Commander X16 YM2151 player

stardust.c

// ym2151 basic player by Barry Yost - sep 25, 2019
// original original tune "Stardust" from C64 game "Lazy Jones"
// ported to C by Frank Buss

#include <stdint.h>
#include <6502.h>

struct YM2151_t {
    uint8_t reg;
    uint8_t data;
};

static uint8_t ofs;
static uint8_t b;
static uint8_t bp;
static uint8_t delay;

static uint8_t patch[] = {
    0x20, 0xe4,
    0x38, 0x00,
    // patches for operator 0
    0x40, 0x31,
    0x60, 0x07,
    0x80, 0x1f,
    0xa0, 0x09,
    0xc0, 0x00,
    0xe0, 0xff,
    // patches for operator 1
    0x48, 0x31,
    0x68, 0x11,
    0x88, 0x1f,
    0xa8, 0x09,
    0xc8, 0x00,
    0xe8, 0xff,
    // patches for operator 2
    0x50, 0x31,
    0x70, 0x08,
    0x90, 0x1f,
    0xb0, 0x09,
    0xd0, 0x00,
    0xf0, 0xff,
    // patches for operator 3
    0x58, 0x30,
    0x78, 0x24,
    0x98, 0x1f,
    0xb8, 0x09,
    0xd8, 0x00,
    0xf8 ,0xff,
    0
};

uint8_t notes[] = {
    0xff, 0xff, 0xff, 0xff, 0xff,
    0x2d, 0x31, 0x34, 0x36, 0x2d, 0xff, 0xff, 0xff,
    0x2d, 0x31, 0x34, 0x36, 0x38, 0x36, 0x31, 0x34, 0xff, 0xff, 0xff,
    0x31, 0xff, 0x36, 0x2d,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0
};

#define YM2151 (*(volatile struct YM2151_t*) 0x9fe0)

static void setReg(uint8_t adr, uint8_t value)
{
    YM2151.reg = adr;
    YM2151.data = value;
}

static void loadPatch(uint8_t ch)
{
    uint8_t i = 0;
    while (1) {
        if (patch[i] == 0) break;
        setReg(patch[i] + ch, patch[i + 1]);
        i += 2;
    }
}

static unsigned char irq()
{
    uint8_t n;
    if (delay++ < 9) return IRQ_HANDLED;
    delay = 0;
    
    // play a note
    n = notes[ofs++];
    if (n == 0) {
        ofs = 0;
        bp = 1;
    } else {
        if (n < 0xfe) setReg(0x28, n);
        if (n != 0xff) {
            setReg(8, 0);
            if (n != 0xfe) setReg(8, 0x78);
        }
        b = (b + 1) & 1;
        if ((ofs == 21) || (ofs == 25)) bp += 2;
        if (ofs == 29) bp = 1;
        setReg(8, b + bp);
        setReg(8, 0x78 + b + bp);
    }
    
    // return from interrupt
    return IRQ_HANDLED;
}

static void main2()
{
    int i;
    uint8_t l, r;
    uint8_t ch;
    uint8_t n;

    // init ym2151
    for (i = 0; i <= 256; i++) setReg(i, 0);

    // load the patch into all voices
    for (i = 0; i <= 7; i++) loadPatch(i);

    // set up bass notes on channels 1-6
    l = 0xe4 - 0xc0 + 0x40;
    r = 0xe4 - 0xc0 + 0x80;

    // settings for left/right audio only
    for (ch = 1; ch <= 5; ch += 2) {
        if (ch == 1) n = 0x0d;
        if (ch == 3) n = 0x14;
        if (ch == 5) n = 0x16;
        setReg(0x20 + ch, l);
        setReg(0x20 + ch + 1, r);
        setReg(0x28 + ch, n);
        setReg(0x28 + ch + 1, n + 0x10);
    }

    ofs = 0;
    bp = 1;
    b = 1;
    delay = 0;

    // set new interrupt function
    // needs different local stack from 0xa000 to 0xa7ff, because the stack functions are not reentrant
    set_irq(irq, (void*) 0xa800, 0x0800);
    
    while (1);
}

int main(void)
{
    // switch back to uppercase character set
    __asm__("lda #$8e");
    __asm__("jsr BSOUT");
    
    // bad hack: redefine CC65 stack to 0x0xa800-0xafff, should be a proper x16 custom target
    *((uint16_t*) 0x02) = 0xb000;
    
    // call new main function, because local variables are not valid anymore after the stack change
    main2();
    
    return 0;
}