This sample program draws dot, line and rectangle.

#include <psx.h>
#include <stdio.h>

unsigned int prim_list[0x4000];
unsigned short padbuf;

volatile int display_is_old = 1;
volatile int time_counter = 0;
int  dbuf=0, hx=0, hy=0;

void prog_vblank_handler() {
    display_is_old = 1;
    time_counter++;
}

int main() {
    PSX_Init();
    GsInit();
    GsSetList(prim_list);
    GsClearMem();
    GsSetVideoMode(320, 240, EXAMPLES_VMODE);
    SetVBlankHandler(prog_vblank_handler);

    while(1) {
        if(display_is_old)  {
            dbuf=!dbuf;
            GsSetDispEnvSimple(0, dbuf ? 0 : 256);
            GsSetDrawEnvSimple(0, dbuf ? 256 : 0, 320, 240);
            GsSortCls(0,0,0);

            // paint start

            // draw dot
            GsDot dot;
            dot.r = 255; dot.g = 255; dot.b = 255;
            dot.x = 10; dot.y = 10;
            dot.attribute = 0;
            GsSortDot(&dot);

            // draw line
            GsLine line;
            line.r = 255; line.g = 255; line.b = 255;
            line.x[0] = 10; line.y[0] = 15;
            line.x[1] = 200; line.y[1] = 35;
            line.attribute = 0;
            GsSortLine(&line);

            // draw rectangle
            GsRectangle rectangle;
            rectangle.r = 255; rectangle.g = 255; rectangle.b = 255;
            rectangle.x = 10; rectangle.y = 60;
            rectangle.w = 50; rectangle.h = 50;
            rectangle.attribute = 0;
            GsSortRectangle(&rectangle);

            // paint end

            GsDrawList();
            while(GsIsDrawing());

            display_is_old=0;
        }
    }

    return 0;
}