Reorganized all code to wait for VBlank, this is before simplifying with VCount

main.c

/* 
 * Main written by Seth Tenembaum beginning Jan 2, 2017
 * 
 * The purpose of this document is to come to a better understanding of the fundamentals of game developmeny by building a simple platformer from scratch at a low level (but not quite down to assembly at the moment).
 * 
 * TODO
 * - fix issue when holding left and right (actually not possible on physical hardware...)
 * - subpixel positioning and detach movement speed from frame rate
 * - add a single platform
 * - implement a global coordinate system, array of platform positions and sizes, and a camera
 * 
 */

#include "gba.h" //memory addresses and type shortcuts
#include "screenmode.h" //video mode bits and SetMode macro
#include "keypad.h" //buttons and input register
#include "gfx.h" //RGB macro, sleep, random

s32 xpos = 10, // must be signed
    ypos = 100,
    xvel = 0,
    yvel = 0,
    xacc = 0,
    yacc = 0;

const s32 GRAV  = 1,
          FLOOR = 30,
          SPEED = 7,
          JUMP  = 10;

void gameLoop();

int main(void) {
    SetMode(MODE_3 | BG2_ENABLE); //screenmode.h
    // 240 px wide, 160 px tall

    // draw floor
    word i;
    for (i = 0; i < 240; i++)
        (VideoBuffer)[(160-FLOOR)*240+i] = RGB(0,10,31);

    word state = 0; // 0 = waiting for VBlank, 1 = waiting for VDraw

    while (1) {

        if (REG_DISPSTAT & 0x01) { // VBlank
            if (state == 0) {
                gameLoop();
                state = 1;
            }
        } else { // VDraw
            state = 0;
        }

    }
}

void gameLoop() {
    // erase old sprite
    drawRect(xpos-5, 160-(ypos+10), 10, 10, 0, 0, 0);

    // get input
    if (KEYS & KEY_LEFT)
        xacc--;
    if (KEYS & KEY_RIGHT)
        xacc++;
    if (!(KEYS & (KEY_LEFT|KEY_RIGHT))) // decelerate if neither pressed
        if (xvel > 0)
            xacc = -1;
        else if (xvel < 0)
            xacc = 1;
        else
            xacc = 0;
    if (KEYS & KEY_UP && ypos == FLOOR) // only jump if on floor
        yvel = JUMP;

    // simulate
    yvel -= GRAV; // apply gravity

    xvel += xacc; // apply acceleration
    yvel += yacc;

    if (xvel > SPEED) // enfore maximum velocities
        xvel = SPEED;
    if (xvel < -1*SPEED)
       xvel = -1*SPEED;

    // apply velocity
    if (ypos > FLOOR)
        xpos += 0.7*xvel; // air resistance
    else
        xpos += xvel; 
    ypos += yvel;

    if (xpos > 240) // wrap x-coordinate
        xpos -= 240;
    if (xpos < 0)
        xpos += 240;
    if (ypos < FLOOR) { // pop out of floor
        ypos = FLOOR;
        yvel = 0;
    }

    // draw sprite
    drawRect(xpos-5, 160-(ypos+10), 10, 10, 31, 31, 31);
}