ES segment of an allocated memory block
BX desired size for the block, in paragraphs (16-byte chunks)
BX largest block available (if a "grow" request fails)
Info: Changes the size of an existing memory block.
When a program receives control, the 4bH EXEC function has
allocated a memory block starting at the PSP that contains all of
available memory. To make memory available to EXEC a child
process, shrink the block starting at your PSP paragraph.
.COM programs: Move your stack down BEFORE shrinking your