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chip8.c
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/* Thank you to http://devernay.free.fr/hacks/chip8/C8TECH10.HTM for being an amazing technical reference. */
#include <stdio.h>
#include <stdint.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#include <time.h>
#include <SDL2/SDL.h>
#define RAMSIZE 4096
#define PROGRAM_START 512
#ifdef DEBUG /* -DDEBUG in cc or debug=1 in the make command. */
#pragma message("Debug mode enabled.")
#endif
#define SPRITES 16
#define SPRITE_SIZE 5
#define SPRITE_OFFSET 0x50
uint8_t sprite_data[SPRITES][SPRITE_SIZE] = /* Sprites are things that can be drawn onto the screen, unlike modern bitmap systems which allow pixel by pixel drawing. */
{
{0xF0,0x90,0x90,0x90,0xF0}, /* 0 */
{0x20,0x60,0x20,0x20,0x70}, /* 1 */
{0xF0,0x10,0xF0,0x80,0xF0}, /* 2 */
{0xF0,0x10,0xF0,0x10,0xF0}, /* 3 */
{0x90,0x90,0xF0,0x10,0x10}, /* 4 */
{0xF0,0x80,0xF0,0x10,0xF0}, /* 5 */
{0xF0,0x80,0xF0,0x90,0xF0}, /* 6 */
{0xF0,0x10,0x20,0x40,0x40}, /* 7 */
{0xF0,0x90,0xF0,0x90,0xF0}, /* 8 */
{0xF0,0x90,0xF0,0x10,0xF0}, /* 9 */
{0xF0,0x90,0xF0,0x90,0x90}, /* A */
{0xE0,0x90,0xE0,0x90,0xE0}, /* B */
{0xF0,0x80,0x80,0x80,0xF0}, /* C */
{0xE0,0x90,0x90,0x90,0xE0}, /* D */
{0xF0,0x80,0xF0,0x80,0xF0}, /* E */
{0xF0,0x80,0xF0,0x80,0x80} /* F */
};
#define KEY_COUNT 16
uint8_t key_data[KEY_COUNT] = /* Keycodes. */
{
SDLK_x,
SDLK_1,
SDLK_2,
SDLK_3,
SDLK_q,
SDLK_w,
SDLK_e,
SDLK_a,
SDLK_s,
SDLK_d,
SDLK_z,
SDLK_c,
SDLK_4,
SDLK_r,
SDLK_f,
SDLK_v
};
#define W 64
#define H 32
#define WINDOW_SCALE 16
int main(int argc, char *argv[])
{
if (argc < 2)
{
fprintf(stderr, "Please provide a Chip8 ROM.\n");
return -1;
}
uint8_t RAM[RAMSIZE]; /* 0x000 - 0x1FF for Interpreter, 0x200 - 0xFFF for program. */
uint8_t REGISTERS[16]; /* V0 - VF (VF is for flags and should not be used by programs.) */
uint16_t I = 0; /* Use lowest 12 bits for storing memory addresses. */
uint16_t PC = PROGRAM_START; /* Stores the currently executing address. */
uint8_t SP = 0; /* Points to the topmost level of the stack. */
uint16_t STACK[16]; /* Stores return point for subroutines. */
uint8_t DT = 0; /* When greater than 0, decrement by 1 at a rate of 60hz. */
uint8_t ST = 0; /* When greater than 0, decrement by 1 at a rate of 60hz and play a sound. */
uint8_t SCREEN[W][H]; /* 64x32 monochrome display, only least significant bit is used because I'm lazy. */
uint8_t KEYS[KEY_COUNT]; /* Every key 0-F. */
memset(RAM, 0, sizeof RAM);
memset(REGISTERS, 0, sizeof REGISTERS);
memset(STACK, 0, sizeof STACK);
memset(SCREEN, 0, sizeof SCREEN);
memset(KEYS, 0, sizeof KEYS);
double ticks = 0; /* Add 60ths of a second taken to calculate the instruction. When >= 1, decrement ticks, ST, and DT. */
#define CALCULATE_TIMERS \
{ \
\
uint32_t currTime = SDL_GetTicks(); \
ticks += (currTime - startTime) / 16.6; \
\
if (ticks >= 1) /* TODO: Make the sound timer actually play sound. */ \
{ \
if (DT > 0) \
--DT; \
if (ST > 0) \
--ST; \
--ticks; \
} \
}
const uint8_t *state = SDL_GetKeyboardState(NULL);
for (int i = 0; i < SPRITES; ++i) /* Load some sprites into memory. */
{
for (int j = 0; j < SPRITE_SIZE; ++j)
{
RAM[i * SPRITE_SIZE + j + SPRITE_OFFSET] = sprite_data[i][j];
}
}
FILE *file = fopen(argv[1], "rb");
fread(RAM + PROGRAM_START /* Pointer arithmetic! */, 1, RAMSIZE - PROGRAM_START, file);
fclose(file);
#ifdef DEBUG
for (int i = 0; i < 0xFFF; i += 1)
{
printf("%02X ", RAM[i]);
if (!((i+1) % 16))
printf("\n");
}
printf("\n\n"); /* Just to offset the later debug things. */
#endif
/* SDL Stuff. */
if (SDL_Init(SDL_INIT_EVERYTHING) < 0)
{
printf("SDL failed to init.\n%s\n", SDL_GetError());
return -1;
}
SDL_Window *window = SDL_CreateWindow("Chip-8 Interpreter", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, W*WINDOW_SCALE, H*WINDOW_SCALE, SDL_WINDOW_SHOWN);
if (!window)
{
printf("Window failed to be created.\n%s\n", SDL_GetError());
return -1;
}
SDL_Renderer *renderer = SDL_CreateRenderer(window, -1, SDL_RENDERER_ACCELERATED);
if (!renderer)
{
printf("Renderer failed to be created.\n%s\n", SDL_GetError());
return -1;
}
uint8_t quit = 0;
srand(time(NULL)); /* Generate random seed. */
while (!quit)
{
uint32_t startTime = SDL_GetTicks();
SDL_Event e;
SDL_PollEvent(&e);
if(e.type == SDL_QUIT)
quit = 1;
SDL_SetRenderDrawColor(renderer, 0, 0, 0, 0xFF);
SDL_RenderClear(renderer);
SDL_PumpEvents();
for (int i = 0; i < KEY_COUNT; ++i)
{
KEYS[i] = 0;
if (state[SDL_GetScancodeFromKey(key_data[i])])
KEYS[i] = 1;
}
/* Back to Chip-8 stuff. */
uint16_t instruction = (RAM[PC] << 8) + RAM[PC+1]; /* The ram is an array of single bytes, while each instruction is 2 bytes long, so we need to combine them into one 16 bit value. */
#ifdef DEBUG
printf("%04X\n", instruction);
#endif
uint8_t broken_instruction[4] = /* All 4 nibbles of the instruction, from left to right. */
{
instruction >> 12,
instruction >> 8 & 15,
instruction >> 4 & 15,
instruction & 15
};
#define UNKNOWN_INSTRUCTION
#ifdef DEBUG
#undef UNKNOWN_INSTRUCTION
#define UNKNOWN_INSTRUCTION printf("Unknown instruction %04X\n", instruction)
#endif
uint8_t jumped = 0;
switch (broken_instruction[0])
{
case 0:
if (instruction == 0x00E0) /* Clear the screen. */
memset(SCREEN, 0, sizeof SCREEN);
else if (instruction == 0x00EE) /* Return from subroutine. */
{
#ifdef DEBUG
printf("Stack pointer: %i\n", SP-1);
#endif
PC = STACK[SP--];
}
else
UNKNOWN_INSTRUCTION;
break;
case 1: /* Jump. */
PC = broken_instruction[3] + (broken_instruction[2] << 4) + (broken_instruction[1] << 8);
#ifdef DEBUG
printf("Jumping to location %03X\n", PC);
#endif
jumped = 1;
break;
case 2: /* Start a subroutine. */
#ifdef DEBUG
printf("Stack pointer: %i\n", SP+1);
#endif
STACK[++SP] = PC;
PC = broken_instruction[3] + (broken_instruction[2] << 4) + (broken_instruction[1] << 8);
#ifdef DEBUG
printf("Starting subroutine at %03X\n", PC);
#endif
jumped = 1;
break;
case 3: /* Skip the next instruction if a register equals a value. */
if (REGISTERS[broken_instruction[1]] == (broken_instruction[2] << 4) + broken_instruction[3])
PC += 2;
break;
case 4: /* Skip the next instruction if a register does not equal a value. */
if (REGISTERS[broken_instruction[1]] != (broken_instruction[2] << 4) + broken_instruction[3])
PC += 2;
break;
case 5: /* Skip the next instruction if a register equals another register. */
if (broken_instruction[3]) /* Needs to end in a zero. */
UNKNOWN_INSTRUCTION;
else if (REGISTERS[broken_instruction[1]] == REGISTERS[broken_instruction[2]])
PC += 2;
break;
case 6: /* Put a value into a register. */
REGISTERS[broken_instruction[1]] = (broken_instruction[2] << 4) + broken_instruction[3];
break;
case 7: /* Adds a value to a register. */
REGISTERS[broken_instruction[1]] += (broken_instruction[2] << 4) + broken_instruction[3];
break;
case 8: /* Operations between two registers. */
switch (broken_instruction[3])
{
case 0: /* Stores the value of a register into another register. */
REGISTERS[broken_instruction[1]] = REGISTERS[broken_instruction[2]];
break;
case 1: /* Bitwise or. */
REGISTERS[broken_instruction[1]] |= REGISTERS[broken_instruction[2]];
REGISTERS[15] = 0;
break;
case 2: /* Bitwise and. */
REGISTERS[broken_instruction[1]] &= REGISTERS[broken_instruction[2]];
REGISTERS[15] = 0;
break;
case 3: /* Bitwise xor. */
REGISTERS[broken_instruction[1]] ^= REGISTERS[broken_instruction[2]];
REGISTERS[15] = 0;
break;
case 4: /* Add. */
REGISTERS[broken_instruction[1]] += REGISTERS[broken_instruction[2]];
REGISTERS[15] = 0;
if (REGISTERS[broken_instruction[1]] < REGISTERS[broken_instruction[2]]) /* Overflow. */
REGISTERS[15] = 1; /* Carry flag. */
break;
case 5: /* Subtract. */
REGISTERS[15] = 0;
if (REGISTERS[broken_instruction[1]] > REGISTERS[broken_instruction[2]])
REGISTERS[15] = 1; /* Carry flag. */
REGISTERS[broken_instruction[1]] -= REGISTERS[broken_instruction[2]];
break;
case 6: /* Right shift. */
REGISTERS[15] = 0;
if (REGISTERS[broken_instruction[2]] & 1) /* Least significant bit. */
REGISTERS[15] = 1; /* Carry flag. */
REGISTERS[broken_instruction[1]] = REGISTERS[broken_instruction[2]] >> 1;
break;
case 7: /* Subtract but different. */
REGISTERS[15] = 0;
if (REGISTERS[broken_instruction[2]] > REGISTERS[broken_instruction[1]])
REGISTERS[15] = 1; /* Carry flag. */
REGISTERS[broken_instruction[1]] = REGISTERS[broken_instruction[2]] - REGISTERS[broken_instruction[1]];
break;
case 14: /* Left shift. */
REGISTERS[15] = 0;
if (REGISTERS[broken_instruction[2]] & 0b10000000) /* Most significant bit. */
REGISTERS[15] = 1; /* Carry flag. */
REGISTERS[broken_instruction[1]] = REGISTERS[broken_instruction[2]] << 1;
break;
default:
UNKNOWN_INSTRUCTION;
break;
}
break;
case 9: /* Skip next instruction if a register does not equal another register. */
if (broken_instruction[3]) /* Needs to end in a zero. */
UNKNOWN_INSTRUCTION;
if (REGISTERS[broken_instruction[1]] != REGISTERS[broken_instruction[2]])
PC += 2;
break;
case 10: /* Set register I to a 12-bit memory address. */
I = broken_instruction[3] + (broken_instruction[2] << 4) + (broken_instruction[1] << 8);
break;
case 11: /* Jump to a value added to a register. */
PC = broken_instruction[3] + (broken_instruction[2] << 4) + (broken_instruction[1] << 8) + REGISTERS[0];
jumped = 1;
break;
case 12: /* Sets a register to a random value and performs a bitwise and operation on it. */
REGISTERS[broken_instruction[1]] = (rand() & 0xFF) & (broken_instruction[3] + (broken_instruction[2] << 4));
#ifdef DEBUG
printf("Random number is %i\n", REGISTERS[broken_instruction[1]]);
#endif
break;
case 13: /* Draw the sprite stored in the memory address in register I. */
REGISTERS[15] = 0;
for (int i = 0; i < broken_instruction[3]; ++i)
{
uint8_t row = RAM[I + i];
for (int j = 0; j < 8; ++j)
{
int x = (REGISTERS[broken_instruction[1]] % 64 + j);
int y = (REGISTERS[broken_instruction[2]] % 32 + i);
if (x < 64 && y < 32)
{
uint8_t pixel = (row >> (7 - j)) & 1;
if (SCREEN[x][y]) REGISTERS[15] = 1;
SCREEN[x][y] ^= pixel;
}
}
}
break;
case 14: /* Skip next instruction if a specific key is/isn't pressed. */
switch (broken_instruction[3])
{
case 14: /* Is pressed. */
if (KEYS[REGISTERS[broken_instruction[1]]])
PC += 2;
break;
case 1: /* Isn't pressed. */
if (!KEYS[REGISTERS[broken_instruction[1]]])
PC += 2;
break;
default:
UNKNOWN_INSTRUCTION;
break;
}
break;
case 15: /* More register operations. */
switch (broken_instruction[2])
{
case 0:
switch (broken_instruction[3])
{
case 7: /* Set a register to the value of the delay timer. */
REGISTERS[broken_instruction[1]] = DT;
break;
case 10: /* Pause execution until a key is pressed, and then store the key value in a register. */
while (1)
{
CALCULATE_TIMERS;
uint8_t done = 0;
SDL_PumpEvents();
for (int i = 0; i < KEY_COUNT; ++i)
{
if (state[SDL_GetScancodeFromKey(key_data[i])])
{
REGISTERS[broken_instruction[1]] = i;
done = 1;
break;
}
}
if (done)
break;
}
break;
default:
UNKNOWN_INSTRUCTION;
break;
}
break;
case 1:
switch (broken_instruction[3])
{
case 5: /* Set the value of the delay timer to a register. */
DT = REGISTERS[broken_instruction[1]];
break;
case 8: /* Set the value of the sound timer to a register. */
ST = REGISTERS[broken_instruction[1]];
break;
case 14: /* Add the value of a register to I. */
I += REGISTERS[broken_instruction[1]];
break;
default:
UNKNOWN_INSTRUCTION;
break;
}
break;
case 2:
if (broken_instruction[3] == 9) /* I is set to the location of one of the default sprites. */
I = SPRITE_OFFSET + REGISTERS[broken_instruction[1]] * SPRITE_SIZE;
else
UNKNOWN_INSTRUCTION;
break;
case 3:
if (broken_instruction[3] == 3) /* Converts a number into its bcd equivalent. */
{
RAM[I] = REGISTERS[broken_instruction[1]]/100;
RAM[I+1] = REGISTERS[broken_instruction[1]]/10%10;
RAM[I+2] = REGISTERS[broken_instruction[1]]%10;
}
else
UNKNOWN_INSTRUCTION;
break;
case 5:
if (broken_instruction[3] == 5) /* Copy n amount of registers starting at register 0 into memory at the location stored in I. */
{
for (int i = 0; i <= broken_instruction[1] ; ++i)
{
RAM[I++] = REGISTERS[i];
}
}
else
UNKNOWN_INSTRUCTION;
break;
case 6:
if (broken_instruction[3] == 5) /* Load n amount of registers starting at register 0 from memory at the location stored in I.*/
{
for (int i = 0; i <= broken_instruction[1]; ++i)
{
REGISTERS[i] = RAM[I++];
}
}
else
UNKNOWN_INSTRUCTION;
break;
default:
UNKNOWN_INSTRUCTION;
break;
}
break;
default:
UNKNOWN_INSTRUCTION;
break;
}
if (!jumped)
PC += 2;
/* Draw the screen. */
SDL_Rect pixel = {0, 0, WINDOW_SCALE, WINDOW_SCALE};
SDL_SetRenderDrawColor(renderer, 0xFF, 0xFF, 0xFF, 0xFF);
for (int x = 0; x < W; ++x)
{
for (int y = 0; y < H; ++y)
{
pixel.x = x * WINDOW_SCALE;
pixel.y = y * WINDOW_SCALE;
if (SCREEN[x][y])
{
SDL_RenderFillRect(renderer, &pixel);
}
}
}
SDL_RenderPresent(renderer);
CALCULATE_TIMERS;
}
return 0;
}