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Terminology:

  Codeflash - The flash chip used to store the Mailstation OS.
  Dataflash - The flash chip used to store user data 
  Slot4000  - The swappable 16KB section of address space between 0x4000 and
              0x7FFF.
  Slot8000  - The swappable 16KB section of address space between 0x8000 and 
              0xBFFF.
  Device    - One of the pieces of hardware capable of being swapped into
              the slot4000 and slot8000 sections of address space.
  Page      - A 16KB section of RAM, codeflash, or dataflash which can be
              switched into addressable space.

Devices:

  Codeflash - 29F080 1MB flash memory (64 pages)
  Dataflash - 28SF040 512KB flash memory (32 pages)
  RAM       - 128KB SRAM (8 pages)
  LCD       - 320x128 monochrome display (two separate Mailstation devices, 160x128 each)
  Modem     - Varies; sometimes RCV336ACFW 33.6kbps

Address space layout:

  0x0000 - 0x3FFF: Always codeflash page 0
  0x4000 - 0x7FFF: Variable device and page (slot4000)
  0x8000 - 0xBFFF: Variable device and page (slot8000)
  0xC000 - 0xFFFF: Always RAM page 0

CPU:

  The CPU (in everything but the Mailstation 350) is an 8-bit Z80-compatible, integrated into a 
  custom ASIC.  The part number is:
    
    RSDRD 
    3SI176 0A
  
  Some investigating shows it was actually developed by Inventec, the same company who developed
  the Z80-based CPUs in devices such as TI graphing calculators, and the Avigo PDA.  The part
  numbers as well as their internal functionality are also similar, though this chip was likely 
  designed particularly for the Mailstation.
  
  Internally it's composed of the Z80 core, address decoding, clock divider, 8-bit I/O ports, 
  various registers, and a real-time clock.  Other potential capabilities include a serial port, 
  LCD controller, and a buzzer output, though these things are fully unexplored, based solely on 
  CPU pin labels found, with no knowledge on how to interface with them at the moment.
  
  The RTC is believed to be (or based on) the TC8521, a 4-bit RTC from Toshiba with alarm and 
  timer functionality.  The time is stored by the system batteries or AC adapter, so it's lost
  when changing them or disconnecting the external power.  The RTC is also used to wake the 
  Mailstation from its dormant state for checking emails on specified intervals.
  
  The CPU can access up to 8MB of address space, although the Mailstation is only using enough
  of the available address lines to access 1MB total (the size of the codeflash).  
  
  There are seven chip-select lines, connected to the various devices listed above, though the 
  last line remains unconnected.