/* Simple microprocessor and fibonacci number generator */

const dw   = 8;       /* Data width    */
const opcw = 4;       /* Op-code width */
const oprw = 4;       /* Operand width */

const rom_aw = 4;     /* Width of ROM address bus */
const ram_aw = 4;     /* Width of RAM address bus */

const HALT   = 0 : opcw;
const LOAD   = 1 : opcw;
const LOADI  = 2 : opcw;
const STORE  = 3 : opcw;
const ADD    = 4 : opcw;
const SUB    = 5 : opcw;
const JUMP   = 6 : opcw;
const JUMPNZ = 7 : opcw;
const INPUT  = 8 : opcw;
const OUTPUT = 9 : opcw;

#define _asm_(opc,opr) ((opc)+((opr)*(1<<opcw)))

void main(chan (in)  STDIN  : dw,
	  chan (out) STDOUT : dw)
{
    /* Locations 0 and 1 hold two consecutive fib numbers,
       whilst location 2 holds the counter variable,
       and 3 holds constant 1 */

    rom program = {
	_asm_(LOADI,  1),  /*  0 */  /* Get a one  */
	_asm_(STORE,  3),  /*  1 */  /* Store this */ 
	_asm_(STORE,  1),  /*  2 */  
        _asm_(INPUT,  0),  /*  3 */  /* Read value */
        _asm_(STORE,  2),  /*  4 */  /* Store this */
	_asm_(LOAD,   1),  /*  5 */  /* Loop entry point */
	_asm_(ADD,    0),  /*  6 */  /* Make a fib number */
	_asm_(STORE,  0),  /*  7 */  /* Store it */
	_asm_(OUTPUT, 0),  /*  8 */  /* Output it */
	_asm_(ADD,    1),  /*  9 */  /* Make a fib number */
	_asm_(STORE,  1),  /* 10 */  /* Store it */
	_asm_(OUTPUT, 0),  /* 11 */  /* Output it */
	_asm_(LOAD,   2),  /* 12 */  /* Decrement counter */
	_asm_(SUB,    3),  /* 13 */  
	_asm_(JUMPNZ, 4),  /* 14 */  /* Repeat if not zero */
	_asm_(HALT,   0)   /* 15 */
    };

    ram data[1 << ram_aw] : dw;
    int pc : rom_aw;
    int ir : opcw + oprw;
    int x  : dw;

    int opcode()  = ir <- opcw;
    int operand() = ir \\ opcw;

    do {
	ir, pc = program[pc], pc + 1;
	case (opcode()) {
	   HALT    : skip;                                   
	   LOAD    : x = data[operand() <- ram_aw];            
	   LOADI   : x = 0 @ operand();
	   STORE   : data[operand() <- ram_aw] = x;            
	   ADD     : x = x + data[operand() <- ram_aw];        
	   SUB     : x = x - data[operand() <- ram_aw];        
	   JUMP    : pc = operand() <- rom_aw;                 
	   JUMPNZ  : if (x != 0) pc = operand() <- rom_aw;     
	   INPUT   : STDIN  ? x;                             
	   OUTPUT  : STDOUT ! x;
	   default : stop;
	}
    } while (opcode() != HALT);
}