// petulant.60p - The PETulant Cursor, a "display hack" for the Commodore 64
// Originally written by Chris Pressey sometime in the late 1980's
// Rewritten in P65 assembly and released March 2008
// Rewritten in SixtyPical in March 2018
// This work is part of the public domain.

// ----- Types -----

typedef routine
  inputs border, blnon, color, background
  outputs border
  trashes a, z, n
    irq_handler

// ----- Addresses -----

vector irq_handler cinv @ $314  // hw irq interrupt, 60x per second

byte blnon            @ $cf     // was last cursor blink on or off?
byte color            @ $286    // current foreground colour for text

byte border           @ $d020   // colour of border of the screen
byte background       @ $d021   // colour of background of the screen

// ----- Variables -----

vector irq_handler save_cinv

// ----- Interrupt Handler -----

define our_cinv irq_handler
{
    ld a, blnon
    if not z {
        ld a, color
    } else {
        ld a, background
    }
    st a, border
    goto save_cinv
}

// ----- Main Program -----

define main routine
  inputs cinv
  outputs cinv, save_cinv
  trashes a, n, z
{
    with interrupts off {
        copy cinv, save_cinv
        copy our_cinv, cinv
    }
}