Paul Gilmartin wrote:
In a recent note, Greg Smith said:
Date: Wed, 6 Jul 2005 12:26:01 -0400
ds 0d
a ds 12x
b ds 16x
But if I code
mvc b(16),a
the overlap is destructive (ie b won't necessarily be an exact copy of
a). Would a
concurrent copy occur? (in this case, a concurrent copy could occur
without affecting
the result).
No. I believe the "destructive" character of the overlap has long
been part of the specification of MVC. In particular, programmers
of old were accustomed to blanking out a buffer with:
MVI C' ',A
MVC A+1(L'A'-1),A
(before padding MVCL).
-- gil
Well that's what I would think. I'm trying to understand concurrent copy
better - as I understand it, if the dest and source begin on the same byte
boundary in a doubleword, doublewords are fetched and stored
concurrently and if they are off by 4 then fullwords are fetched and stored
concurrently (once you get to the proper boundary). However, if there is
destructive overlap, then bytes are moved 1 at a time.
But, I cannot get the program below to fail. Either I don't quite
understand
destructive overlap (tfm: ` Destructive overlap is said to exist when
the result
location is used as a source after the result has been stored, assuming
processing to be performed one byte at a time') or there is concurrent
copy if the source and dest are far enough apart.
Greg
MVCTEST CSECT
SAVE (14,12)
lr 12,15
using MVCTEST,12
IDENTIFY EP=SUBTASK,ENTRY=SUBTASK
ATTACH EP=SUBTASK
loop mvc a(32),c
mvc a(16),b
mvc b(16),a
cli sw,0
be loop
STIMER WAIT,BINTVL==f'5'
RETURN (14,12),RC=0
SUBTASK SAVE (14,12)
lr 12,15
ahi 12,MVCTEST-SUBTASK
l 0,=a(N)
loop2 l 1,b
cl 1,c+12
be cont
cl 1,c+24
be cont
cl 1,c
be cont
mvi sw,1
dc h'0'
cont bct 0,loop2
mvi sw,1
RETURN (14,12),RC=0
N EQU 500000000
sw dc f'0'
ds d
a ds 12x
b ds 20x
c dc x'01020304',x'05060708',x'090a0b0c',x'0d0e0f10'
dc x'11121314',x'15161718',x'191a1b1c',x'1d1e1f20'
end
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