Line By Line
In an earlier post, I mentioned that the video memory in the Apple II has a peculiar layout. For example, the data for line six is offset in memory from the data for line seven by a different amount than the offset between the data for lines seven and eight. This makes moving an object on the screen from line to line more complicated than simply adding or subtracting a fixed offset from the previous address.
Previously I wrote a routine that would convert a screen line number to a base address in video memory. I have since rewritten that routine by replacing a shift-and-add multiplication with some bitwise operations, thereby improving its performance. Nevertheless, the routine is long enough that I would like to limit how much it is used.
I was able to implement somewhat simpler routines for moving up or down one line at a time. These routines combine simple offset adjustments with checks and adjustments for crossing 8-line boundaries. The results are somewhat simpler than the direct calculation routine, with corresponding performance improvements. A single-line offset is simple enough to be practical and common enough to be useful, especially for Fahrfall.
The 6502 only has a handful of registers for use by the programmer, none of which are large enough to hold an actual address. Addresses for data access can be embedded in the program code, but either that would limit a given function to accessing only predetermined memory locations or it would require the use of self-modifying code that rewrites the addresses whenever different memory locations are used -- ugh!
Fortunately, the 6502 does provide a couple of means for storing an address in memory and then using that address as the object of an indirect memory access. Unfortunately, those options require storing the address somewhere within the 256-byte "zero page". Those locations are in high demand, and the Apple II firmware pre-allocates most of that memory for its own use. Fortunately I was able to find enough space to fill my immediate need for a drawing pointer.
When I only had platforms on the screen, I could get by with a single drawing pointer. Even though there are three rows of platforms, their relative positions makes it easy to adjust the drawing pointer to point to each platform in turn. Unfortunately, the player object needs its own drawing pointer value which is independent of the platform drawing pointer. Of course, the player drawing pointer needs the same sorts of manipulations as the platform drawing pointer needs.
One option would be to use separate "zero page" locations for each pointer and to duplicate the pointer manipulation routines for the player object drawing pointer. A better alternative is to use a single "zero page" location for the active drawing pointer and to save/restore this value to/from separate memory locations for different uses. The second option effectively treats the "zero page" drawing pointer as a 16-bit register, providing me with the flexibility I need for what I want to do.
Ode To Joy
Fahrfall is an arcade-style game, so it makes sense to use a joystick to play the game. The joysticks on the Apple II use an analog circuit based on an RC time delay. The actual timing is done at the CPU by counting the number of cycles that are spent executing a tight loop while a capacitor is charged. Coding such a loop seems simple enough, but doing so is unnecessary -- the Apple II firmware already contains such a function at a well-known location.
Reviewing the actual code of the firmware's joystick reading function reveals that there is little opportunity for improving its performance. Consequently, I used the Apple II firmware to read the Y-axis of the joystick. This returns an 8-bit number which represents the position of the joystick on that axis. Fahrfall only needs to know the general direction for movement, not the actual position. So, I added code to take the number representing the joystick position and to test it for left and right ranges. I also included a dead zone in the middle to represent "no movement".
Well, that covers a few micro-topics related to my port of Fahrfall to the Apple II. I also finished reading Assembly Lines: The Complete Book, so I guess I am now a trained Apple II programmer...
Things are coming together, but there is still more to come. I hope you will stay tuned!