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https://github.com/pnx/m16vm
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52 lines
1.5 KiB
C
52 lines
1.5 KiB
C
/* instr_decode.c
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*
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* Copyright (C) 2012,2014 Henrik Hautakoski <henrik.hautakoski@gmail.com>
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
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* MA 02110-1301, USA.
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*/
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#include "instr_decode.h"
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void instr_decode(unsigned char *instr, struct instr *out) {
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out->opcode = *instr >> 4;
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if (out->opcode == OP_NOOP)
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return;
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// J-Type
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if (out->opcode == OP_JMP) {
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out->j.addr = ((*instr & 0xF) << 8) + *(instr + 1);
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// if MSB (bit 12) is set
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// perform 2s complement by setting bit 13-16 to 1
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/*
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if (out->j.addr & 0x0800)
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out->j.addr |= 0xF000; */
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} else {
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out->r.rs = *instr & 0xF;
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// I-Type
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if (out->opcode == OP_MOVL || out->opcode == OP_MOVH || out->opcode == OP_INT) {
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out->i.imm = *(instr + 1);
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}
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// R/RI-Type
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else {
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out->r.r0 = *(instr + 1) >> 4;
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out->r.r1 = *(instr + 1) & 0xF;
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}
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}
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}
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