That CPU halting only applies for PGB mode 1. In PGB mode 2 the CPU is still able to run (within a limited address range) and can use register FF75 "PGBIO" for

That CPU halting only applies for PGB mode 1. In PGB mode 2 the CPU is still able to run (within a limited address range) and can use register FF75 "PGBIO" for limited input and output on some cartridge pins (and/or the link port for IO).