The gameboy is having a 16bit address bus, that is used to address ROM, RAM, and I/O registers.
General Memory Map
0000-3FFF 16KB ROM Bank 00 (in cartridge, fixed at bank 00) 4000-7FFF 16KB ROM Bank 01..NN (in cartridge, switchable bank number) 8000-9FFF 8KB Video RAM (VRAM) (switchable bank 0-1 in CGB Mode) A000-BFFF 8KB External RAM (in cartridge, switchable bank, if any) C000-CFFF 4KB Work RAM Bank 0 (WRAM) D000-DFFF 4KB Work RAM Bank 1 (WRAM) (switchable bank 1-7 in CGB Mode) E000-FDFF Same as C000-DDFF (ECHO) (typically not used) FE00-FE9F Sprite Attribute Table (OAM) FEA0-FEFF Not Usable FF00-FF7F I/O Ports FF80-FFFE High RAM (HRAM) FFFF Interrupt Enable Register
Jump Vectors in First ROM Bank
The following addresses are supposed to be used as jump vectors:
0000,0008,0010,0018,0020,0028,0030,0038 for RST commands 0040,0048,0050,0058,0060 for Interrupts
However, the memory may be used for any other purpose in case that your program doesn't use any (or only some) RST commands or Interrupts. RST commands are 1-byte opcodes that work similiar to CALL opcodes, except that the destination address is fixed.
Cartridge Header in First ROM Bank
The memory at 0100-014F contains the cartridge header. This area contains information about the program, its entry point, checksums, information about the used MBC chip, the ROM and RAM sizes, etc. Most of the bytes in this area are required to be specified correctly. For more information read the chapter about The Cartridge Header.
External Memory and Hardware
The areas from 0000-7FFF and A000-BFFF may be used to connect external hardware. The first area is typically used to address ROM (read only, of course), cartridges with Memory Bank Controllers (MBCs) are additionally using this area to output data (write only) to the MBC chip. The second area is often used to address external RAM, or to address other external hardware (Real Time Clock, etc). External memory is often battery buffered, and may hold saved game positions and high score tables (etc.) even when the gameboy is turned of, or when the cartridge is removed. For specific information read the chapter about Memory Bank Controllers.