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Amiga Format CD 23
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Amiga Format AFCD23 (Feb 1998, Issue 107).iso
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-in_the_mag-
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emulation
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consoles
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vision-8
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sources
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chip8.c
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C/C++ Source or Header
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1997-12-12
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383 lines
/** Vision8: CHIP8 emulator *************************************************/
/** **/
/** CHIP8.c **/
/** **/
/** This file contains the portable CHIP8 emulation engine **/
/** **/
/** Copyright (C) Marcel de Kogel 1997 **/
/** You are not allowed to distribute this software commercially **/
/** Please, notify me, if you make any changes to this file **/
/****************************************************************************/
#include "CHIP8.h"
#include <stdlib.h>
#include <string.h>
byte chip8_mem[4096];
#define read_mem(a) (chip8_mem[(a)&4095])
#define write_mem(a,v) (chip8_mem[(a)&4095]=(v))
byte chip8_iperiod;
#ifdef DEBUG
byte chip8_trace;
word chip8_trap;
#endif
struct chip8_regs_struct chip8_regs;
static byte chip8_key_pressed;
byte chip8_keys[16];
byte chip8_display[64*32];
byte chip8_running;
#define get_reg_offset(opcode) (chip8_regs.alg+(opcode>>8))
#define get_reg_value(opcode) (*get_reg_offset(opcode))
#define get_reg_offset_2(opcode) (chip8_regs.alg+((opcode>>4)&0x0f))
#define get_reg_value_2(opcode) (*get_reg_offset_2(opcode))
typedef void (*opcode_fn) (word opcode);
typedef void (*math_fn) (byte *reg1,byte reg2);
static void op_call (word opcode)
{
chip8_regs.sp--;
write_mem (chip8_regs.sp,chip8_regs.pc&0xff);
chip8_regs.sp--;
write_mem (chip8_regs.sp,chip8_regs.pc>>8);
chip8_regs.pc=opcode;
}
static void op_jmp (word opcode)
{
chip8_regs.pc=opcode;
}
static void op_key (word opcode)
{
byte key_value,cp_value;
if ((opcode&0xff)==0x9e)
cp_value=1;
else if ((opcode&0xff)==0xa1)
cp_value=0;
else
return;
key_value=chip8_keys[get_reg_value(opcode)&0x0f];
if (cp_value==key_value) chip8_regs.pc+=2;
}
static void op_skeq_const (word opcode)
{
if (get_reg_value(opcode)==(opcode&0xff)) chip8_regs.pc+=2;
}
static void op_skne_const (word opcode)
{
if (get_reg_value(opcode)!=(opcode&0xff)) chip8_regs.pc+=2;
}
static void op_skeq_reg (word opcode)
{
if (get_reg_value(opcode)==get_reg_value_2(opcode)) chip8_regs.pc+=2;
}
static void op_skne_reg (word opcode)
{
if (get_reg_value(opcode)!=get_reg_value_2(opcode)) chip8_regs.pc+=2;
}
static void op_mov_const (word opcode)
{
*get_reg_offset(opcode)=opcode&0xff;
}
static void op_add_const (word opcode)
{
*get_reg_offset(opcode)+=opcode&0xff;
}
static void op_mvi (word opcode)
{
chip8_regs.i=opcode;
}
static void op_jmi (word opcode)
{
chip8_regs.pc=opcode+chip8_regs.alg[0];
}
static void op_rand (word opcode)
{
*get_reg_offset(opcode)=rand()&(opcode&0xff);
}
static void math_or (byte *reg1,byte reg2)
{
*reg1|=reg2;
}
static void math_mov (byte *reg1,byte reg2)
{
*reg1=reg2;
}
static void math_nop (byte *reg1,byte reg2)
{
}
static void math_and (byte *reg1,byte reg2)
{
*reg1&=reg2;
}
static void math_xor (byte *reg1,byte reg2)
{
*reg1^=reg2;
}
static void math_add (byte *reg1,byte reg2)
{
word tmp;
tmp=*reg1+reg2;
*reg1=(byte)tmp;
chip8_regs.alg[15]=tmp>>8;
}
static void math_sub (byte *reg1,byte reg2)
{
word tmp;
tmp=*reg1-reg2;
*reg1=(byte)tmp;
chip8_regs.alg[15]=((byte)(tmp>>8))+1;
}
static void math_shr (byte *reg1,byte reg2)
{
chip8_regs.alg[15]=*reg1&1;
*reg1>>=1;
}
static void math_shl (byte *reg1,byte reg2)
{
chip8_regs.alg[15]=*reg1>>7;
*reg1<<=1;
}
static void math_rsb (byte *reg1,byte reg2)
{
word tmp;
tmp=reg2-*reg1;
*reg1=(byte)tmp;
chip8_regs.alg[15]=((byte)(tmp>>8))+1;
}
static void op_system (word opcode)
{
switch ((byte)opcode)
{
case 0xe0:
memset (chip8_display,0,sizeof(chip8_display));
break;
case 0xee:
chip8_regs.pc=read_mem(chip8_regs.sp)<<8;
chip8_regs.sp++;
chip8_regs.pc+=read_mem(chip8_regs.sp);
chip8_regs.sp++;
break;
}
}
static void op_misc (word opcode)
{
byte *reg,i,j;
reg=get_reg_offset(opcode);
switch ((byte)opcode)
{
case 0x07:
*reg=chip8_regs.delay;
break;
case 0x0a:
if (chip8_key_pressed)
*reg=chip8_key_pressed-1;
else
chip8_regs.pc-=2;
break;
case 0x15:
chip8_regs.delay=*reg;
break;
case 0x18:
chip8_regs.sound=*reg;
if (chip8_regs.sound) chip8_sound_on();
break;
case 0x1e:
chip8_regs.i+=(*reg);
break;
case 0x29:
chip8_regs.i=((word)(*reg&0x0f))*5;
break;
case 0x33:
i=*reg;
for (j=0;i>=100;i-=100) j++;
write_mem (chip8_regs.i,j);
for (j=0;i>=10;i-=10) j++;
write_mem (chip8_regs.i+1,j);
write_mem (chip8_regs.i+2,i);
break;
case 0x55:
for (i=0,j=(opcode>>8)&0x0f;i<=j;++i)
write_mem(chip8_regs.i+i,chip8_regs.alg[i]);
break;
case 0x65:
for (i=0,j=(opcode>>8)&0x0f;i<=j;++i)
chip8_regs.alg[i]=read_mem(chip8_regs.i+i);
break;
}
}
static void op_sprite (word opcode)
{
byte *q;
byte n,x,x2,y,collision;
word p;
x=get_reg_value(opcode)&63;
y=get_reg_value_2(opcode)&31;
p=chip8_regs.i;
q=chip8_display+y*64;
n=opcode&0x0f;
if (n+y>32) n=32-y;
for (collision=1;n;--n,q+=64)
{
for (y=read_mem(p++),x2=x;y;y<<=1,x2=(x2+1)&63)
if (y&0x80) collision&=(q[x2]^=0xff);
}
chip8_regs.alg[15]=collision^1;
}
static math_fn math_opcodes[16]=
{
math_mov,
math_or,
math_and,
math_xor,
math_add,
math_sub,
math_shr,
math_rsb,
math_nop,
math_nop,
math_nop,
math_nop,
math_nop,
math_nop,
math_shl,
math_nop
};
static void op_math (word opcode)
{
(*(math_opcodes[opcode&0x0f]))
(get_reg_offset(opcode),get_reg_value_2(opcode));
}
static opcode_fn main_opcodes[16]=
{
op_system,
op_jmp,
op_call,
op_skeq_const,
op_skne_const,
op_skeq_reg,
op_mov_const,
op_add_const,
op_math,
op_skne_reg,
op_mvi,
op_jmi,
op_rand,
op_sprite,
op_key,
op_misc
};
/****************************************************************************/
/* Execute chip8_iperiod opcodes */
/****************************************************************************/
void chip8_execute (void)
{
byte i;
byte key_pressed=0;
word opcode;
for (i=chip8_iperiod;i;--i)
{
/* Fetch the opcode */
opcode=(read_mem(chip8_regs.pc)<<8)+read_mem(chip8_regs.pc+1);
#ifdef DEBUG
/* Check if trap address has been reached */
if ((chip8_regs.pc&4095)==chip8_trap) chip8_trace=1;
/* Call the debugger if chip8_trace!=0 */
if (chip8_trace) chip8_debug (opcode,&chip8_regs);
#endif
chip8_regs.pc+=2;
/* Emulate this opcode */
(*(main_opcodes[opcode>>12]))(opcode&0x0fff);
}
/* Update timers */
if (chip8_regs.delay) --chip8_regs.delay;
if (chip8_regs.sound)
if (!--chip8_regs.sound) chip8_sound_off();
/* Update the machine status */
chip8_interrupt ();
for (i=key_pressed=0;i<16;++i) /* check if a key was first */
if (chip8_keys[i]) key_pressed=i+1; /* pressed */
if (key_pressed && key_pressed!=chip8_key_pressed)
chip8_key_pressed=key_pressed;
else
chip8_key_pressed=0;
}
/****************************************************************************/
/* Reset the virtual chip8 machine */
/****************************************************************************/
void chip8_reset (void)
{
static byte chip8_sprites[16*5]=
{
0xf9,0x99,0xf2,0x62,0x27,
0xf1,0xf8,0xff,0x1f,0x1f,
0x99,0xf1,0x1f,0x8f,0x1f,
0xf8,0xf9,0xff,0x12,0x44,
0xf9,0xf9,0xff,0x9f,0x1f,
0xf9,0xf9,0x9e,0x9e,0x9e,
0xf8,0x88,0xfe,0x99,0x9e,
0xf8,0xf8,0xff,0x8f,0x88,
};
byte i;
for (i=0;i<16*5;++i)
{
write_mem (i<<1,chip8_sprites[i]&0xf0);
write_mem ((i<<1)+1,chip8_sprites[i]<<4);
}
memset (chip8_regs.alg,0,sizeof(chip8_regs.alg));
memset (chip8_keys,0,sizeof(chip8_keys));
chip8_key_pressed=0;
memset (chip8_display,0,sizeof(chip8_display));
chip8_regs.delay=chip8_regs.sound=chip8_regs.i=0;
chip8_regs.sp=0x1e0;
chip8_regs.pc=0x200;
chip8_sound_off ();
chip8_running=1;
#ifdef DEBUG
chip8_trace=0;
#endif
}
/****************************************************************************/
/* Start CHIP8 emulation */
/****************************************************************************/
void chip8 (void)
{
chip8_reset ();
while (chip8_running) chip8_execute ();
}