358 lines
6.9 KiB
C
358 lines
6.9 KiB
C
#include <stdio.h>
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#include <stdint.h>
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#include <string.h>
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#include <stdlib.h>
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uint8_t mem[64*1024];
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union regset {
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uint8_t r8[8]; // AL, AH, CL, CH...
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uint16_t r16[8]; // AX, CX, DX, BX, SP, BP, SI, DI
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} regset;
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int carry, zero;
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uint16_t ip = 0x100;
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uint16_t flags = 0;
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uint16_t scratch;
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uint16_t reg;
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int off;
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void* rmptr;
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#define RM8 *(uint8_t*)rmptr
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#define RM16 *(uint16_t*)rmptr
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#define R8(x) regset.r8[fixr8ref(x)]
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#define R16(x) regset.r16[x]
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#define AL regset.r8[0]
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#define CL regset.r8[2]
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#define DL regset.r8[4]
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#define AX regset.r16[0]
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#define BX regset.r16[3]
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#define SP regset.r16[4]
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#define BP regset.r16[5]
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#define SI regset.r16[6]
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#define DI regset.r16[7]
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#define ADD 0
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#define OR 1
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#define ADC 2
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#define SBB 3
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#define AND 4
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#define SUB 5
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#define XOR 6
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#define CMP 7
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#define TEST 8
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const char* const regnames[] = { "AX", "CX", "DX", "BX", "SP", "BP", "SI", "DI", 0 };
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// Dump registers
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void dump() {
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printf("\n");
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int i;
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for (i=0; i<8; i++) {
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printf("%s=%04X ", regnames[i], regset.r16[i]);
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}
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printf("\n");
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printf("IP=%04X : %02X %02X\n", ip, mem[ip], mem[ip+1]);
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}
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void arith8(int op, int8_t* dst, int src) {
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// I'll just assume this sign-extends
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int t = *dst;
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int p = *dst;
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switch (op) {
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case ADD: t += src; break;
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case OR: t |= src; break;
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case ADC: t += (src + carry); break;
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case SBB: t -= (src + carry); break;
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case AND: t &= src; break;
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case SUB: t -= src; break;
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case XOR: t ^= src; break;
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case CMP: t -= src; break;
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case TEST: t &= src; break;
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}
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zero=!t;
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// I dont think the following is correct
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carry=((t ^ p) >> 8) & 1;
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if (op==CMP || op==TEST) return;
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*dst = t;
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}
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void arith16(int op, int16_t* dst, int src) {
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// I'll just assume this sign-extends
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int t = *dst;
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int p = *dst;
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switch (op) {
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case ADD: t += src; break;
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case OR: t |= src; break;
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case ADC: t += (src + carry); break;
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case SBB: t -= (src + carry); break;
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case AND: t &= src; break;
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case SUB: t -= src; break;
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case XOR: t ^= src; break;
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case CMP: t -= src; break;
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case TEST: t &= src; break;
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}
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zero=!t;
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// I dont think the following is correct
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carry=((t ^ p) >> 16) & 1;
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if (op==CMP || op==TEST) return;
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*dst = t;
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}
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uint16_t get_flags() {
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return (carry & 1) | ((zero & 1)<< 6);
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}
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int fixr8ref(int r) {
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return ((r << 1) & 0b110) | ((r >> 2) & 0b1);
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}
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uint8_t imm8() {
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uint8_t r = *(uint8_t*)&mem[ip];
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ip++;
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return r;
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}
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uint16_t imm16() {
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uint16_t r = *(uint16_t*)&mem[ip];
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ip+=2;
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return r;
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}
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#define BIT8 8
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#define BIT16 16
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void modrm(int size) {
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uint8_t mod = (mem[ip] >> 6) & 0b11; // highest 2 bits
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reg = (mem[ip] >> 3) & 0b111;
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uint8_t rm = (mem[ip]) & 0b111;
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ip++;
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if (mod == 3) {
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if (size == BIT16) {
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rmptr = &R16(rm);
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} else {
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rmptr = &R8(rm);
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}
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return;
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}
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if (mod == 0 && rm == 6) {
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rmptr = mem + (int16_t)imm16();
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return;
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}
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switch(rm) {
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case 0: rmptr = mem + BX + SI; break;
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case 1: rmptr = mem + BX + DI; break;
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case 2: rmptr = mem + BP + SI; break;
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case 3: rmptr = mem + BP + DI; break;
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case 4: rmptr = mem + SI; break;
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case 5: rmptr = mem + DI; break;
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case 6: rmptr = mem + BP; break;
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case 7: rmptr = mem + BX; break;
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}
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if (mod == 1) rmptr += (int8_t)imm8();
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if (mod == 2) rmptr += (int16_t)imm16();
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}
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int cond(int num) {
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int r = 0;
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switch(num >> 1) {
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case 2:
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r = !scratch;
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break;
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default:
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fprintf(stderr, "Condition code %d not implemented\n", num);
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exit(1);
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break;
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}
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// Each odd condition is the negation of the previous
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if (num & 1) r=1-r;
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return r;
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}
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void push(uint16_t v) {
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SP -= 2;
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*(uint16_t*)&mem[SP] = v;
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}
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uint16_t pop() {
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uint16_t v = *(uint16_t*)&mem[SP];
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SP += 2;
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return v;
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}
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void handle_syscall(int number) {
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switch(number) {
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case 0:
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exit(0);
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break;
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case 2:
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printf("%c", DL);
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break;
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default:
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fprintf(stderr, "Fatal: Unhandled syscall CL=%Xh\n", number);
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exit(1);
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}
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}
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void handle_intr(int number) {
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switch(number) {
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case 0x20:
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handle_syscall(0);
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break;
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case 0xE0:
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handle_syscall(CL);
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break;
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default:
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fprintf(stderr, "Fatal: Unhandled interrupt %Xh\n", number);
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exit(1);
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}
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}
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// Execute a single instruction
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void step() {
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uint8_t opcode = mem[ip];
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ip++;
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switch(opcode) {
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case 0x00: // ARITH r/m8,reg8
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case 0x08:
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case 0x10:
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case 0x18:
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case 0x20:
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case 0x28:
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case 0x30:
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case 0x38:
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modrm(BIT8);
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arith8(opcode >> 3, &RM8, R8(reg));
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break;
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case 0x01: // ARITH r/m16,reg16
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case 0x09:
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case 0x11:
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case 0x19:
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case 0x21:
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case 0x29:
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case 0x31:
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case 0x39:
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modrm(BIT16);
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arith16(opcode >> 3, &RM16, R16(reg));
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break;
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case 0x50 ... 0x57: // PUSH reg16
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push(R16(opcode-0x50));
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break;
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case 0x58 ... 0x5F: // POP reg16
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R16(opcode-0x58)=pop();
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break;
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case 0x70 ... 0x7F: // Jcc
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reg = ip + (int8_t)imm8();
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if (cond(opcode-0x70)) {
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ip = reg;
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}
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break;
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case 0x83: // ARITH r/m16,imm8
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modrm(BIT16);
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arith16(reg, &RM16, imm8());
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break;
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case 0x84: // TEST r/m8,reg8
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modrm(BIT8);
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scratch = R8(reg) & RM8;
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break;
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case 0x88: // MOV r/m8,reg8
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modrm(BIT8);
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RM8 = R8(reg);
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break;
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case 0x89: // MOV r/m16,reg16
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modrm(BIT16);
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RM16 = R16(reg);
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break;
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case 0x8B: // MOV reg16,r/m16
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modrm(BIT16);
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R16(reg) = RM16;
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break;
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case 0x9C: // PUSHF
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push(get_flags());
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break;
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case 0xA1: // MOV AX,memoffs16
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AX = *(uint16_t*)&mem[imm16()];
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break;
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case 0xAC: // LODSB
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AL = mem[SI];
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SI++;
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break;
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case 0xB0 ... 0xB7: // MOV reg8,imm8
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reg = opcode-0xB0;
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R8(reg) = imm8();
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break;
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case 0xB8 ... 0xBF: // MOV reg16,imm16
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reg = opcode-0xB8;
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R16(reg) = imm16();
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break;
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case 0xC3: // RET
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ip = pop();
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break;
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case 0xCC: // INT3
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dump();
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break;
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case 0xCD: // INT imm8
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handle_intr(imm8());
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break;
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case 0xE8: // CALL rw
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reg = ip + (int16_t)imm16();
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if (reg == 5) {
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handle_syscall(CL);
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} else {
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push(ip);
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ip = reg;
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}
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break;
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case 0xEB: // JMP rb
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ip = ip + (int8_t)imm8();
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break;
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case 0xFA: // CLI
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case 0xFB: // STI
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break; // no-op, since we dont have interrupts
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default:
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ip--;
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dump();
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fprintf(stderr, "Invalid opcode at IP=%04X\n", ip);
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exit(1);
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break;
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}
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}
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void copy_cmdline(char* str) {
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int i, c;
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uint8_t *len = &mem[0x80];
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char* ptr = (char*)&mem[0x81];
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c = strlen(str);
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// Clip at max length
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if (c>0x7E) {
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fprintf(stderr, "Command line too long, max is 126 bytes\n");
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exit(1);
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}
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memcpy(ptr, str, c);
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ptr[c]=0x0D;
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*len=c;
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}
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int main(int argc, char** argv) {
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memset(&mem, sizeof(mem), 0);
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memset(®set, sizeof(regset), 0);
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mem[0]=0xCD;
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mem[1]=0x20;
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push(0);
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if (argc>2) {
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copy_cmdline(argv[2]);
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} else {
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copy_cmdline("");
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}
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FILE* fd = fopen(argv[1], "r");
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fread(mem + ip, 1, sizeof(mem) - ip, fd);
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while(1) {
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step();
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dump();
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}
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}
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