Lunar V 

Stan Ockers, Illionois, U.S.A.

Issue 4

Jul/Aug 83

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With Player Missile Graphics it is possible to move images you create around the screen independent of what is displayed as background. This is done with very little calculation. Changing a single memory location will move the entire image horizontally while moving bytes up and down in memory will move the image vertically.

I've developed a set of machine language routines which can be called from Basic to assist in missile graphic movement. These are not necessarily the best and certainly not the only way of doing missile graphics, but I hope what I've come up with will help others in developing programs.

I'll cover only enough to get started in this article. I placed the routines in the unused portion of memory in page six and have reserved some locations at the top of page six. You often have to remember vertical and horizontal positions of players. Table 1 lists locations for this. Also listed are locations storing pointers to areas of memory used to hold player images. These have to be calculated because they depend on the size of memory in your machine.

	MISS	Plyr. 3	Plyr. 2	Plyr. 1	Plyr. 0
Horizontal Position	1788 53252	1787 53251	1786 53250	1785 53249	1784 53248
Vertical Position	1783	1782	1781	1780	1779
High Byte Memory Vector	1778	1777	1776	1775	1774
Low Byte Memory Vector	1773	1772	1771	1770	1769

Perhaps the best way to explain what is going on is through an actual program. The listing is from a program called Lunar V. The machine language subroutines are poked into memory in lines 30-44. Line 36 contains a subroutine used to load zero page locations OOCB and OOCC with the start of memory assigned to player X. Players are numbered 0 to 4 with player 4 being missiles. At line 38 is a routine which places bytes representing player X into memory at the appropriate spot. It is called using the USR function in the form A=USR(1606,X,ADDR). ADDR is the decimal address where player image bytes are stored. The routine will keep loading bytes until a zero byte is found. It picks up the vertical position (increasing down from the top of the screen) from 1779,X. The load player routine does not erase what is already there. This is done by a routine poked in at line 44 and called by A=USR(1706,X).

Major routines move the player vertically. A=USR(1637,X) poked into memory by lines 40 and 41, moves him downscreen and A=USR(1673,X) in lines 42 and 43 move him upscreen. Traps are included to keep from roaming into another player's area of memory. These can also be used to limit the vertical motion of players. Change the 126 at 1678 (make it lower), or the 1 at 1683 (make it higher).

Setting pointers for player memory is done in line 50. These are for double line resolution meaning each byte will cover two scan lines on the TV. You could get smoother motion using single line resolution, but double line resolution is simpler for now. The 4 page gap (1 K) between the end of player memory locations and the end of RAM is used by the operating system for display purposes, the amount used depends on the graphics mode.

Lines 60 and 63 poke data for images into memory. The images (in order) are the lander, the flame and the crater. Various portions of the flame are used depending on the thrust (see line 195). Images are stored in an unused portion of memory at the beginning of the missile graphics area.

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I believe that Lunar V was Stan Ocker's first program submitted to the A.C.E. Newsletter in June 1981. Stan has come a long way since then but I hope that this article and program will provide readers who do not understand yet about Vertical Blank Interrupts, with a better understanding of player missile graphics. See if you can improve the program, there is plenty of scope, and send in your revised efforts.

AtariLister - requires Java

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