Memory Mapping: The CPU itself builds a memoy map that lets it address (using numbers from 0x0000 to 0xFFFF) various different chips like the two SRAM shown earlier as well as memory on the cartridge/game-pak inserted , there are also many specific addresses that act as hardware registers and this is how it interfaces with the serial port, lcd display, sound synthesis etc.
Interrupts: The CPU also supports interrupts via the interrupt controller. These are triggered by special hardware events, during code execution the program-counter can jump to execute an interrupt routine to handle the event before returning back.
DMA: There is also a feature called Direct Memory Access which enables it to copy blocks of data around while executing other instructions.
Here are microscope photographs of one of these CPU chips decapped:
taken by Epop
and this fantastic image from tinytransistors (the link includes zooms of specific parts and descriptions of what they are)
- Game Boy Programming Manual
- DMG-CPU decap by Epop from wikipedia
- tinytransistors feature on the Game Boy
- not gb related http://www.cpu-world.com/forum/viewtopic.php?p=176441
- not gb related http://siliconpr0n.org/map/