There is a 256 byte limitation when moving
character data with MVC. If a field is
larger than 256 bytes, one solution to moving it in memory is to divide it into
multiple parts and code multiple MVC’s. A more elegant solution involves the Move (Characters)
Long instruction (MVCL) which was designed for fields longer than 256
bytes. In fact, up to 224 =
16,777,216 bytes can be moved with one MVCL instruction. This instruction is unusual because it
requires the use of four registers.
Operand 1 and operand 2, denoted by R1 and R2, each represent the even
register of an even-odd consecutive register pair (2-3, 4-5, 6-7, …). For example,
if you coded “MVCL R4,R8” the registers involved would be R4, R5, R8,
and R9. The even registers contain the
address of target and source fields, while the odd registers contain the
lengths of target and source fields. Additionally,
a pad character can be place in the high order byte of the odd register which
contains the length of the source field. The pad character is used to fill up
the target field when the source field is shorter and all of its contents have
been transmitted. Here is a simple
example in which we want to move FIELDB to FIELDA:
LA R4,FIELDA POINT AT TARGET FIELD WITH EVEN REG
L R5,LENGTHA PUT LENGTH OF TARGET IN ODD REG
LA R6,FIELDB POINT AT SOURCE FIELD WITH EVEN REG
L R7,LENGTHB PUT
LENGTH OF SOURCE IN ODD REG
ICM R7,B’1000’,BLANK INSERT A SINGLE BLANK PAD CHAR IN ODD REG
MVC R4,R6
…
FIELDA DC CL2000’ ’
FIELDB DC
CL1000’X’
LENGTHA DC
A(L’FIELDA) CREATE AN
ADDRESS CONSTANT THAT IS A LENGTH
LENGTHB DC
A(L’FIELDB) CREATE AN
ADDRESS CONSTANT THAT IS A LENGTH
BLANK DC
C’ ’
The above
code copies 1000 X’s from FIELDB into FIELDA.
Since FIELDA was larger than FIELDB, the last 1000 bytes of FIELDA are
filled with the blank pad character. If
the length of FIELDA were equal to or smaller than the length of FIELDB, the
pad character would not be used.
Bytes are transferred one at a time in a
fashion similar to MVC, by MVCL. As each
byte is copied from source to target, the even registers are incremented by one
and the odd registers are decremented by one until one of the lengths in the
odd registers becomes zero.
Consider the following scenarios with MVCL
in which we are copying from field A to field B. Assume the length of A is L1 and the length
of B is L2 .
Further assume A(A) and A(B) are the addresses
of A and B. Lengths are specified in
decimal and we assume a blank pad character in R7.
Case 1: L1 > L2
Before execution:
R4 R5 R6 R7
A(A)
1000
A(B) 500
After execution:
R4 R5 R6 R7
A(A) + 1000
0 A(B) + 500 40
0
Case 2:
L1 = L2
Before execution:
R4
R5 R6 R7
A(A) 1000 A(B) 1000
After execution:
R4 R5 R6 R7
A(A) + 1000
0 A(B) +
1000 40 0
Case 3:
L1 < L2
Before execution:
R4 R5 R6 R7
A(A)
500
A(B)
1000
After execution:
R4 R5 R6 R7
A(A) + 500
0 A(B) + 500 40 500
The pad
byte is only used in case 1 where 500 blank characters are added to
FIELDB. In case 2, all 1000 characters
of FIELDB are copied to FIELDA. In case
3, only the first 500 bytes of FEILDA are copied to FIELDB.
A destructive overlap occurs when the target
and source fields overlap and the address of the source is lower than the
address of the target. This is
illustrated below.
MVCL detects destructive overlap conditions
and prevents any bytes from being moved.
Instead, MVCL sets the condition code to indicate destructive
overlap. This condition can be tested
with BO.
MVCL sets the condition code to 0 (equal) if
L1 = L2, to 1 (low) if L1 < L2, to 2 (high) if L1 > L2, and 3 (overflow)
if no bytes are moved due to destructive overlap.
You can use MVCL to blank out a target field
by setting the length of the source to 0.
This forces the pad character to be copied to each byte of the source:
LA R8,TARGET
L R9,TARLEN
LA R4,SOURCE SOURCE DOESN’T PARTICIPATE
LA R5,0 SET THE LENGTH OF SOURCE TO 0
ICM R5,B’1000’,BLANK SET PAD BY TO A BLANK
MVCL R8,R4 COPY
BLANKS
- It is often difficult to find
four available registers when coding MVCL.
A simple solution to this problem is to store four consecutive
registers in a save area before using MVCL and restore them afterwards:
STM R4,R7,SAVE4
(use MVCL here…)
LM R4,R7,SAVE4
…
SAVE4 DS 4F FOUR FULLWORDS FOR R4,R5,R6, AND R7
- Use length attributes for the
lengths of the source and target fields:
LENGTHA DC
A(L’FIELDA) TURN THE LENGTH
INTO A FULLWORD “ADDRESS”