address hai. Humein student.grade waali location chahiye.

1. Index (displacement) addressing: X(R1)

   • Yahaan hum R1 ko base maante hain aur usme ek offset (X) add karke particular field pe pahunch jaate hain.
   • Offset 16-bit 2’s complement representation mein diya hota hai, jisse hum structure ke andar koi field direct access kar sakte hain.

 

Post-increment (R1)+

• Isme pehle R1 se memory access hoti hai, fir R1 ko increment kiya jaata hai.
• Zyada tar ye array traversal mein kaam aata hai (element by element).

Pre-decrement –(R1)

• Pehle R1 decrement hota hai, fir memory access.
• Ye stack push/pop jaisi cheezo ke liye convenient hai.

Register direct addressing (R1)

• Isme hum R1 ko hi data ki tarah treat karte hain.
• Par yahan R1 me base address hai, field offset handle karne ke liye koi facility nahi.

Kyun “Index addressing mode” Sahi Hai?

• student.grade structure ke andar 400 bytes baad (yani marks[100] ke baad) aata hai.
• Humein R1 (base) + offset (400) pe jana hai.
• Post-increment ya pre-decrement use karne se R1 badal jaayega, jo hum structure ke baaki fields ke liye nahi chahte.
• Register direct me toh offset handle nahi kar sakte.

Isliye sabse efficient tariqa hai “Index addressing mode, X(R1)”, jahan X = grade field ka offset ho

 

Comments

well explained !!

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🐐

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Very good explanation

Answer 3

Actually answer can be both C or D.It depends on the architecture.If the machine is byte addressable then option D is correct.

For instance, say int takes 4 bytes and char takes 8 bytes.If it is byte addressable then the memory layout will be somewhat shown in the figure.If we are using for e.g decimal addressing and lets take the base address as 1000,then we need displacement of 51 to access student.grade.

Now if the machine is word addressable and say 1 word=100*4(for marks array)+8(for grade)+4(for cnumber)=412bytes .Then the picture will look something like..

In this case we dont need displacement...it can be done with direct addressing mode only.

Comments

can u tell me wat is register direct addressing ??? and if u consider C as answer hw r u going to retrieve student.grade ??

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register direct addressing mode  ADDRESS OF THE DATA IS DIRECTLY PRESENT IN THE REGISTER.

Answer 4

sruct data
{
int marks[100];
char grade;
int cnumber;
}; struct data student
Base Address of student is available in R1.
So student.grade can be accessed efficiently by Relative Indexed Addressing Mode.
It is clearly mentioned X is the offset address to be summed with Base Address of R1.

Hence Index Addressing mode X(R1), where X is an offset represented in 2’s complement 16-bit representation.
⇾ Relative, Base Indexed & all subtypes of Indirect addressing modes are used with Arrays.

Comments

Why not a option

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Because they asked for efficiently

Answer 5

Ans D) The struct student has a marks[100] array (400 bytes), followed by a grade field (1 byte). To access student.grade, we need to address it 400 bytes after the base address of student.

Indexed addressing mode is the most efficient here because we know the exact offset of 400 bytes from the base. This mode allows us to compute the address as base_address + offset efficiently.

Other modes like post-increment and pre-decrement are suited for sequential access, not for direct field access. Register direct would require loading the precise address, making it less efficient. Hence, indexed addressing mode is ideal for accessing student.grade using the known offset.

Answer 6

 

sruct data
{
int marks[100];
char grade;
int cnumber;
}; struct data student
Base Address of student is available in R1.
So student.grade can be accessed efficiently by Relative Indexed Addressing Mode.
It is clearly mentioned X is the offset address to be summed with Base Address of R1.

And Index Addressing mode X(R1), where X is an offset represented in 2’s complement 16-bit representation.And Relative, Base Indexed & all subtypes of Indirect addressing modes are used with Arrays.