Hello, Display!

In this chapter, we're going to learn the basics of compiling and testing a WonderSwan homebrew project; by the end of it, we will write basic code to display something on the screen.

First, set up a new project: 

$ mkdir 1-hellodisplay/ # (1)!
$ wf-wswantool project new 1-hellodisplay/ # (2)!
  1. Create the directory “1-hellodisplay” in the terminal's current working directory.
  2. Create a new wswan targetting project with the directory and name “1-hellodisplay”.

Next, compile the project: 

$ cd 1-hellodisplay/
$ make

This will output the following lines: 

CC      src/main.c
LD      build/1-hellodisplay_stage1.elf
ROM     1-hellodisplay.wsc
MERGE   compile_commands.json
  1. The C compiler is compiling the source file src/main.c.
  2. The ELF file build/1-hellodisplay_stage1.elf is being linked. This contains all of the program's source code and assets, but not yet at the correct memory locations.
  3. The ROM file 1-hellodisplay.wsc is being created, now with the correct memory locations.
  4. The compile_commands.json file is being generated. This is used by your IDE to provide syntax highlighting; as a consequence, it is a good idea to run `make` after adding a new .c file to the project.

Finally, you can run the ROM file using your emulator of choice. However, it's not displaying anything - after all, there's no code written yet.

Let's examine the src/main.c file: 

// SPDX-License-Identifier: CC0-1.0
//
// SPDX-FileContributor: Adrian "asie" Siekierka, 2023
// (1)!
#include <wonderful.h> // (2)!
#include <ws.h> // (3)!

void main(void) {
    while(1);
}

  1. The default copyright header for the template file. This is required to inform you that you can use the example code contained without restrictions. However, before writing your own code, if you wish to put different terms on your code, you should remove it.
  2. The wonderful.h include contains common CPU/target definitions. It's a good idea to put it in essentially every C file.
  3. The ws.h include contains hardware definitions specific to the WonderSwan, as well as many lower-level hardware abstraction functions.

Pay special attention to the `while(1);` at the end - this is an infinite loop.

You may be used to ending `main()` with a `return;` of some type. On bare metal, however, one should not return from main, as there's no operating system to return to. Removing it will cause the system to jump to arbitrary code with unforeseen consequences!

With only an infinite loop in main, it's clear that the code won't do anything. Let's make it do something!

For now, we're going to assume a “mono” program - we'll introduce the color mode in a later chapter. On the “mono” WonderSwan, the palette pipeline that converts tile information to display shades consists of two parts: the palette, mapping each of the four possible palette indexes to one of eight color values; and a shade look-up table, mapping each of the color values to one of the sixteen total shades which the panel can display.

graph LR A[Palette index
one of four colors] -->|palette
one of sixteen| B[Color value
one of eight values] B -->|shade look-up table
global| C[Shade
one of sixteen values]

First, we need to initialize the shade look-up table. At the beginning of main, add the following function call: 

ws_display_set_shade_lut(SHADE_LUT_DEFAULT);

This sets a default table, with the color value 0 corresponding to the brightest shade (0) and 7 to the darkest shade (15).

As the “mono” WonderSwan's panel is a monochrome LCD, it's shades with higher values which are darker; note that the opposite is true for the WonderSwan Color's RGB values.

Next, we need to enable the display. We're going to enable the “screen 1” layer - as the WonderSwan's memory is zeroed by default, this should not cause anything to be drawn to the screen, but will enable viewing the background color - which, by default, is the first of eight color values: 

outportw(IO_DISPLAY_CTRL, DISPLAY_SCR1_ENABLE);

Finally, we're going to do some changes to the display. Edit the while(1) loop as follows to cycle the first palette: 

while (1) {
    outportw(IO_SCR_PAL_0, inportw(IO_SCR_PAL_0) + 1); // (1)!
    ws_busywait(65535); // (2)!
}

  1. This loads the first palette, adds one to it, and saves it. As the lowest bits constitute the color value for the first palette index, this will cause the background color to change on every execution.
  2. As we don't have any interrupts set up yet, the only thing we can do is busy-wait - that is, stall the CPU for a specified number of microseconds. While this is not recommended for production code, it's simple enough to prevent rapid flicker in the demonstration.

That's all! Compile the code using make and run it in your emulator of choice. If done correctly, you should see the display slowly change colors from brightest to darkest and back again.