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chip8.cpp
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///////////////////////////////////////////////////////////////////////////////
// Project description
// Name: myChip8
//
// Author: Laurence Muller
// Contact: laurence.muller@gmail.com
//
// License: GNU General Public License (GPL) v2
// ( http://www.gnu.org/licenses/old-licenses/gpl-2.0.html )
//
// Copyright (C) 2011 Laurence Muller / www.multigesture.net
///////////////////////////////////////////////////////////////////////////////
#include "chip8.h"
#include <cstdio>
#include <cstdlib>
#include <ctime>
unsigned char chip8_fontset[80] =
{
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
};
chip8::chip8()
{
// empty
}
chip8::~chip8()
{
// empty
}
void chip8::init()
{
pc = 0x200; // Program counter starts at 0x200 (Start adress program)
opcode = 0; // Reset current opcode
I = 0; // Reset index register
sp = 0; // Reset stack pointer
// Clear display
for(int i = 0; i < 2048; ++i)
gfx[i] = 0;
// Clear stack
for(int i = 0; i < 16; ++i)
stack[i] = 0;
for(int i = 0; i < 16; ++i)
key[i] = V[i] = 0;
// Clear memory
for(int i = 0; i < 4096; ++i)
memory[i] = 0;
// Load fontset
for(int i = 0; i < 80; ++i)
memory[i] = chip8_fontset[i];
// Reset timers
delay_timer = 0;
sound_timer = 0;
// Clear screen once
drawFlag = true;
srand (time(NULL));
}
void chip8::emulateCycle()
{
// Fetch opcode
opcode = memory[pc] << 8 | memory[pc + 1];
// Process opcode
switch(opcode & 0xF000)
{
case 0x0000:
switch(opcode & 0x000F)
{
case 0x0000: // 0x00E0: Clears the screen
for(int i = 0; i < 2048; ++i)
gfx[i] = 0x0;
drawFlag = true;
pc += 2;
break;
case 0x000E: // 0x00EE: Returns from subroutine
--sp; // 16 levels of stack, decrease stack pointer to prevent overwrite
pc = stack[sp]; // Put the stored return address from the stack back into the program counter
pc += 2; // Don't forget to increase the program counter!
break;
default:
printf ("Unknown opcode [0x0000]: 0x%X\n", opcode);
}
break;
case 0x1000: // 0x1NNN: Jumps to address NNN
pc = opcode & 0x0FFF;
break;
case 0x2000: // 0x2NNN: Calls subroutine at NNN.
stack[sp] = pc; // Store current address in stack
++sp; // Increment stack pointer
pc = opcode & 0x0FFF; // Set the program counter to the address at NNN
break;
case 0x3000: // 0x3XNN: Skips the next instruction if VX equals NN
if(V[(opcode & 0x0F00) >> 8] == (opcode & 0x00FF))
pc += 4;
else
pc += 2;
break;
case 0x4000: // 0x4XNN: Skips the next instruction if VX doesn't equal NN
if(V[(opcode & 0x0F00) >> 8] != (opcode & 0x00FF))
pc += 4;
else
pc += 2;
break;
case 0x5000: // 0x5XY0: Skips the next instruction if VX equals VY.
if(V[(opcode & 0x0F00) >> 8] == V[(opcode & 0x00F0) >> 4])
pc += 4;
else
pc += 2;
break;
case 0x6000: // 0x6XNN: Sets VX to NN.
V[(opcode & 0x0F00) >> 8] = opcode & 0x00FF;
pc += 2;
break;
case 0x7000: // 0x7XNN: Adds NN to VX.
V[(opcode & 0x0F00) >> 8] += opcode & 0x00FF;
pc += 2;
break;
case 0x8000:
switch(opcode & 0x000F)
{
case 0x0000: // 0x8XY0: Sets VX to the value of VY
V[(opcode & 0x0F00) >> 8] = V[(opcode & 0x00F0) >> 4];
pc += 2;
break;
case 0x0001: // 0x8XY1: Sets VX to "VX OR VY"
V[(opcode & 0x0F00) >> 8] |= V[(opcode & 0x00F0) >> 4];
pc += 2;
break;
case 0x0002: // 0x8XY2: Sets VX to "VX AND VY"
V[(opcode & 0x0F00) >> 8] &= V[(opcode & 0x00F0) >> 4];
pc += 2;
break;
case 0x0003: // 0x8XY3: Sets VX to "VX XOR VY"
V[(opcode & 0x0F00) >> 8] ^= V[(opcode & 0x00F0) >> 4];
pc += 2;
break;
case 0x0004: // 0x8XY4: Adds VY to VX. VF is set to 1 when there's a carry, and to 0 when there isn't
if(V[(opcode & 0x00F0) >> 4] > (0xFF - V[(opcode & 0x0F00) >> 8]))
V[0xF] = 1; //carry
else
V[0xF] = 0;
V[(opcode & 0x0F00) >> 8] += V[(opcode & 0x00F0) >> 4];
pc += 2;
break;
case 0x0005: // 0x8XY5: VY is subtracted from VX. VF is set to 0 when there's a borrow, and 1 when there isn't
if(V[(opcode & 0x00F0) >> 4] > V[(opcode & 0x0F00) >> 8])
V[0xF] = 0; // there is a borrow
else
V[0xF] = 1;
V[(opcode & 0x0F00) >> 8] -= V[(opcode & 0x00F0) >> 4];
pc += 2;
break;
case 0x0006: // 0x8XY6: Shifts VX right by one. VF is set to the value of the least significant bit of VX before the shift
V[0xF] = V[(opcode & 0x0F00) >> 8] & 0x1;
V[(opcode & 0x0F00) >> 8] >>= 1;
pc += 2;
break;
case 0x0007: // 0x8XY7: Sets VX to VY minus VX. VF is set to 0 when there's a borrow, and 1 when there isn't
if(V[(opcode & 0x0F00) >> 8] > V[(opcode & 0x00F0) >> 4]) // VY-VX
V[0xF] = 0; // there is a borrow
else
V[0xF] = 1;
V[(opcode & 0x0F00) >> 8] = V[(opcode & 0x00F0) >> 4] - V[(opcode & 0x0F00) >> 8];
pc += 2;
break;
case 0x000E: // 0x8XYE: Shifts VX left by one. VF is set to the value of the most significant bit of VX before the shift
V[0xF] = V[(opcode & 0x0F00) >> 8] >> 7;
V[(opcode & 0x0F00) >> 8] <<= 1;
pc += 2;
break;
default:
printf ("Unknown opcode [0x8000]: 0x%X\n", opcode);
}
break;
case 0x9000: // 0x9XY0: Skips the next instruction if VX doesn't equal VY
if(V[(opcode & 0x0F00) >> 8] != V[(opcode & 0x00F0) >> 4])
pc += 4;
else
pc += 2;
break;
case 0xA000: // ANNN: Sets I to the address NNN
I = opcode & 0x0FFF;
pc += 2;
break;
case 0xB000: // BNNN: Jumps to the address NNN plus V0
pc = (opcode & 0x0FFF) + V[0];
break;
case 0xC000: // CXNN: Sets VX to a random number and NN
V[(opcode & 0x0F00) >> 8] = (rand() % 0xFF) & (opcode & 0x00FF);
pc += 2;
break;
case 0xD000: // DXYN: Draws a sprite at coordinate (VX, VY) that has a width of 8 pixels and a height of N pixels.
// Each row of 8 pixels is read as bit-coded starting from memory location I;
// I value doesn't change after the execution of this instruction.
// VF is set to 1 if any screen pixels are flipped from set to unset when the sprite is drawn,
// and to 0 if that doesn't happen
{
unsigned short x = V[(opcode & 0x0F00) >> 8];
unsigned short y = V[(opcode & 0x00F0) >> 4];
unsigned short height = opcode & 0x000F;
unsigned short pixel;
V[0xF] = 0;
for (int yline = 0; yline < height; yline++)
{
pixel = memory[I + yline];
for(int xline = 0; xline < 8; xline++)
{
if((pixel & (0x80 >> xline)) != 0)
{
if(gfx[(x + xline + ((y + yline) * 64))] == 1)
{
V[0xF] = 1;
}
gfx[x + xline + ((y + yline) * 64)] ^= 1;
}
}
}
drawFlag = true;
pc += 2;
}
break;
case 0xE000:
switch(opcode & 0x00FF)
{
case 0x009E: // EX9E: Skips the next instruction if the key stored in VX is pressed
if(key[V[(opcode & 0x0F00) >> 8]] != 0)
pc += 4;
else
pc += 2;
break;
case 0x00A1: // EXA1: Skips the next instruction if the key stored in VX isn't pressed
if(key[V[(opcode & 0x0F00) >> 8]] == 0)
pc += 4;
else
pc += 2;
break;
default:
printf ("Unknown opcode [0xE000]: 0x%X\n", opcode);
}
break;
case 0xF000:
switch(opcode & 0x00FF)
{
case 0x0007: // FX07: Sets VX to the value of the delay timer
V[(opcode & 0x0F00) >> 8] = delay_timer;
pc += 2;
break;
case 0x000A: // FX0A: A key press is awaited, and then stored in VX
{
bool keyPress = false;
for(int i = 0; i < 16; ++i)
{
if(key[i] != 0)
{
V[(opcode & 0x0F00) >> 8] = i;
keyPress = true;
}
}
// If we didn't received a keypress, skip this cycle and try again.
if(!keyPress)
return;
pc += 2;
}
break;
case 0x0015: // FX15: Sets the delay timer to VX
delay_timer = V[(opcode & 0x0F00) >> 8];
pc += 2;
break;
case 0x0018: // FX18: Sets the sound timer to VX
sound_timer = V[(opcode & 0x0F00) >> 8];
pc += 2;
break;
case 0x001E: // FX1E: Adds VX to I
if(I + V[(opcode & 0x0F00) >> 8] > 0xFFF) // VF is set to 1 when range overflow (I+VX>0xFFF), and 0 when there isn't.
V[0xF] = 1;
else
V[0xF] = 0;
I += V[(opcode & 0x0F00) >> 8];
pc += 2;
break;
case 0x0029: // FX29: Sets I to the location of the sprite for the character in VX. Characters 0-F (in hexadecimal) are represented by a 4x5 font
I = V[(opcode & 0x0F00) >> 8] * 0x5;
pc += 2;
break;
case 0x0033: // FX33: Stores the Binary-coded decimal representation of VX at the addresses I, I plus 1, and I plus 2
memory[I] = V[(opcode & 0x0F00) >> 8] / 100;
memory[I + 1] = (V[(opcode & 0x0F00) >> 8] / 10) % 10;
memory[I + 2] = (V[(opcode & 0x0F00) >> 8] % 100) % 10;
pc += 2;
break;
case 0x0055: // FX55: Stores V0 to VX in memory starting at address I
for (int i = 0; i <= ((opcode & 0x0F00) >> 8); ++i)
memory[I + i] = V[i];
// On the original interpreter, when the operation is done, I = I + X + 1.
I += ((opcode & 0x0F00) >> 8) + 1;
pc += 2;
break;
case 0x0065: // FX65: Fills V0 to VX with values from memory starting at address I
for (int i = 0; i <= ((opcode & 0x0F00) >> 8); ++i)
V[i] = memory[I + i];
// On the original interpreter, when the operation is done, I = I + X + 1.
I += ((opcode & 0x0F00) >> 8) + 1;
pc += 2;
break;
default:
printf ("Unknown opcode [0xF000]: 0x%X\n", opcode);
}
break;
default:
printf ("Unknown opcode: 0x%X\n", opcode);
}
// Update timers
if(delay_timer > 0)
--delay_timer;
if(sound_timer > 0)
{
if(sound_timer == 1)
printf("BEEP!\n");
--sound_timer;
}
}
void chip8::debugRender()
{
// Draw
for(int y = 0; y < 32; ++y)
{
for(int x = 0; x < 64; ++x)
{
if(gfx[(y*64) + x] == 0)
printf("O");
else
printf(" ");
}
printf("\n");
}
printf("\n");
}
bool chip8::loadApplication(const char * filename)
{
init();
printf("Loading: %s\n", filename);
// Open file
FILE * pFile = fopen(filename, "rb");
if (pFile == nullptr)
{
fputs ("File error", stderr);
return false;
}
// Check file size
fseek(pFile , 0 , SEEK_END);
long lSize = ftell(pFile);
rewind(pFile);
printf("Filesize: %d\n", (int)lSize);
// Allocate memory to contain the whole file
char * buffer = (char*)malloc(sizeof(char) * lSize);
if (buffer == nullptr)
{
fputs ("Memory error", stderr);
return false;
}
// Copy the file into the buffer
size_t result = fread (buffer, 1, lSize, pFile);
if (result != lSize)
{
fputs("Reading error",stderr);
return false;
}
// Copy buffer to Chip8 memory
if((4096-512) > lSize)
{
for(int i = 0; i < lSize; ++i)
memory[i + 512] = buffer[i];
}
else
printf("Error: ROM too big for memory");
// Close file, free buffer
fclose(pFile);
free(buffer);
return true;
}