Nuts and Volts - October 2015

Vintage Computing

One of the earliest things we do in the code is to see the random number generator with this line of code:

notepad random.c
 srand( PEEKW(0x00a1));

The “jiffy” clock that is updated by interrupt routines is stored at locations $A0, $A1, and $A2 in zero page memory. This is the clock used by the BASIC meta-variable TIME$. In the line of code above, I use the last two bytes of the clock as an unsigned int initializer for srand() since we’re unlikely to have the same values there each time we run the program.

FIGURE 7. Changing the screen colors.

Random.c contains two function prototypes near the beginning of the program:

int roll one die(int);
int roll dice();

The colorhello.c program first sets the border and background colors to black. I used the hexadecimal constants 0xd020 and 0xd021 because any number over 32767 expressed in decimal is assumed to be a “long” by the compiler.

This allows functions that appear earlier in the code to understand the parameter list requirements and return values of each function.

Note that I used a decimal constant for location 646 which contains the current character color.

When you load and run the colorhello program, you should see something like the screen in Figure 7.

Our function roll_one_die() accepts an integer parameter. This parameter is named “sides” in the body of the function code itself. The variables side’s visibility is limited to use within the function roll_one_die(). Parameters are local variables.

If you try to LIST the program, you’ll see a line similar to the following:

704 sys2061

The above is a BASIC stub that’s just long enough to invoke the compiled machine-language payload that follows it in memory.

The function roll_one_die() is intended to simulate a singe die roll. Many role-playing games use dice with varying numbers of sides. In the code, we call roll_one_die() with a parameter of 6 when we want to simulate rolling a six-sided die. We call the same function with a parameter of 20 when we want to simulate a 20- sided die.

Fun and Games

The function roll_dice() simulates rolling two six-sided dice. Note that it calls roll_one_die() twice and then returns the sum of those values.

Part of the fun of owning a microcomputer in the 1980s was typing in games from listings in books and magazines. Let’s take a look at a few simple game-related programs.

Also note that in the body of main(), I issue a “return” command with the value zero. I did not do this in earlier examples and received no warnings from the compiler. It’s good form to supply the return value from main().

Most single-player games require the use of a random number generator function. CC65 provides the rand() function which returns a random unsigned integer in the range of 0 to 32767 inclusive. The randomizer should be seeded by the srand() function at the beginning of a program to ensure that the series of random numbers will be different each time the program is executed.

Let’s compile and run random.c. From a command prompt, enter the following:

cc random

You should see the verbose compilation messages.

Then, load and run “random” from WinVICE. You might see output similar to that in Figure 8.

Let’s take a look at random.c. From a command prompt in the C:\c64\dev directory, enter the following: If you run it again just by typing the word “run” and enter, you should see different sets of random dice rolls.

Let’s take a look at guess.c:

notepad guess.c

The random.c code isn’t a game by itself, but it illustrates a simulation of dice rolls. In addition, it illustrates a few CC65 concepts that we can study.