* Revision 1.0 10 September 2003
'
' Ship moving smoothly and missile working correctly
'
' * Revision 1.1 10 September 2003
'
' Invaders moving left to right and down (got a problem with alignment when moving down)
' Invaders become faster as they move down
' Invader erased when missile hits, and hit counter incremented
' Bases require five hits per section before they dissapear
' There are 9 sections, three in each base
' A new sheet of invaders is created when all of them are hit
' The new sheet is slightly faster than the previous one
'
' * Revision 1.2 11 September 2003
'
' Invaders now move down the screen without alignment problems
' Started placing subroutines to enable the invaders to fire back at the ship
' Placed the base hit check as a subroutine so that the invader's missile can also
' be checked for a base hit.
' Invaders can now fire back, but cannot detect any hits yet.
' Both the ship's and the invader's missiles are slowed down but still need some work.
'
' * Revision 1.3 12 September 2003
'
' Both the ship's and the invaders missiles have independant speed controls
' Invader's missile now detect whether it has hit a base, and alters the base accordingly
' Still a problem with the invader's missile hitting the base, sometimes destroys it with one blow
' Game now detects whether the invaders have reached the bottom of the screen. In which case, GAME OVER
' If the invaders reach the bases, then the bases are disabled
' Invaders now have animated characters
' Bases also have animated characters that destroy with every hit with a missile
' The ship now only moves one pixel at a time instead of two. This slow the ship to a reasonable speed
' The invaders speed up whenever one is hit, and also speed up by a factor of 3 when there are 4 or less invaders left
' Invaders also speed up whenever they move down the screen
' Added simple sound effects when invaders move. This will be altered later.
'
' * Revision 1.4 13 September 2003
'
' Flying saucer now moves right to left at the top of the screen
' Missile to missile collisions are now detected
'
' * Revision 1.5 14 September 2003
'
' Missiles are now implemented using the PLOT command for smooth movement
' As the invaders move down the screen their missile speed slows down slightly
' Game now detects a hit on the flying saucer at the top of the screen
' There is still a problem with the invaders and the ship hitting the bases
' The base counter is decremented too quickly, or destroyed with a single hit
'
' * Revision 1.6 18 September 2003
'
' Added scoring for invaders and saucer
' Invaders score 20 points for top row, 10 points for middle row, and 5 points for bottom row
' Saucer scores 100 points if hit
' A small animation of the points scored is also included if the saucer or invaders are hit.
' Added simple sound effects for invaders, saucer and missile shot.
'
' * Revision 1.7 27 September 2003
'
' Added second missile for space invaders, but still requires some work
' because the spread of missiles is uneven.
'
' * Revision 1.8 28 September 2003
'
' Replaced sound using the SOUND command with interrupt driven sounds
' There are three independant sound channels from PORTA0,1,2
' Each is capable of producing a different frequency, and channel one (INVADER channel) also has a duration.
' With the interrupt driven sound, I have had to increase the crystal to 8MHz.
' Each layer of invader now has a different invader sprite, each consisting of two characters.
' Created new user defined shape for saucer and ship.
' Saucer now scores 100+ points if hit. More points gained the closer to the right hand side of the screen.
' Added levels to the game. After level 10 the invaders start firing two missiles.
'
' * Revision 1.9 01 October 2003
'
' Invader to Ship missile collision detection is now implemented
' Each time the ship is hit a life is decremented.
' After three lives the game is over.
' Added a splash screen when the game first starts
'
' * Revision 2 04 October 2003
'
' Score increases by one if a missile hits a missile
' If there are only 1 or 2 invaders left on the screen their speed increases dramatically
' When all the ship's lives are lost, a large circular explosion occurs.