I don't know of any instructions that accept a hard-coded negative
displacement. You can't do MVI 0,-6(R3)
The closest you can come is a negative index. For the indexed
instructions, the following will work.
LHI R2,-6
STH R0,0(R2,R3) Store R0 -6 from R3
Probably not worth the bother for your situation.
/CM/
On 2/10/2026 2:41 PM, David Clark wrote:
Let me preface this by saying I'm not really that familiar with newer
instructions. But, I have a 40-year old subroutine that provided 15 string
manipulation functions for COBOL programs. Now I am adding three more (one
of which came up in my previous post). At the same time...
The previous support was for one function at a time, even though multiple
input strings were supported (i.e., a variable-length parm list; with the
first for a header work area [R8], the second for a large "master" buffer
[R9], and the third for the beginning of one or more "working"
string parameters[R10]; where R11 is the code base register but only needed
for the fixed storage area LOCTR'd to the beginning of the subroutine).
I would like to add a "mini-scripting" feature that allows one of 16
different functions to be performed on a string-by-string basis and in a
single CALL. Naturally, this would have to be backward-compatible. To do
this I changed the string DSECT to have a "redefined" layout--as follows:
TXTINPT DSECT
USING TXTINPT,R10 ESTABLISH REG 10 AS BASE
TXTSREQU DS CL1 SCRIPTING REQUEST CODE
TXTSRETN DS CL1 SCRIPTING RETURN CODE
TXTSSTRL DS H SCRIPTING INPUT/OUTPUT STRING LENGTH
TXTSSTRG DS 256CL1 SCRIPTING INPUT/OUTPUT STRING
ORG TXTINPT
TXTSTRL DS H LEGACY INPUT/OUTPUT STRING LENGTH
TXTSTRG DS 256CL1 LEGACY INPUT/OUTPUT STRING
So as not to have to duplicate *all* of the code, I figured I would just
save off the first two bytes and then increment R10 by 2 such that the last
two fields would work for both modes (and the last two fields are the ones
that are referenced by the remainder of the subroutine).
That is fine, but when I want to save the return code for the current
string and function, I initially coded it this way (I presume it is OK to
use regular arithmetic on an address)--and then continue on with whatever
else needs to be done next.
IF TXTREQU,EQ,C'S' IF IN SCRIPTING MODE
SHI R10,2 POINT TO SCRIPTING PARM AREA
MVC TXTSRETN,HOLDRETN SAVE STRING RETURN CODE
AHI R10,2 POINT TO LEGACY LENGTH AND STRING
ENDIF ENDIF
But then I had a thought... Is there any instruction that will let you
move (and examine) data to/in a location prior to a given address? Now, I
saw where an AI generated a sample MVC instruction that used a negative
displacement, but I didn't know if that was legitimate? Recommendations?
Sincerely,
Dave Clark
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