/* FCE Ultra - NES/Famicom Emulator
 *
 * Copyright notice for this file:
 *  Copyright (C) 2005 Jason Oster
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 */


/*
Pseudo Machine Code (PMC) engine

Simple Low-Level specs:
 Two 16-bit registers named r0 and r1.
 Program counter named i. 
 Full access to NES memory via READ instruction.
 Access to NES registers and non-memory data via COPY instruction.
 Immediate value loading via LOAD instruction.
 Jump instructions with integrated unsigned comparison against r0 and r1.
 Subroutine return instructions with integrated return value.

Compiler specs:
 Work register (operand) defined in LSB of all instructions.
 Instructions without register operands make no current use of LSB.
 Jumps are compiled on the first pass with a 'pointer' to indicate where to jump
 MSB signifies to second pass that the byte is a label for jumps.
 On second pass, jump 'pointer' is replaced with an i-relative pointer, based on where the pointed label is found.
 Labels are removed on second pass in favor of above behavior.

Virtual syntax:
 READ: 2-byte operand for little endian pointer
 LOAD: 2-byte operand for little endian value
 COPY: 1-byte operand for source
 JUMP: 1-byte operand for i-relative increment (unsigned)

Notes:
 For i-relative jumps, i = next instruction. A jump operand of 0 will literally never jump.
*/


//PMC opcodes
#define PMC_READ		0x00 /* Read NES memory into r0 or r1 */
#define PMC_LOAD		0x02 /* Load immediate value into r0 or r1 */
#define PMC_COPY		0x04 /* Copy NES register into r0 or r1 */
#define PMC_JMPE		0x10 /* Jump if r0 == r1 (unsigned) */
#define PMC_JMPN		0x12 /* Jump if r0 != r1 (unsigned) */
#define PMC_JMPG		0x14 /* Jump if r0 > r1 (unsigned) */
#define PMC_JMPL		0x16 /* Jump if r0 < r1 (unsigned) */
#define PMC_JUMP		0x18 /* Unconditional jump */
#define PMC_GOOD		0x20 /* End routine with success */
#define PMC_EROR		0x22 /* End routine with error */

//defines for PMC registers
//#define PMC_R0			0x00
//#define PMC_R1			0x01

//defines for COPY instruction
#define PMC_COPY_A		0x00 /* NES A register */
#define PMC_COPY_X		0x01 /* NES X register */
#define PMC_COPY_Y		0x02 /* NES Y register */
#define PMC_COPY_PC		0x03 /* NES PC register */
#define PMC_COPY_S		0x04 /* NES Stack register */
#define PMC_COPY_SCAN	0x05 /* NES Scanline counter */

02 20 00  LOAD r0,$0020  ;load 32 into r0
05 05     COPY r1,SCAN   ;copy scanline counter into r1
10 01     JMPE i+$01     ;if r0 == r1, jump to GOOD
22        EROR           ;exit with error (no break)
20        GOOD           ;exit with success (cause break)

Originally posted by Parasyte

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Work register (operand) defined in LSB of all instructions.
Instructions without register operands make no current use of LSB.

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