SHIFT instructions

alokagarwaljgd · New User Joined: 02 Jun 2006 Posts: 28

Hi,

Can any body help me out with below two questions. It was asked in one of the quiz.

1) What will happen if we shift n bits to left using SLL and then again if we use SRL to shift n bits to right?

2) What does the following instruction do?

EX 0,SIGNWK

SIGNWK MVC VAR+4(2),=X'000C'
XC VAR2(5),VAR3

where VAR, VAR2 and VAR3 can be assumed as already defined valid variables.

Any help for the above is highly appreciated.

Thanks,
Alok

Bill Woodger · Posted: Tue Aug 02, 2011 5:58 pm

Try a bit of experimentation.

Open a new member in your editor.

Fill a single line with a single character, all the way to the end of the line.

Do a shift left of a number of characters, like "<5". Then do a shift right ">5".

Same thing is happening. Try it swapping the commands around as well if you like.

For your second question, look up the instructions in the POP, if you don't know them, and get an idea there. Search the Assembler forum for EX (Execute). If you get stuck, let use know where. If you get an answer, let us know as well.

alokagarwaljgd · New User Joined: 02 Jun 2006 Posts: 28

1) I did the left shift and right shift as suggested by you. I filled the line completely with 1s. Then on doing left shift (let's say 3 times), first time, 3 charcters (say 111) are removed from the line on extreme left side and 3 blank spaces are filled on extreme right side of the line. Again after doing right shift ( 3 times), the three blank spaces are replaced by three ones and again three blank spaces get created on extreme left side. So, basically it is creating 3 blank spaces (or insert 3 zeroes, in case of SLL/SRL) when we shift it either to right or left.

After one time execution of SRL and SLL, it will contain the same number of ones and zeroes. However, the resultant value of the number will change in each case. I understand the above logic and the way SRL and SLL works but does shifting of n bits to left and then to right make sense in any case or is there any situation in real life, when we make use of it?

2) Coming to my second question, I understand that EX with register 0 as first operand doesn't change anything, it will simply execute the target instruction, (MVC in this case). Since, it works on single instruction, the control after executing the MVC will not execute the XC instruction and will be going back to execute the instruction after the EX instruction. Please let me know if my understanding is correct.

Thanks,
Alok

Bill Woodger · Posted: Wed Aug 03, 2011 2:06 pm

Yep, I think that is about it. Just a little more work on the EX to understand whether the register is relevant for the "doesn't change anything" and the trickiest bit is understanding the "doesn't change anything". Absolutely correct about the XC.

As to uses of the Shifts, and the use like described, they are many.

Take a -ve binary halfword. Want to make it positive with the same absolute value? Shift left one bit, shift right one bit.

enrico-sorichetti · Posted: Wed Aug 03, 2011 2:57 pm

Bill Woodger · Posted: Wed Aug 03, 2011 7:33 pm

As enrico is alluding, I've been a little slack with my terminology. Sorry about that.

PeterHolland · Posted: Wed Aug 03, 2011 9:13 pm

Dont use the shift too much else people are going to say :
There are some shifty fellows on this forum.

Especially when you shift leftish.

alokagarwaljgd · New User Joined: 02 Jun 2006 Posts: 28

Bill Woodger · Posted: Thu Aug 04, 2011 11:47 am

Alok,

Because I was unthinking, I'm sorry. I was struggling for an example of why you'd you SLL followed by SRL and thought "aha, clobber the sign", without thinking that you'd end up with a huge positive bearing no useful relationship to the original number. My mistake.

Peter's comment was just a word-play joke, nothing to do with anything.

dbzTHEdinosauer · Posted: Thu Aug 04, 2011 12:18 pm

enrico already explained that there is an important difference between the shift bits logical and shift bits instructions.

attention to detail......

enrico-sorichetti · Posted: Thu Aug 04, 2011 7:20 pm

just plain stubborness..
Your example of not losing 0/1 s is a specific case, not the general one

and ... I forgot to clarify a shift right ( arithmetical shift right will propagate the sign
(remember negative numbers are in 2s complement )

no shifting will change the sign, read about LNR
again before posting Your wrong conclusions read for the umpteen time the manual

to save some of Your precious time here is the link for the download
www-304.ibm.com/support/docview.wss?uid=isg2b9de5f05a9d57819852571c500428f9a

alokagarwaljgd · New User Joined: 02 Jun 2006 Posts: 28

Enrico,

I never said that shifting to left and right will not lose 0/1. It will definitely do that and in my example also it is doing that.

Let me give another example, say
if we have R2 = 1110 and if we execute the following instructions

SLL 2,1
After execution, R2 = 1100, which contains 2 ones and 2 zeros
Then if we execute the below instruction

SRL 2,1
After execution, R2 = 0110, which again contains 2 ones and 2 zeros

In the above example, shifting to left resulted in loss of 1 and introduction of 0 at end and then shifting to right resulted in loss of 0 at extreme right and introduction of 0 in the beginning.
Hence, in each of the above case, we can see that after the execution of SRL and SLL, they have same number of ones and zero
i.e. 2 and 2 respectively.

I think there was some confusion here, when I am saying "After one time execution of SRL and SLL, it will contain the same number of ones and zeroes" I am not comparing the number of 0/1 s produced in the register after execution with the first initial value of the register( 3 ones and 1 zero in this example). I am only comparing the number of 0/1 s produced in the register (R2 = 1100, which contains 2 ones and 2 zeros, in this example) after once SLL executes with that of number of 0/1 s produced in the register (R2 = 0110, which again contains 2 ones and 2 zeros, in this example) after a successive SRL executes.

I also understand the sign propagation in case of arithmetic shifts. But, here my main focus from the beginning was logical shifts (n bit shifts of SLL followed by n bit shifts of SRL). And since, Bill suggested me to take a -ve binary halfword and try out the shifts. I did the same as suggested by him to understand if that can act as a practical use/purpose of shifting n bits to left using SLL and then immediately followed by SRL for shifting n bits to right.

Thanks,
Alok

Ed Goodman · Active Member Joined: 08 Jun 2011 Posts: 556 Location: USA

good use for bit shifting: reading/writing flag bits.

flag byte has 10010001

do this loop
right-most bit = AND with 00000001
shift right