since the machine-code for MVC moves up to 256 bytes, you would need to do a series of MVCs to initialize 20000 bytes. this requires that you maintain a register or two to keep track of how far you've progressed through initialization.
possibly, you could get MVCL to do it; i've never tried...
MVCL uses 2 sets of even-odd pairs of registers to do the move.
you specify source address, destination address, length of source, length of destination, and fill character in the registers. the fill-character goes into the high order byte of (I THINK...) the destination length register (in this case r4)
To initialize a 20000 byte field you have to use MVCL
instruction. Let's say VAR1 is defined as " VAR1 DS XL20000"
For using MVCL instruction you require two even-odd regiter
pair, even registers will contain the destination and
source address respectively, first odd register will
contain the destination leght and from 2nd to 4th byte of
the second odd register will contain the source length and
the first byte will contain filler(it will come into play
if destination lenghth is shorter than source). Now if we
want to initialize VAR1 with spaces we have to code like
Here source length is 0 and destination length is 20000. So
no move ment will occur and all the 20000 byte will be
filled by the filler wich is space here. So as result of
this instruction all 20000 byte will be initialized by