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Consider a new instruction named branch-on-bit-set (mnemonic bbs). The  instruction “bbs reg, pos, label” jumps to label if bit in position pos of register  operand reg is one. A register is $32 -bits$ wide and the bits are numbered 0 to 31,  bit in position 0 being the least significant. Consider the following emulation of  this instruction on a processor that does not have bbs implemented.

$temp\leftarrow reg \& mask$

Branch to label if temp is non-zero. The variable temp is a temporary register. For correct emulation, the variable  mask must be generated by

1. $mask\leftarrow \text{0x1} << pos$
2. $mask\leftarrow \text{0xffffffff} << pos$
3. $mask\leftarrow pos$
4. $mask\leftarrow \text{0xf}$

edited | 3k views

1. $mask\leftarrow \text{0x1} << pos$

We want to check for a particular bit position say $2$ (third from right). Let the number be $0xA2A7$ (last $4$ bits being $0111$). Here, the bit at position $2$ from right is $1$. So, we have to AND this with $0x0004$ as any other flag would give wrong value (may count other bits or discard the bit at position "$pos$"). And $0x0004$ is obtained by $0x1 << 2$ (by shifting $1$ "$pos$" times to the left we get a flag with $1$ being set only for the "$pos$" bit position).

by Veteran (420k points)
edited by
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My problem is i did not get the example given by you properly. What is the meaning of wrong value here ? It would be better for me if you can explain this again.

And option a. mask <- 0x1 << pos means mask will get the value ( of 0x1 after left shift by pos times)

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I have edited the answer- is it clear now?
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yes sir Thank you.
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I am not getting the question, what it wants to explain. It's tough for me to understand the answer.
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@Gupta731 In Layman terms given a bit string, we have to check if string[pos] is set or not. How it is done?

By ANDig the value of that particular position by 1. Option A left shifts one by the position we want to check and it will act as mask.

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$\text{0xA2A7}$ equivalent binary is

$\text{1010 0010 10}\color{Magenta}{1}\text{0 0111}$

Let's say we want to find whether bit at position 5 from right is 1 or not.

That means we need to left shift 5 by 1 then it will become 10 whose equivalent binary is $1010$ then how it is going check bit at position 5

This question can be simply frame into bit manipulation question.

We can say that "find the bit is set or not at position "pos" in given register "reg".

So finding the bit is set or not simply we need to perform an "AND" operation with '1' of that bit if result is 0 then bit is not set and if result is not 0 then bit is set. Now if given "pos" is not 0 then we need to left shift 1 by pos ( 0x1 << pos ).

by Active (2.4k points)
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Is this in gate syllabus??????
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If Gate ppl start mentioning such things in Gate syllabus, we will get a 300-400 pages book of syllabus. :)
here, bbs reg ,pos, label is given instruction , suppose i m taking position =2 in pos register ,

then, "bbs , reg, pos,label" will take branch when 2nd bit of binary pattern stored in register is 1.

but how to check 2nd bit of 32 bit binary pattern in register reg is 1 or not ?

for that we will perform an operation

reg : ..........0......11 (32 bit operand in binary pattern)

mask: 000........0010(32 bit binary pattern in which 2nd bit is 1)

if AND operation is done b/w reg and mask and 2nd bit of result is 1 and all are 0's then branch is taken otherwise brach is not taken

mask <----- 0x1 i.e. (0001)<<(left shift by given position) POS

after shifting 0010 will be stored in mask (32 bit register ) for that option (a) is correct option
by Active (5k points)
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Can anybody explain the other options given in the Question?
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The mask obtained by the other options when anded with register they will not give the POS. Because of which they are incorrect.

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