
How to install and use :

iceprog icebreaker.bin
iceprog -o 1M mecrisp-quintus-icebreaker-with-tools.bin
./terminal

Memory map:

$00000000 - $00000FFF:  4 kb RAM
$00001000 - $0000AFFF: 40 kb RAM, containing mirrored Flash contents
$00100000 - $00200000: SPI Flash memory, slow.

IO Registers:

  $20000010: PORTA IN
  $20000020: PORTA OUT
  $20000040: PORTA DIR
  $20000080: LEDs

  $20000100: PORTB IN
  $20000200: PORTB OUT
  $20000400: PORTB DIR

  $20001000: PORTC IN
  $20002000: PORTC OUT
  $20004000: PORTC DIR

  $20010000: UART RX / TX
  $20020000: Random, TX Flag, RX Flag
  $20040000: Cycle counter
  $20080000: Cycle counter reload value on overflow

PicoRV is configured without qreg support, and I changed the registers
for interrupt return and interrupt cause to x30 and x31, respectively.

The cycle counter counts up, triggers an interrupt on overflow and
loads the value from the reload register, which defaults to 0.
