Brian Lewis saves Harrisburg from nuclear catastrophe

Jump Jet Assault is a scrolling game written for the Acorn Electron and originally published in Your Computer Vol. 5 No. 7 (July 1985). In it, you have to defend Three Mile Island from the onslaught of enemy aircraft. To defend against the invaders you are armed with a Harrier Jump Jet which has the air-to-air missile capacity of 50. Once you have fired all your missiles, or if at any time you want to reload, you simply land your Harrier on a building similar to the one you started from.

Your actual fighting area is eight times larger than the screen. However, enemy planes have a wrap-around capacity which allows them to keep flying while you have run out of air space. You lose a life if you crash into the horizon, crash into an enemy plane or get hit by one of their bombs. However, as you are defending Three Mile Island there is a greater danger than losing your three lives - the danger of a nuclear explosion! On the main screen display you will see the word "Techs" and beneath it a number of small figures. If the enemy planes drop a bomb on your fuel dump building you lose a technician; lose all your techs and the resulting nuclear blast destroys everything, including your remaining lives.

To further the difficulty of the task set for you the enemy has invented a new heat seeking missile which, unlike their attack planes can travel faster than you. This missile only appears after you have killed three enemy planes without losing a life.

This is one of the few scrolling games written for the Electron - the lack of scrolling games is mainly due to the large size of the screen memory. BBC users can get over this problem by using the 6845 chip - not present in the Electron - control full screen scrolling via registers 12 and 13. To counter this problem, the author has used a screen window of exactly 32 bytes wide, which enables fast and easy handling of the screen display since, when displaying the contents of the window the computer does not have an X position number larger than 255 thus enabling a simple loop involving only one of the computer's registers.

There are two listings. The first program is the main machine-code program containing most of the graphics, sound definitions and all the machine-code routines while the second handles setting up the screen display, keeping the score, storing the high score table, etc. The program is quite long but the finished product gives some pleasing screen displays and presents a very playable game.

If BBC users wish to play Jump Jet Assault then they merely have to increase the contents of the x and y registers in line 460 of program 1 to a suitable value found through experiment.

Brian Lewis