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+23 votes

Which of the following is/are true of the auto-increment addressing mode?

  1. It is useful in creating self-relocating code

  2. If it is included in an Instruction Set Architecture, then an additional ALU is required for effective address calculation

  3. The amount of increment depends on the size of the data item accessed

  1. I only
  2. II only
  3. III only
  4. II and III only
asked in CO & Architecture by Veteran (59.5k points)
retagged by | 4k views

3 Answers

+40 votes
Best answer

In auto increment addressing mode, the base address is incremented after operand fetch. This is useful in fetching elements from an array. But this has no effect in self-relocating code (where code can be loaded to any address) as this works on the basis of an initial base address. 

An additional ALU is desirable for better execution especially with pipelining, but never a necessity. 

Amount of increment depends on the size of the data item accessed as there is no need to fetch a part of a data. 

So, answer must be C only. 

answered by Veteran (355k points)
edited by
Sir, without ALU, how ++ or -- operations will be performed
it is said "additional ALU"
by using counter we can do ++ and -- so no additional circuit is necessary
we know control unit of CPU has both decoder and counter to provide the timing signal
but counter is also part of ALU operation in execution phase?
sir i think think additional ALU means caacity of alu to perform addition. so accordingly b should be true. How c is true?  

Amount of increment depends on the size of the data item accessed as there is no need to fetch a part of a data.

1.self reallocating code required for displacement addressing mode. auto increment/decrement addressing mode , Register will be automatically incremented or decremented So, ALU is required. But  additional ALU is not required.

Some people are saying that we can use counter for increment.

1.So will that counter increment with step size always?

2.We always increment by one or by step size in auto incr addressing mode?

rahul sharma 5 , in auto-increment/decrement mode the address of operand register is incremented/decremented by 1,2,4,8 or 16 depending on the opcode.

So counter can handle this?
"Autoincrement mode automatically increments the contents of a register by a value that corresponds to the size of the accessed operand. Thus , the increment is 1 for byte-sized operands, 2 for 16-bit operands, and 4 for 32-bit operands. The size of the operand is usually specified as part of the operation code of an instruction."

--Computer Organisation by Carl Hamacher
a register can be automatically incremented by control signal INC  hence no additional ALU is required
+8 votes
Correct answer (C)

Auto increment mode is used for incrementing and depends on size of data.
answered by Loyal (6.1k points)
–3 votes

Auto increment mode: 

The effective address of the operand is the contents of a register specified in the instruction. After accessing the operand, the contents of this register are automatically incremented to the next value. This increment is 1 for byte sized operands, 2 for 16 bit operands and so on.

E.g. Add (R2) +, R0

Here are the contents of R2 are first used as an E.A. then they are incremented.

 Answer should be (C)

answered by Loyal (7.8k points)

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