63981: Initialise an uninterruptible subcommand if possible
Used by the routines at 64162 and 64338. Places the address of an uninterruptible subcommand routine into bytes 18 and 19 of a character's buffer if there's not one there already, and returns with the zero flag set in that case.
Input
A
Character number (215-229)
BC
Uninterruptible subcommand routine address
Output
63981
PUSH HL
63982
LD H,A
Point HL at byte 19 of the character's buffer
63983
LD L,19
63985
LD A,(HL)
Is there already an uninterruptible subcommand routine address MSB there?
63986
AND A
63987
JR NZ,63992
Jump if so
63989
LD (HL),B
Otherwise copy the routine address from BC into bytes 18 and 19 of the character's buffer, thus setting his uninterruptible subcommand