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praveensinha
Warnings : 1
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Joined: 02 Oct 2009
Posts: 64
Location: Hyd
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| why condition codes are in even only like 4,8,12,16 ???any specific reason |
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enrico-sorichetti
Superior Member
Joined: 14 Mar 2007
Posts: 10873
Location: italy
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not a requirement...
just a habit |
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praveensinha
Warnings : 1
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Joined: 02 Oct 2009
Posts: 64
Location: Hyd
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| not clear ...... |
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Robert Sample
Global Moderator
Joined: 06 Jun 2008
Posts: 8696
Location: Dubuque, Iowa, USA
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| IBM uses 4, 8, 12,etc to designate specific levels of problem with their compilers and utilities. What IBM does, many people copy. Since a code can be anything from 0 to 4095, there is no requirement to use even numbers (much less numbers divisible by 4). However, since IBM tends to be standard, a lot of people picked up the habit of using 4, 8, 12, etc and continue it through today. |
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dick scherrer
Moderator Emeritus
Joined: 23 Nov 2006
Posts: 19244
Location: Inside the Matrix
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Hello,
Some systems i've worked on have a standard that locally developed code must use condition codes of 1000 and greater so the only times a cc of 4, 8, etc will occur is from an IBM (or other vendor) utility. |
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vasanthz
Global Moderator
Joined: 28 Aug 2007
Posts: 1742
Location: Tirupur, India
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Hi,
4:8-12 PASSAGE from Bible says, 
| Quote: |
There was no end to his toil,
yet his eyes were not content with his wealth.
"For whom am I toiling," he asked,
"and why am I depriving myself of enjoyment?"
This too is meaningless—
a miserable business! |
Its so synonymous to what a Software professional is,
Maybe thats why IBM chose 4,8 & 12.
BTW, really nice observation to ask this question.
P.S. - No harm intended. 
Regards, |
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Pedro
Global Moderator
Joined: 01 Sep 2006
Posts: 2547
Location: Silicon Valley
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I think this was decided a long time ago, before I started on z/OS.
But I have seen some colleagues coding some kind of branch table. That is, 4, 8, 12, 16, etc... fall on fullword boundaries and could be used to pick something out of a previously setup table; for example, the address of an appropriate message. |
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Bill Dennis
Active Member
Joined: 17 Aug 2007
Posts: 562
Location: Iowa, USA
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| Many times it was the offset of the next instruction. The return code from a subroutine was added to the return address to find the next instruction, so it needed to be on a fullword boundary. |
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MBabu
Active User
Joined: 03 Aug 2008
Posts: 400
Location: Mumbai
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Wow. I didn't realize that so many people would know this bit of trivia originated with branch tables, let alone how to code one. Bravo. Have an extra beer (or Metamucil) on me  |
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mmwife
Super Moderator
Joined: 30 May 2003
Posts: 1592
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As I recall, reg 15 contained the RC and was used to increment a Branch instr to a branch table [B BRTBL(R15)] that, in turn, contained 4 byte branch instrs to processing routines, e.g. B RC00RTN, B RC04RTN, B RC08RTN, etc.
Don't recall the Assembler coding format at all anymore - it's been a while. |
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dick scherrer
Moderator Emeritus
Joined: 23 Nov 2006
Posts: 19244
Location: Inside the Matrix
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Hello,
Something like:
| Code: |
B *+4(R15) BRANCH DEPENDING ON RETURN CODE
B ITSZERO 00 - OK, CONTINUE
B ITS04 04 - DO THE 4 STUFF
B ITS08 08 - DO THE 8 STUFF
B ITS12 12 - DO THE 12 STUFF |
Yup, it has been a while 
Nope, didn't assemble it. . .
Of course if R15 has some other value, it could get "entertaining". . . |
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