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chip8.c
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chip8.c
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#include "chip8.h"
#define STACK_SIZE 48
#define MEMORY_SIZE 4096
#define SIZE ROWS*COLS
#define MAX_GAME_SIZE (0x1000 - START_PROGRAM_ADDRESS)
#define START_PROGRAM_ADDRESS 0x200
#define FONT_OFFSET 0x00
#define START_CALL_STACK 0xEA0
#define END_CALL_STACK 0xEFF
#define START_DISPLAY_REFRESH_OFFSET 0xF00
#define END_DISPLAY_REFRESH_OFFSET 0xFFF
#define REGISTER_SIZE 16
#define KEY_SIZE 16
uint16_t PC;
uint16_t opcode;
uint16_t I;
uint16_t SP;
uint16_t stack[STACK_SIZE];
uint8_t video_memory[ROWS][COLS];
uint8_t memory[MEMORY_SIZE];
uint8_t V[REGISTERS_SIZE];
uint8_t delay_timer;
uint8_t sound_timer;
uint8_t KEY_STATE[NUMBERS_OF_KEYS];
bool DrawFlag;
/* Standard 4x5 font */
static uint16_t font[FONT_SIZE] = {
/* '0' */ 0xF0, 0x90, 0x90, 0x90, 0xF0,
/* '1' */ 0x20, 0x60, 0x20, 0x20, 0x70,
/* '2' */ 0xF0, 0x10, 0xF0, 0x80, 0xF0,
/* '3' */ 0xF0, 0x10, 0xF0, 0x10, 0xF0,
/* '4' */ 0x90, 0x90, 0xF0, 0x10, 0x10,
/* '5' */ 0xF0, 0x80, 0xF0, 0x10, 0xF0,
/* '6' */ 0xF0, 0x80, 0xF0, 0x90, 0xF0,
/* '7' */ 0xF0, 0x10, 0x20, 0x40, 0x40,
/* '8' */ 0xF0, 0x90, 0xF0, 0x90, 0xF0,
/* '9' */ 0xF0, 0x90, 0xF0, 0x10, 0xF0,
/* 'A' */ 0xF0, 0x90, 0xF0, 0x90, 0x90,
/* 'B' */ 0xE0, 0x90, 0xE0, 0x90, 0xE0,
/* 'C' */ 0xF0, 0x80, 0x80, 0x80, 0xF0,
/* 'D' */ 0xE0, 0x80, 0x80, 0x80, 0xE0,
/* 'E' */ 0xF0, 0x80, 0xF0, 0x80, 0xF0,
/* 'F' */ 0xF0, 0x80, 0xF0, 0x80, 0x80,
};
void
opcode_cycle(void)
{
uint8_t a;
uint8_t X;
uint8_t Y;
uint8_t Z;
uint16_t NN;
uint16_t NNN;
int i;
/*
In caso non andasse ricordati di vedere se fread ritorna
quantitativo di byte letti e scritti nella memoria apposto di usare MAX_GAME_SIZE
*/
opcode = memory[PC] << 8 | memory[PC + 1];
/*Take the first digit of the opcode*/
a = (opcode & 0xF000) >> 0xC;
NNN = (opcode & 0x0FFF);
NN = (opcode & 0x00FF);
X = (opcode & 0x0F00) >> 0x8; /*Second Digit of the opcode*/
Y = (opcode & 0x00F0) >> 0x4; /*Third Digit of the opcode*/
Z = (opcode & 0x000F); /*Last Digit of the opcode*/
#if DEBUG
printf("%x \n",opcode);
#endif
// Ok, i REALLY don't know why i used if/elif/else statemente instead of switch case
// there is a major difference in the two (che the asm)
// but it's a boring correction and i don't want to do it :-).
if(a == 0){
/*
Clear Istruction
*/
if(NNN == 0x0E0){
memset(video_memory, 0, sizeof(uint8_t) * (SIZE));
DrawFlag = true;
PC += 2;
}
/*
Return Istruction
*/
else if(NNN == 0x0EE)
PC = stack[--SP];
}
/*
Jump to address NNN
*/
else if(a == 1)
PC = NNN;
/*
Call Adrress NNN
*/
else if(a == 2){
stack[SP++] = PC + 2;
PC = NNN;
}
/*
Skips the next instruction if VX equals NN. (Usually the next instruction is a jump to skip a code block)
*/
else if(a == 3){
if(V[X] == NN)
PC += 4;
else
PC += 2;
}
/*
Skips the next instruction if VX doesn't equal NN. (Usually the next instruction is a jump to skip a code block)
*/
else if(a == 4){
if(V[X] != NN)
PC += 4;
else
PC += 2;
}
/*
Skips the next instruction if VX equals VY. (Usually the next instruction is a jump to skip a code block)
*/
else if(a == 5){
if(V[X] == V[Y])
PC += 4;
else
PC += 2;
}
/*
Sets VX to NN.
*/
else if(a == 6){
V[X] = NN;
PC += 2;
}
/*
ADDS NN to VX
*/
else if(a == 7){
V[X] += NN;
PC += 2;
}
/*
Sets VX to the value of VY.
*/
if(a == 8){
if(Z == 0){
V[X] = V[Y];
PC += 2;
}
/*
Sets VX to VX or VY. (Bitwise OR operation)
*/
else if(Z == 1){
V[X] |= V[Y];
PC += 2;
}
/*
Sets VX to VX and VY. (Bitwise AND operation)
*/
else if(Z == 2){
V[X] &= V[Y];
PC += 2;
}
/*
Sets VX to VX xor VY.
*/
else if(Z == 3){
V[X] ^= V[Y];
PC += 2;
}
/*
Adds VY to VX. VF is set to 1 when there's a carry, and to 0 when there isn't.
*/
else if(Z == 4){
V[0xF] = ((int) V[X] + (int) V[Y]) > 255 ? 1 : 0;
V[X] += V[Y];
PC += 2;
}
/*
VY is subtracted from VX. VF is set to 0 when there's a borrow, and 1 when there isn't.
*/
else if(Z == 5){
V[0xF] = (V[X] > V[Y]) ? 1 : 0;
V[X] -= V[Y];
PC += 2;
}
/*
Stores the least significant bit of VX in VF and then shifts VX to the right by 1
*/
else if(Z == 6){
V[0xF] = V[X] & 0x1;
V[X] >>= 1;
PC += 2;
}
/*
Sets VX to VY minus VX. VF is set to 0 when there's a borrow, and 1 when there isn't.
*/
else if(Z == 7){
V[0xF] = (V[Y] > V[X]) ? 1 : 0;
V[X] = V[Y] - V[X];
PC += 2;
}
/*
Stores the most significant bit of VX in VF and then shifts VX to the left by 1
*/
else if(Z == 0xE){
V[0xF] = (V[X] >> 7) & 0x1;
V[X] <<= 1;
PC += 2;
}
}
/*
Skips the next instruction if VX doesn't equal VY. (Usually the next instruction is a jump to skip a code block)
*/
else if(a == 9){
if(V[X] != V[Y])
PC += 4;
else
PC += 2;
}
/*
Sets I to the address NNN.
*/
else if(a == 0xA){
I = NNN;
PC += 2;
}
/*
Jumps to the address NNN plus V0.
*/
else if(a == 0xB)
PC = V[0] + NNN;
/*
Sets VX to the result of a bitwise and operation on a random number (Typically: 0 to 255) and NN.
*/
else if(a == 0xC){
V[X] = rand() & NN;
PC += 2;
}
/*
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. As described above, 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
*/
else if(a == 0xD){
DrawSprite(V[X],V[Y],Z); /*X,Y,HEIGHT*/
PC += 2;
DrawFlag = true;
}
else if(a == 0xE){
/*
Skips the next instruction if the key stored in VX is pressed. (Usually the next
instruction is a jump to skip a code block)
*/
if(NN == 0x9E){
if(KEY_STATE[V[X]])
PC += 4;
else
PC += 2;
}
/*
Skips the next instruction if the key stored in VX isn't pressed. (Usually the next
instruction is a jump to skip a code block)
*/
else if(NN == 0xA1){
if(!KEY_STATE[V[X]])
PC += 4;
else
PC += 2;
}
}
else if(a == 0xF){
/*
Sets VX to the value of the delay timer.
*/
if(NN == 0x07){
V[X] = delay_timer;
PC += 2;
}
/*
A key press is awaited, and then stored in VX. (Blocking
Operation.All instruction halted until next key event)
*/
else if(NN == 0x0A){
V[X] = gfx_wait();
PC += 2;
}
/*
Sets the delay timer to VX.
*/
else if(NN == 0x15){
delay_timer = V[X];
PC += 2;
}
/*
Sets the sound timer to VX
*/
else if(NN == 0x18){
sound_timer = V[X];
PC += 2;
}
/*
Adds VX to I
*/
else if(NN == 0x1E){
I += V[X];
PC += 2;
}
/*
Sets I to the location of the sprite for the character in VX.
Characters 0-F (in hexadecimal) are represented by a 4x5 font.
*/
else if(NN == 0x29){
I = 5 * V[X];
PC += 2;
}
/*
Stores the binary-coded decimal representation of VX, with the most significant of three digits at the address in I, the middle digit at I plus 1, and the least significant digit at I plus 2.
(In other words, take the decimal representation of VX, place the hundreds digit in memory at location in I, the tens digit at location I+1, and the ones digit at location I+2.)
*/
else if(NN == 0x33){
memory[I] = (V[X] % 1000) / 100;
memory[I+1] = (V[X] % 100) / 10;
memory[I+2] = (V[X] % 10);
PC += 2;
/*
Stores V0 to VX (including VX) in memory starting at address I. The offset from I is
increased by 1 for each value written, but I itself is left unmodified.
*/
}else if(NN == 0x55){
for(i = 0; i <= X; i++)
memory[I+i] = V[i];
I = I + ( X + 1);
PC += 2;
/*
Fills V0 to VX (including VX) with values from memory starting at address I.
The offset from I is increased by 1 for each value written, but I itself is left unmodified.
*/
}else if(NN == 0x65){
for(i = 0; i <= X; i++)
V[i] = memory[I + i];
I = I + (X + 1);
PC += 2;
}
Tick();
}
}
void
LoadRom(char *game){
FILE *fp;
fp = fopen(game, "rb");
if (fp == NULL) {
fprintf(stderr, "failed to open The game: %s\n", game);
exit(EXIT_FAILURE);
}
fread(&memory[START_PROGRAM_ADDRESS], 1, MAX_GAME_SIZE, fp);
fclose(fp);
}
void
DrawSprite(uint8_t x, uint8_t y, uint8_t nibble)
{
unsigned row = y, col = x;
unsigned byte_index;
unsigned bit_index;
V[0xF] = 0;
for (byte_index = 0; byte_index < nibble; byte_index++) {
uint8_t byte = memory[I + byte_index];
for (bit_index = 0; bit_index < 8; bit_index++) {
uint8_t bit = (byte >> bit_index) & 0x1;
uint8_t *pixelp = &video_memory[(row + byte_index) % ROWS]
[(col + (7 - bit_index)) % COLS];
if (bit == 1 && *pixelp ==1) V[0xF] = 1;
*pixelp = *pixelp ^ bit;
}
}
}
void Init(void)
{
int i;
PC = 0x200;
opcode = 0;
I = 0;
SP = 0;
memset(stack, 0, sizeof(uint16_t) * STACK_SIZE);
memset(V, 0, sizeof(uint8_t) * REGISTER_SIZE);
memset(memory, 0, sizeof(uint8_t) * MEMORY_SIZE);
memset(video_memory, 0, sizeof(uint8_t) * SIZE);
memset(font, 0, sizeof(uint8_t) * KEY_SIZE);
for (i = 0; i < 80; i++)
memory[FONT_OFFSET + i] = font[i];
DrawFlag = true;
delay_timer = 0;
sound_timer = 0;
srand(time(NULL));
}
void Tick(void) {
if(delay_timer > 0)
--delay_timer;
if(sound_timer > 0){
--sound_timer;
#if DEBUG
if(sound_timer == 0)
printf("\aBeep");
#endif
}
}