The Multiplatform series covers general 8086 code which will work on any system. Each lesson will cover a concept, and provide code which you can use in your project if you wish

Lesson M1 - Random Numbers and Ranges Let's write some simple routines we may need in a game! We'll write a psuedo random number generator, which can create repeatable 16 bit random numbers. We'll also create a 'Range checker' which can be used as a collision detection routine for unsigned 8 bit numbers

The Random number generator shown today is far from the 'Best'... but it does work! It's used in these tutorials by all the games as it's been tested without any problems, and has been ported to many different systems and CPUs, and given the same random seed, it gives the same results... meaning it can be used for generating random levels in your game, and the level will be the same on every system.

Random number generation

To allow our random number generator to create good varied results, we use a pair of 16 byte lookup tables
Our random number generator produces a 16 bit result (DX) from a 16 bit random seed (BX) The 'High byte' of the 16 bit pair is created by bitshifting and combining the two.
We need the 'Low byte' to be very different, or the 16 bit values won't be very random! We use the lookup tables to get some new data, and combine these to produce the second value
We have two routines we can use to get values "DoRandomWord" will get a 16 bit value DX from seed CX, this can be used to produce 'psuedorandom' data for times we want repeatable results. "DoRandom" is an easy function to return an 8 bit byte in the AL accumulator... it automatically updates its seed every time.

To be usable, a random number generator needs to produce every possible random number (0-255 or 0-65535)... otherwise you could have serious problems with your program (if you're waiting for a result that never happens). The more 'random' the data the better... that is, if you plotted the values on a graph there should be no patterns present.

Range checking

We need to do 'Collision detection'. We specify a two targets and a 'range' ... if target 1 is within 'Range' of target 2, we return A=1... if it's out of range we return A=0... This means we can do a BNE or BEQ on return.

The range check is simple... we test X and Y axis We test the X axis first. we subtract the 'RANGE' from each axis of the object, and check if we went below Zero (if so that direction is over the limit)... we then compare to the position... if it's lower we're out of range... if not we need to test more! Next we add 'RANGE' twice... once to move back to the center and once to move to the right... we then compare to the position... If it's higher we're out of range... if not we need to test the Y axis! We repeat the same tests for the Y axis.

Lesson M2 - Binary Coded Decimal Working in HEX is great for programming, but converting 16 or 32 bit hex to numbers for display is a pain! We'll want to show an 8 digit 'highscore' in our games... and for this we'll use Binary Coded Decimal!

BCDTest_ASCII.asm BCDTest_PACKED.asm