I/O Interface
-------------

The I/O interface is character-based and memory-mapped.  Three 16-bit
word-addressable memory addresses are used: 1) the input/output
control register, REG_IOCONTROL, 2) the input/output character buffer
#1, REG_IOBUFFER_1, and 3) the input/output character buffer #2,
REG_IOBUFFER_2.  The three memory addresses may also be read and/or
written using byte-mode operations rather than word-mode operations.


The memory layout is as follows:

Address   Name             Description
0x00FF00  REG_IOCONTROL    Read/write control register
0x00FF04  REG_IOBUFFER_1   I/O buffer
0x00FF08  REG_IOBUFFER_2   I/O buffer


The bits read from REG_IOCONTROL are:

Bit  Name                    Description
0    BIT_SERIAL_INPUTREADY   If set, there is a character ready to be read
     			     from the serial port device (RS-232) via
			     REG_IOBUFFER_1, i.e., the user has sent at
			     least one character to the Altera DE2-70 (or
			     DE2-115) via the serial port.

1    BIT_SERIAL_OUTPUTREADY  If set, the serial port device (RS-232) is ready
     			     to accept a character by writing that character
			     to REG_IOBUFFER_1.  After the character is
			     written to REG_IOBUFFER_1, it will be
			     transmitted from the Altera DE2-70 (or DE2-115)
			     via the serial port.

2    BIT_PS2_INPUTREADY      If set, there is a character ready to be read
     			     from the PS/2 port device via
			     REG_IOBUFFER_2, i.e., the user has sent at
			     least one character to the Altera DE2-70 (or
			     DE2-115) via the PS/2 port.

3    BIT_LCD_OUTPUTREADY     If set, the LCD device is ready to accept a
     			     character by writing that character
			     to REG_IOBUFFER_2.  After the character is
			     written to REG_IOBUFFER_2, it will be
			     displayed on the Altera DE2-70's (or DE2-115's)
			     LCD display.


The bits written to REG_IOCONTROL are:

Bit  Name                    Description
0    BIT_SERIAL_INPUTFLUSH   Flush the serial port (RS-232) input queue
     			     when this bit is set.  That is, delete
			     any characters in the serial port input
			     queue (those that have been received
			     (i.e., entered or typed at the
			     peripheral), but not already read).  This
			     operation causes characters in the input
			     queue (that have not already been read)
			     to *never* be read.

1    BIT_SERIAL_OUTPUTFLUSH  Flush the serial port (RS-232) output queue
     			     when this bit is set.  That is, delete
			     any characters in the serial port output
			     queue (those that have been successfully
			     written to REG_IOBUFFER_1, but not
			     already sent via the serial port).  This
			     operation causes characters in the output
			     queue (that have not already been
			     written) to *never* be written.

2    BIT_PS2_INPUTFLUSH      Flush the PS/2 port input queue when this
     			     bit is set.  That is, delete any
			     characters in the PS/2 port input queue
			     (those that have been received (i.e.,
			     entered or typed at the peripheral), but
			     not already read).  This operation causes
			     characters in the input queue (that have
			     not already been read) to *never* be
			     read.

3    BIT_LCD_OUTPUTFLUSH     Flush the LCD device output queue when this
     			     bit is set.  That is, delete any
			     characters in the LCD device output queue
			     (those that have been successfully
			     written to REG_IOBUFFER_2, but not
			     already sent to the LCD device).  This
			     operation causes characters in the output
			     queue (that have not already been
			     written) to *never* be written.


Note: "read" bits are set by the memory subsystem, "write" bits change
the state of the I/O device when the processor writes onto them.
Undocumented bits are reserved for future expansion and they should
not be set; therefore, all unused bits should be set to zero when the
REG_IOCONTROL register is written.  Writing a one bit to them may
cause indeterminate behavior.

Reading REG_IOBUFFER_1 when the BIT_SERIAL_INPUTREADY bit in the
REG_IOCONTROL register is not set produces undefined results.

Writing to REG_IOBUFFER_1 when the BIT_SERIAL_OUTPUTREADY bit in the
REG_IOCONTROL register is not set produces undefined results.

Reading REG_IOBUFFER_2 when the BIT_PS2_INPUTREADY bit in the
REG_IOCONTROL register is not set produces undefined results.

Writing to REG_IOBUFFER_2 when the BIT_LCD_OUTPUTREADY bit in the
REG_IOCONTROL register is not set produces undefined results.