X86 (subset) interpreter, prototype. Reads instructions from plaintext file, instructions separated by spaces.

vm.c

/*
 * vm.c - Simple X86 interpreter
 *
 * Copyright (c) 2013, Jeremie Miserez <[email protected]>
 */

/*
 * Highlights/Features not specified in assignment
 * ===============================================
 * - Memory simulation of all 4GB with paging
 *   - Endianness is ensured by correctly reading and writing all bytes
 *     in the correct order (little endian) byte-by-byte.
 *
 * - Program is loaded into simulated memory and executed from there
 *   -> The interpreter terminates only once the instruction at %eip is no
 *      longer readable/valid (e.g. all zeros). Thus, self-modifying code
 *      is possible!
 *
 * - Graphical visualization of memory/registers/instructions:
 *   -> The memory around EBP and ESP is shown as in textbooks.
 *   -> The individual bytes can be seen in a similar fashion as in a HEX-editor,
 *      with interpretations as bytes, 16bit integers, 32bit integers.
 *
 * - Disassembler supports decoding of all addressing modes and possible bytes.
 *   -> New opcodes can be easily added to the decoder with having to change any of
 *     the parts not directly concerned with the opcode itself.
 *
 * - Dispatcher/Interpreter supports generalized interpretation of addresses and
 *   registers:
 *   -> Supports all addressing modes. Although not all opcodes are implemented for
 *      addressing modes, the LEA instruction is implemented fully (32/16bits) as an
 *      example. Note that the functions used for LEA could also be reused for
 *      all other operations.
 *   -> Concept of an vm_effective_location, which can be a register or an actual
 *      memory address. This makes it easy to specify a "source" and a "target",
 *      similar to the descriptions in the Intel x86 manual.
 *   -> This makes the interpreter very general, new opcodes can more or less
 *      be implemented without modification of the framework.
 *  
 *  - Support for JNZ, NOP, and additional XOR functions.
 */

/*
 * Memory simulation: 4GB of virtual memory using on-demand paging
 * ===============================================================
 * General idea:
 * - Provide a 4GiB 32bit address space while only allocating the memory actually needed.
 * - Use a form of paging with 64KiB pages and a fixed page table with 65536 entries.
 * - Whenever a memory address is accessed, look up the appropriate memory address and if
 *   necessary malloc the appropriate space.
 *
 * Pagetable size:
 * - 65536 entries of 64bits each = 512KiB pagetable
 *
 * Reading and writing is done byte by byte in order to ensure correct access across page
 * boundaries. A virtual address (between 0 and 4GB) as used by the X86 program is used
 * to access the contents of the simulated memory.
 *
 *
 *
 *
 *
 * A note about segments selectors/descriptor tables:
 * - As the segment descriptor tables are not available and need not be implemented, we can
 *   (implicitely) assume that our segments look as follows (same as usermode/Linux)
 * - Code segment: Starts at 0x0, limit 4GB, (Permissions (unchecked): Read/Execute)
 *   -> CS register
 * - Data segment: Starts at 0x0, limit 4GB, (Permissions (unchecked): Read/Write)
 *   -> SS, DS, ES registers.
 * - Unused/Unimplemented: Segments pointed to by:
 *   -> FS, GS registers
 * Also, we assume that the D/B flags are set to 1 (32bit addressing mode)
 *
 * Therefore, no translation between logical addresses and effective addresses is necessary,
 * as the base address is always 0. Also, no check is necessary to determine whether the
 * address is inside the correct segment, as they all have a 32bit address space.
 *
 */

#include <stdlib.h>
#include <stdint.h> // data types
#include <stdio.h> // IO etc.
#include <ctype.h> //characters
#include <stdarg.h> //variadic functions
#include <inttypes.h> //PRI macros
#include <string.h>
#include <errno.h>
#include <unistd.h>

/*
 * Constants and macros
 */
//Decoder constants
#define MAX_INSTRUCTION_LENGTH 15
#define DECODE_STATE_END 0
#define DECODE_STATE_PREFIX 1
#define DECODE_STATE_OPCODE 2
#define DECODE_STATE_CODE_OFFSET 3
#define DECODE_STATE_MODRM 4
#define DECODE_STATE_SIB 5
#define DECODE_STATE_DISPLACEMENT 6
#define DECODE_STATE_IMMEDIATE 7

//Instructions
#define OP_NOP 0
#define OP_LEA_8D 1
#define OP_AND_83_4 2
#define OP_PUSH_50 3
#define OP_SAR_C1_7 4
#define OP_XOR_34 5
#define OP_MOV_89 6
#define OP_INC_40 7
#define OP_SUB_83_5 8
#define OP_CMP_80_7 9
#define OP_POP_58 10
#define OP_JNZ_75 11
#define OP_XOR_31 12

//Shared opcodes
#define OP_0F 100
#define OP_83 101
#define OP_C1 102
#define OP_80 103

//Register numbers
#define EAX 0
#define ECX 1
#define EDX 2
#define EBX 3
#define ESP 4
#define EBP 5
#define ESI 6
#define EDI 7
//For 16bit addressing
#define AL 0
#define CL 1
#define DL 2
#define BL 3
#define AH 4
#define CH 5
#define DH 6
#define BH 7

//Segment registers
#define CS 0
#define SS 1
#define DS 2
#define ES 3
#define FS 4
#define GS 5

//EFLAGS bits
#define CF 0 //Carry
#define PF 2 //Parity
#define AF 4 //Adjust flag, not implemented/simulated
#define ZF 6 //Zero
#define SF 7 //Sign
#define OF 11 //Overflow

//Printing of the stack (how much memory around the stack frame should be output)
#define PRINT_MEM_STACK_OUTSIDE_LINES 4
#define PRINT_MEM_STACK_INSIDE_LINES 4
//Vitual memory constants
#define MMU_PAGESIZE 0x10000 // =64KiB
#define MMU_PAGES 0x10000 // =(4GiB/64KiB) = 65536 pages = 0x100000000 / 0x10000

/*
 * Types
 */
//Singleton: cmdline arguments
typedef struct {
	int loglevel;
	int interactive;
	char *inputfile;
} vm_cmdline;

//Singleton: File input (not used after loading into virtual memory)
typedef struct {
	int length;
	uint8_t* buffer;
} vm_input;

//Singleton: Register state
typedef struct {
	uint32_t gp[8];
	uint32_t eip;
	uint32_t eflags;
	uint32_t seg[6];
} vm_registerfile;

//Singleton: Virtual memory state
typedef struct {
	uint8_t** pagetable; //pointer to array of pointers to memory addresses
} vm_mmu;

//Singleton: Current instruction being executed
typedef struct {
	int halt;
	int op;
	uint8_t text[MAX_INSTRUCTION_LENGTH];
	uint32_t length;
	uint8_t operandsizeoverride;
	uint8_t addrsizeoverride;
	uint16_t code_offset_1;
	uint32_t code_offset_2;
	uint8_t rbrwrd; //hardcoded register as part of opcode (e.g. PUSH 50-57)
	uint8_t mod;
	uint8_t rm;
	uint8_t reg;
	uint32_t ss;
	uint8_t index;
	uint8_t base;
	uint32_t displacement;
	uint32_t immediate;
	int has_modrm;
	int has_sib;
	int disp_size;
	int immed_size;
	int code_offset_size;
	int modrm_contains_opcode_extension;
} vm_op;

//Either a register or memory location
typedef struct {
	int is_gp_register; // is this a register address or a memory address?
	int reg; //general purpose register number
	uint32_t address; //virtual address
} vm_effective_location;

/********************
 * Global variables
 *******************/
vm_cmdline g_cmdline;
vm_input g_input;
vm_op g_op;
vm_registerfile g_regs;
vm_mmu g_mmu;
int print_counter= 0;

/*********************
 * Function prototypes
 *********************/

void run_interpreter();
void usage();

void read_input();
void load_program_into_memory();
void decode();
void dispatch();

void set_subtract_eflags32(uint32_t old, uint32_t new, uint32_t value);
void set_SF_ZF_PF_eflags32(uint32_t new);

vm_effective_location null_location();
vm_effective_location effective_loc32();
vm_effective_location effective_loc16();
uint32_t sib_addr32();
uint16_t rm_addr16();

vm_effective_location register_location(int reg);
vm_effective_location memory_location(uint32_t address);
uint32_t read_effective_loc32(vm_effective_location src);
uint32_t read_effective_loc16(vm_effective_location src);
uint32_t read_effective_loc8(vm_effective_location src);
void write_effective_loc32(vm_effective_location dest, uint32_t value);
void write_effective_loc16(vm_effective_location dest, uint32_t value);
void write_effective8(vm_effective_location dest, uint32_t value);

void init_g_regs();
void init_g_op();
void print_g_regs();
void print_g_op();
void print_memory();
void print_memory_location(uint32_t address);
void print_input();

uint8_t extract_bits8(uint8_t input, int lsb, int num);
uint32_t extract_bits32(uint32_t input, int lsb, int num);

uint32_t sign_extend8(uint32_t input);
uint32_t sign_extend16(uint32_t input);
uint32_t zero_extend8(uint32_t input);
uint32_t zero_extend16(uint32_t input);

uint8_t get_eflag(int flag);
uint32_t get_eflag_bit(int flag);
void set_eflag(int flag);
void write_eflag(int flag, int value);
void clear_eflag(int flag);
void toggle_eflag(int flag);
int parity_lsb(uint32_t value);

void init_g_mmu();
void mmu_allocate_page(int pagenumber);
uint32_t mmu_read(uint32_t addr, int bytes);
uint8_t mmu_readbyte(int pagenumber, uint32_t offset);
void mmu_writebyte(int pagenumber, uint32_t offset, uint8_t value);
void mmu_write(uint32_t addr, int bytes, uint32_t value);

void internal_log(char *tag, char *template, va_list variadic_args, int level);
void log_fatal_error(char *template, ...);
void log_debug(char *template, ...);

/*
 * Main function, handles cmdline arguments
 */
int main(int argc, char **argv) {
	char opt;

	g_cmdline.loglevel= 0;
	g_cmdline.interactive= 0;
	g_cmdline.inputfile= NULL;

	int show_usage= 1;
	// Read and interpret the command line arguments
	while ((opt= getopt(argc, argv, "f:si")) != EOF) {
		switch (opt) {
			case 'f': // input file
				show_usage= 0;
				g_cmdline.inputfile= optarg;
				log_debug("Input file is %s", g_cmdline.inputfile);
				break;
			case 's': // stdin
				show_usage= 0;
				log_debug("Input is stdin");
				break;
			case 'i': // interactive mode
				g_cmdline.interactive= 1;
				log_debug("Interactive mode");
				break;
			default:
				break;
		}
	}
	if (show_usage) {
		usage();
	} else {
		run_interpreter();
	}
	exit(0);
}

/*
 * Prints usage
 */
void usage() {
	fprintf(stderr, "Usage: vm [i] [-f <file>] [-s] < input \n");
	fprintf(stderr, "Options\n");
	fprintf(stderr, "\t-i         Interactive mode (wait for ENTER after every step)\n");
	fprintf(stderr, "\t-f <file>  Use file for input\n");
	fprintf(stderr, "\t-s         Use stdin for input\n");
	fprintf(stderr, "Example: vm -f myinput.txt\n");
	fprintf(stderr, "Example: vm -s < myinput.txt\n");
}

/*
 * Waits for the user to press enter if in interactive mode.
 */
void interactive_wait() {
	if (g_cmdline.interactive) {
		printf("(Press ENTER to continue...)");
		getchar(); //wait for user input
	}
}

/*
 * Allocate memory for page and add entry to pagetable
 */
void mmu_allocate_page(int pagenumber) {
	//allocate the page
	void* page= (void*) calloc(1, MMU_PAGESIZE); //cleared to zeros
	if (page == NULL ) {
		log_fatal_error("Out of memory");
	}
	//add pagetable entry
	g_mmu.pagetable[pagenumber]= page;
}

/*
 * Read from virtual memory
 */
uint32_t mmu_read(uint32_t addr, int bytes) { //bytes must be 0..4
	int pagenumber;
	uint32_t offset;
	uint32_t value= 0;
	uint32_t tmp;
	int total= bytes;
	int remaining= bytes;
	while (remaining > 0) {
		//might need to read across page boundaries, therefore read 1 byte at a time
		pagenumber= addr / MMU_PAGESIZE;
		offset= addr % MMU_PAGESIZE;
		tmp= (uint32_t) mmu_readbyte(pagenumber, offset);
		// Endianness!
		// uint32_t:  0xAABBCCDD  0xDEADBEEF
		// uint16_t:  0xCCDD 0xAABB 0xBEEF 0xDEAD
		// in memory; DD CC BB AA EF BE AD DE
		value= value | (tmp << (8 * (total - remaining)));
		remaining--;
		addr++;
	}
	return value;
}

/*
 * Read byte from virtual memory
 */
uint8_t mmu_readbyte(int pagenumber, uint32_t offset) {
	uint8_t* page= g_mmu.pagetable[pagenumber];
	if (page == NULL ) {
		return 0; //read 0x0 if page is unallocated
	} else {
		uint8_t* mem= page + offset;
		return *mem;
	}
}

/*
 * Write byte to virtual memory
 */
void mmu_writebyte(int pagenumber, uint32_t offset, uint8_t value) {
	uint8_t* page= g_mmu.pagetable[pagenumber];
	if (page == NULL ) {
		mmu_allocate_page(pagenumber);
		page= g_mmu.pagetable[pagenumber];
	}
	uint8_t* mem= page + offset;
	*mem= value;
}

/*
 * Write to virtual memory
 */
void mmu_write(uint32_t addr, int bytes, uint32_t value) {
	int pagenumber;
	uint32_t offset;
	int remaining= bytes;
	int tmp= value;
	while (remaining > 0) {
		pagenumber= addr / MMU_PAGESIZE;
		offset= addr % MMU_PAGESIZE;
		mmu_writebyte(pagenumber, offset, tmp & ((1 << 8) - 1)); //write only lsb
		tmp= tmp >> 8;
		remaining--;
		addr++;
	}
}

/*
 * Logger using variadic functions, checks loglevel before printing
 */
void internal_log(char *tag, char *template, va_list variadic_args, int level) {
	if (level <= g_cmdline.loglevel) {
		printf("%s: ", tag);
		vprintf(template, variadic_args);
		printf("\n");
	}
}
/*
 * Log error and terminate
 */
void log_fatal_error(char *template, ...) {
	va_list args;
	va_start(args, template);
	internal_log("Fatal error", template, args, 0);
	va_end(args);
	exit(1);
}
/*
 * Log debug
 */
void log_debug(char *template, ...) {
	va_list args;
	va_start(args, template);
	internal_log("DEBUG", template, args, 1);
	va_end(args);
}

/*
 * Reset global singleton
 */
void init_g_regs() {
	g_regs.gp[EAX]= 0xbf8db144;
	g_regs.gp[ECX]= 0x88c5cffb;
	g_regs.gp[EDX]= 0x1;
	g_regs.gp[EBX]= 0xae5ff4;
	g_regs.gp[ESP]= 0xbf8db0bc;
	g_regs.gp[EBP]= 0xbf8db118;
	g_regs.gp[ESI]= 0x9a0ca0;
	g_regs.gp[EDI]= 0x0;
	g_regs.eip= 0x8048354;
	g_regs.eflags= 0x246;
	g_regs.seg[CS]= 0x73;
	g_regs.seg[SS]= 0x7b;
	g_regs.seg[DS]= 0x7b;
	g_regs.seg[ES]= 0x7b;
	g_regs.seg[FS]= 0x0;
	g_regs.seg[GS]= 0x33;
}

/*
 * Reset global singleton
 */
void init_g_op() {
	g_op.halt= 0;
	g_op.op= 0;
	g_op.length= 0;
	g_op.operandsizeoverride= 0;
	g_op.addrsizeoverride= 0;
	g_op.code_offset_1= 0;
	g_op.code_offset_2= 0;
	g_op.rbrwrd= 0;
	g_op.mod= 0;
	g_op.rm= 0;
	g_op.reg= 0;
	g_op.ss= 0;
	g_op.index= 0;
	g_op.base= 0;
	g_op.displacement= 0;
	g_op.immediate= 0;
	g_op.has_modrm= 0;
	g_op.has_sib= 0;
	g_op.disp_size= 0;
	g_op.immed_size= 0;
	g_op.code_offset_size= 0;
	g_op.modrm_contains_opcode_extension= 0;
}

/*
 * Initialize virtual memory, allocate memory for pagetable
 */
void init_g_mmu() {
	//allocate the pagetable with 4096 pointers.
	g_mmu.pagetable= (uint8_t**) calloc(MMU_PAGES, sizeof(uint8_t*)); //cleared to zeros
	if (g_mmu.pagetable == NULL ) {
		log_fatal_error("Out of memory");
	}
}

/*
 * Convert int to binary string
 */
const char *to_binary8(uint8_t x) {
	static char bin[9]; //initialized to zeros on program start due to static-ness
	bin[0]= '\0';
	int mask;
	int bit;
	for (bit= 7; bit >= 0; bit--) {
		mask= (1 << bit);
		strcat(bin, (x & mask) ? "1" : "0");
	}
	return bin;
}
/*
 * Convert int to binary string
 */
const char *to_binary32(uint32_t x) {
	static char bin[33]; //initialized to zeros on program start due to static-ness
	bin[0]= '\0';
	strcat(bin, to_binary8(x >> 24));
	strcat(bin, to_binary8(x >> 16));
	strcat(bin, to_binary8(x >> 8));
	strcat(bin, to_binary8(x));
	return bin;
}

/*
 * Print registers
 *
 * |31..16|15-8|7-0|
 *         |AH.|AL.|
 *         |AX.....|
 * |EAX............|
 */
void print_g_regs() {
	printf("+---------------------------------------------------------------------------------------+\n");
	printf("| EAX_AHAL | ECX_CHCL | EDX_DHDL | EBX_BHBL | ESP_SP__ | EBP_BP__ | ESI_SI__ | EDI_DI__ |\n");
	printf("| %8x | %8x | %8x | %8x | %8x | %8x | %8x | %8x |\n", g_regs.gp[EAX], g_regs.gp[ECX], g_regs.gp[EDX], g_regs.gp[EBX], g_regs.gp[ESP], g_regs.gp[EBP], g_regs.gp[ESI], g_regs.gp[EDI]);
	printf("+---------------------------------------------------------------------------------------+\n");
	printf("| EFLAGS__ | ____________________O___SZ_A_P_C |");
	printf("      C: carry  P: parity  (A: adjust)   |\n");
	printf("| %8x | %s |", g_regs.eflags, to_binary32(g_regs.eflags));
	printf("      Z: zero   S: sign    O: overflow   |\n");
	printf("+---------------------------------------------------------------------------------------+\n");
	printf("| CS______ | SS______ | DS______ | ES______ | FS______ | GS______ |          | EIP_____ |\n");
	printf("| %8x | %8x | %8x | %8x | %8x | %8x |          | %8x |\n",
			g_regs.seg[CS], g_regs.seg[SS], g_regs.seg[DS], g_regs.seg[ES], g_regs.seg[FS], g_regs.seg[GS], g_regs.eip);
	printf("+---------------------------------------------------------------------------------------+\n");
}

/*
 * Print stack
 *
 * ebp+8  esp+40 (memory contents)
 * ebp+4  esp+44
 * ebp+0  esp+48
 * ebp-4  esp+4C
 * ebp-8  esp+50
 * ebp-12 esp+54
 * ...    ...
 * ebp-80 esp+12
 * ebp-84 esp+8
 * ebp-88 esp+4
 * ebp-9C esp+0
 * ebp-90 esp-4
 * ebp-94 esp-8
 *
 */
void print_memory() {
	printf("+--------------------------------------------------------------------------------+\n");
	printf("| Memory | ADDRESS_ | +/- %%esp  | +/- %%ebp  | Bytes______ | as_int32 | as int16s |\n");
	printf("+--------------------------------------------------------------------------------+\n");
	printf("| (%%eax) ");
	print_memory_location(g_regs.gp[EAX]);
	printf("| (%%ecx) ");
	print_memory_location(g_regs.gp[ECX]);
	printf("| (%%edx) ");
	print_memory_location(g_regs.gp[EDX]);
	printf("| (%%ebx) ");
	print_memory_location(g_regs.gp[EBX]);
	printf("| (%%esp) ");
	print_memory_location(g_regs.gp[ESP]);
	printf("| (%%ebp) ");
	print_memory_location(g_regs.gp[EBP]);
	printf("| (%%esi) ");
	print_memory_location(g_regs.gp[ESI]);
	printf("| (%%edi) ");
	print_memory_location(g_regs.gp[ESI]);
	printf("| (%%eip) ");
	print_memory_location(g_regs.eip);
	printf("| (%%cs)  ");
	print_memory_location(g_regs.seg[CS]);
	printf("| (%%ss)  ");
	print_memory_location(g_regs.seg[SS]);
	printf("| (%%ds)  ");
	print_memory_location(g_regs.seg[DS]);
	printf("| (%%es)  ");
	print_memory_location(g_regs.seg[ES]);
	printf("| (%%fs)  ");
	print_memory_location(g_regs.seg[FS]);
	printf("| (%%gs)  ");
	print_memory_location(g_regs.seg[GS]);
	printf("|--------------------------------------------------------------------------------|\n");

	//start with highest address, work our way down.
	uint32_t esp= g_regs.gp[ESP];
	uint32_t ebp= g_regs.gp[EBP];

	uint32_t higher;
	uint32_t lower;
	if (ebp >= esp) {
		higher= ebp;
		lower= esp;
	} else {
		higher= esp;
		lower= ebp;
	}
	int i;
	for (i= PRINT_MEM_STACK_OUTSIDE_LINES; i > 0; i--) {
		printf("| Stack  ");
		print_memory_location(higher + 4 * i);
	}
	uint32_t curr= higher;
	while (curr > lower) {
		printf("| Stack  ");
		print_memory_location(curr);
		curr= curr - 4;
		//if there is a gap, print "..." and skip ahead
		if ((higher - curr) / 4 > PRINT_MEM_STACK_INSIDE_LINES && ((curr - lower) / 4 > PRINT_MEM_STACK_INSIDE_LINES)) {
			printf("| ...    ");
			printf("| ...      | ...       | ...       | ...         | ...      | ...       |\n");
			curr= lower + 4 * PRINT_MEM_STACK_INSIDE_LINES;
		}
	}
	printf("| Stack  ");
	print_memory_location(lower);
	for (i= 1; i <= PRINT_MEM_STACK_OUTSIDE_LINES; i++) {
		printf("| Stack  ");
		print_memory_location(lower - 4 * i);
	}
	printf("+--------------------------------------------------------------------------------+\n");
}
void print_memory_location(uint32_t address) {
	char sign_espoff= ((int32_t) (address - g_regs.gp[ESP])) >= 0 ? (address == g_regs.gp[ESP] ? ' ' : '+') : '-';
	char sign_ebpoff= ((int32_t) (address - g_regs.gp[EBP])) >= 0 ? (address == g_regs.gp[EBP] ? ' ' : '+') : '-';
	uint32_t abs_espoff= abs((int32_t) (address - g_regs.gp[ESP]));
	uint32_t abs_ebpoff= abs((int32_t) (address - g_regs.gp[EBP]));
	//Read multiple times, do not convert after read to enable spotting memory read bugs.
	uint32_t byte0= mmu_read(address, 1);
	uint32_t byte1= mmu_read(address + 1, 1);
	uint32_t byte2= mmu_read(address + 2, 1);
	uint32_t byte3= mmu_read(address + 3, 1);
	uint32_t asint32= mmu_read(address, 4);
	uint32_t asint16_0= mmu_read(address, 2);
	uint32_t asint16_1= mmu_read(address + 2, 2);
	printf("| %8x | %c%8x | %c%8x | %02x %02x %02x %02x | %8x | %4x %4x |\n", address, sign_espoff, abs_espoff, sign_ebpoff, abs_ebpoff, byte0, byte1, byte2, byte3, asint32, asint16_0, asint16_1);

}

/*
 * Print decoded instruction
 */
void print_g_op() {
	printf("Instruction: ");
	int i;
	for (i= 0; i < g_op.length; i++) {
		printf("%02x ", g_op.text[i]);
	}
	printf("\n");

	printf("  - Length:                %x (decimal: %d)\n", g_op.length, g_op.length);
	printf("  - Operand size override: ");
	printf(g_op.operandsizeoverride ? "yes\n" : "no\n");
	printf("  - Address size override: ");
	printf(g_op.addrsizeoverride ? "yes\n" : "no\n");
	printf("  - Code offset:           %x:%x\n", g_op.code_offset_1, g_op.code_offset_2);
	printf("  - rb/rw/rd register:     %x\n", g_op.rbrwrd);
	printf("  - modRM:                 ");
	printf(g_op.has_modrm ? "yes\n" : "no\n");
	printf("    * mod:                 %x\n", g_op.mod);
	printf("    * r/m:                 %x\n", g_op.rm);
	printf("    * reg:                 %x\n", g_op.reg);
	printf("      - is part of opcode: ");
	printf(g_op.modrm_contains_opcode_extension ? "yes\n" : "no\n");
	printf("  - SIB:                   ");
	printf(g_op.has_sib ? "yes\n" : "no\n");
	printf("    * ss:                  %x \n", g_op.ss);
	printf("    * index:               %x \n", g_op.index);
	printf("    * base:                %x \n", g_op.base);
	printf("  - Displacement bytes:    %x \n", g_op.disp_size);
	printf("    * disp32/disp16:       %x (decimal: %d)\n", (int32_t) g_op.displacement, (int32_t) g_op.displacement);
	printf("    * disp8:               %x (decimal: %d)\n", (int32_t) sign_extend8(g_op.displacement), (int32_t) sign_extend8(g_op.displacement));
	printf("  - Immediate bytes:       %x (decimal: %d)\n", g_op.immed_size, g_op.immed_size);
	printf("    * 32bit, unsigned:     %x (decimal: %d)\n", g_op.immediate, g_op.immediate);
	printf("    * 32bit, signed:       %x (decimal: %d)\n", ((int32_t) g_op.immediate), ((int32_t) g_op.immediate));
	printf("    * 16bit, unsigned:     %x (decimal: %d)\n", ((uint16_t) g_op.immediate), ((uint16_t) g_op.immediate));
	printf("    * 16bit, signed:       %x (decimal: %d)\n", ((int16_t) g_op.immediate), ((int16_t) g_op.immediate));
	printf("    * 8bit, unsigned:      %x (decimal: %d)\n", ((uint8_t) g_op.immediate), ((uint8_t) g_op.immediate));
	printf("    * 8bit, signed:        %x (decimal: %d)\n", ((int8_t) g_op.immediate), ((int8_t) g_op.immediate));

	log_debug("op:\t%x", g_op.op);
}

/*
 * Print input read from file
 */
void print_input() {
	int i;
	for (i= 0; i < g_input.length; i++) {
		if (i % 16 == 0) {
			printf("\n");
		}
		printf("%02x ", g_input.buffer[i]);
	}
	printf("\n");
}

/*
 * Read file
 */
void read_input() {
	g_input.buffer= NULL;
	g_input.length= 0;

	int buffer_length= MAX_INSTRUCTION_LENGTH;

	g_input.buffer= (uint8_t*) malloc(sizeof(uint8_t) * buffer_length);
	if (g_input.buffer == NULL ) {
		log_fatal_error("Out of memory.");
	}

	FILE *in_file= NULL;

	if (g_cmdline.inputfile != NULL ) {
		in_file= fopen(g_cmdline.inputfile, "rt");
		if (in_file == NULL ) {
			log_fatal_error("Error opening file.");
		} else {
			log_debug("Opened file for reading.");
		}
	} else {
		log_debug("Reading from stdin.");
	}

	char c;
	char tmp[3];
	int tmppos= 0;
	tmp[2]= '\0';
	do {
		if (in_file != NULL ) {
			c= fgetc(in_file);
		} else {
			c= getchar();
		}
		if (isxdigit(c)) {
			tmp[tmppos++]= c;
			if (tmppos == 2) { //2 characters read
				tmppos= 0;
				if (g_input.length >= buffer_length) {
					buffer_length= buffer_length * 2;
					g_input.buffer= (uint8_t*) realloc(g_input.buffer, sizeof(uint8_t) * buffer_length);
					if (g_input.buffer == NULL ) {
						log_fatal_error("Out of memory: Could not realloc buffer.");
					}
				}
				g_input.buffer[g_input.length]= (uint8_t) strtol(&tmp[0], (char **) NULL, 16);
				log_debug("Read: %x", g_input.buffer[g_input.length]);
				g_input.length++;
			}
		}
	} while (c != EOF);
	log_debug("End of input reached");
	if (in_file != NULL ) {
		fclose(in_file);
	}
}

/*
 * Extract num bits out of input, starting at lsb
 */
uint32_t extract_bits32(uint32_t input, int lsb, int num) {
	return (input >> lsb) & ((1 << num) - 1);
}
uint8_t extract_bits8(uint8_t input, int lsb, int num) {
	return (input >> lsb) & ((1 << num) - 1);
}

/*
 * Sign extend.
 * 0000 0000 0000 1000 -> 1111 1111 1111 1000
 */
uint32_t sign_extend8(uint32_t input) {
	int8_t value= input;
	return value;
}
uint32_t sign_extend16(uint32_t input) {
	int16_t value= input;
	return value;
}
/*
 * Zero extend
 */
uint32_t zero_extend8(uint32_t input) {
	uint8_t value= input;
	return value;
}
uint32_t zero_extend16(uint32_t input) {
	uint16_t value= input;
	return value;
}

/*
 * EFLAG manipulation
 */

uint8_t get_eflag(int flag) {
	return extract_bits32(g_regs.eflags, flag, 1);
}
uint32_t get_eflag_bit(int flag) {
	return g_regs.eflags & (1 << flag);
}
void set_eflag(int flag) {
	g_regs.eflags|= (1 << flag);
}
void clear_eflag(int flag) {
	g_regs.eflags&= ~(1 << flag);
}
void toggle_eflag(int flag) {
	g_regs.eflags^= (1 << flag);
}
void write_eflag(int flag, int value) {
	if (value) {
		set_eflag(flag);
	} else {
		clear_eflag(flag);
	}
}

/*
 * Calculate parity of LSB
 * Note:
 * - There are fast ways to calculate parity using bit operations and multiply only.
 * - X86 processors only calculate the parity of the LSB for the parity flag
 */
int parity_lsb(uint32_t value) {
	int parity= 0;
	value= value & 0xFF; //select LSB
	while (value) {
		value= value & (value - 1); //remove lowest remaining 1
		parity= parity ^ 1; //toggle parity
	}
	return parity;
}

/*
 * Load program into virtual memory at %eip
 */
void load_program_into_memory() {
	int i;
	for (i= 0; i < g_input.length; i++) {
		mmu_write(g_regs.eip + i, 1, g_input.buffer[i]);
	}
}

/*
 * Decode and produce an abstract operation which contains all the information necessary to execute it.
 */
void decode() {
	int state= DECODE_STATE_PREFIX;
	int numread_disp= 0;
	int numread_immed= 0;
	int numread_code_offset= 0;
	int reread_byte= 0;
	uint32_t current_address;
	uint8_t current_byte;

	init_g_op();

	current_address= g_regs.eip;

	while (state != DECODE_STATE_END) {
		if (!reread_byte) {
			current_byte= mmu_read(current_address, 1);
			g_op.text[g_op.length]= current_byte; //store original bytes
			g_op.length++;
		}
		reread_byte= 0;

		switch (state) {
			case DECODE_STATE_PREFIX:
				switch (current_byte) {
					case 0xf0: //Group 1: Lock and repeat prefixes
					case 0xf2:
					case 0xf3:
					case 0x2e: //Group 2: Segment override prefixes / branch hints
					case 0x36:
					case 0x3e:
					case 0x26:
					case 0x64:
					case 0x65:
						//(fallthrough of all cases above)
						break;
					case 0x66: //Group 3: Operand size override prefix
						g_op.operandsizeoverride= 1; //use e.g. AX instead of EAX (reg)
						break;
					case 0x67: //Group 4: Address size override prefix
						g_op.addrsizeoverride= 1; //use 16bit addressing table for mod/rm
						break;
					default:
						//no more prefixes found, go to next mode
						reread_byte= 1;
						state= DECODE_STATE_OPCODE;
						break;
				}
				break;
			case DECODE_STATE_OPCODE:
				switch (current_byte) {
					case 0x0f: //2 or 3 byte opcode
						g_op.op= OP_0F;
						break;
					case 0x90: //NOP
						g_op.op= OP_NOP;
						state= DECODE_STATE_END;
						break;
					case 0x8d: //LEA
						g_op.op= OP_LEA_8D;
						state= DECODE_STATE_MODRM;
						break;
					case 0x83: //83
						g_op.op= OP_83;
						g_op.modrm_contains_opcode_extension= 1;
						state= DECODE_STATE_MODRM;
						break;
					case 0x50: //PUSH
					case 0x51:
					case 0x52:
					case 0x53:
					case 0x54:
					case 0x55:
					case 0x56:
					case 0x57:
						g_op.op= OP_PUSH_50;
						g_op.rbrwrd= current_byte - 0x50; //hardcoded register
						state= DECODE_STATE_END;
						break;
					case 0xC1: //C1
						g_op.op= OP_C1;
						g_op.modrm_contains_opcode_extension= 1;
						state= DECODE_STATE_MODRM;
						break;
					case 0x34: //XOR
						g_op.op= OP_XOR_34;
						g_op.immed_size= 1;
						state= DECODE_STATE_IMMEDIATE;
						break;
					case 0x31: //XOR
						g_op.op= OP_XOR_31;
						state= DECODE_STATE_MODRM;
						break;
					case 0x89: //MOV
						g_op.op= OP_MOV_89;
						state= DECODE_STATE_MODRM;
						break;
					case 0x40: //INC
					case 0x41:
					case 0x42:
					case 0x43:
					case 0x44:
					case 0x45:
					case 0x46:
					case 0x47:
						g_op.op= OP_INC_40;
						g_op.rbrwrd= current_byte - 0x40;
						state= DECODE_STATE_END;
						break;
					case 0x80: //80
						g_op.op= OP_80;
						g_op.modrm_contains_opcode_extension= 1;
						state= DECODE_STATE_MODRM;
						break;
					case 0x58: //POP
					case 0x59:
					case 0x5A:
					case 0x5B:
					case 0x5C:
					case 0x5D:
					case 0x5E:
					case 0x5F:
						g_op.op= OP_POP_58;
						g_op.rbrwrd= current_byte - 0x58;
						state= DECODE_STATE_END;
						break;
					case 0x75:
						g_op.op= OP_JNZ_75;
						state= DECODE_STATE_CODE_OFFSET;
						break;
					default:
						g_op.halt= 1; //invalid instruction
						state= DECODE_STATE_END;
						break;
				}
				break;
			case DECODE_STATE_CODE_OFFSET:
				if (numread_code_offset < 2) {
					g_op.code_offset_1|= (current_byte << (8 * numread_code_offset));
				} else {
					g_op.code_offset_2|= (current_byte << (8 * numread_code_offset));
				}
				numread_code_offset++;
				if (numread_code_offset < g_op.code_offset_size) {
					state= DECODE_STATE_CODE_OFFSET;
				} else {
					switch (g_op.op) {
						default:
							state= DECODE_STATE_END;
					}
				}
				break;

			case DECODE_STATE_MODRM:
				g_op.has_modrm= 1;
				g_op.mod= extract_bits8(current_byte, 6, 2);
				g_op.rm= extract_bits8(current_byte, 0, 3);
				g_op.reg= extract_bits8(current_byte, 3, 3);

				int general_opcode= g_op.op;

				if (g_op.modrm_contains_opcode_extension) {
					switch (g_op.op) {
						case OP_80:
							switch (g_op.reg) {
								case 7: //CMP
									g_op.op= OP_CMP_80_7;
									g_op.immed_size= 1;
									break;
							}
							break;
						case OP_83:
							switch (g_op.reg) {
								case 4: //AND
									g_op.op= OP_AND_83_4;
									g_op.immed_size= 1;
									break;
								case 5: //SUB
									g_op.op= OP_SUB_83_5;
									g_op.immed_size= 1;
									break;
							}
							break;
						case OP_C1:
							switch (g_op.reg) {
								case 7: //SAR
									g_op.op= OP_SAR_C1_7;
									g_op.immed_size= 1;
									break;
							}
							break;
					}
					if (g_op.op == general_opcode) {
						g_op.halt= 1; //no instruction found, thus invalid op
					}
				}

				//32bit addressing
				//General case
				switch (g_op.mod) {
					case 0:
					case 3:
						g_op.disp_size= 0;
						break;
					case 1:
						g_op.disp_size= 1;
						break;
					case 2:
						if (g_op.addrsizeoverride) {
							g_op.disp_size= 2; //16bits
						} else {
							g_op.disp_size= 4; //32bits
						}
						break;
				}
				// Special cases, exceptions to the rule
				if (g_op.addrsizeoverride) { //16bit addressing
					if (g_op.mod == 0 && g_op.rm == 6) {
						g_op.disp_size= 2;
					}
				} else { //32bit addressing
					if (g_op.rm == 4 && (g_op.mod == 0 || g_op.mod == 1 || g_op.mod == 2)) {
						g_op.has_sib= 1;
					}
					if (g_op.mod == 0 && g_op.rm == 5) {
						g_op.disp_size= 4;
					}
				}
				if (g_op.has_sib) {
					state= DECODE_STATE_SIB;
				} else if (numread_disp < g_op.disp_size) {
					state= DECODE_STATE_DISPLACEMENT;
				} else if (numread_immed < g_op.immed_size) {
					state= DECODE_STATE_IMMEDIATE;
				} else {
					state= DECODE_STATE_END;
				}
				break;
			case DECODE_STATE_SIB:
				g_op.ss= extract_bits8(current_byte, 6, 2);
				g_op.index= extract_bits8(current_byte, 0, 3);
				g_op.base= extract_bits8(current_byte, 3, 3);
				if (numread_disp < g_op.disp_size) {
					state= DECODE_STATE_DISPLACEMENT;
				} else if (numread_immed < g_op.immed_size) {
					state= DECODE_STATE_IMMEDIATE;
				} else {
					state= DECODE_STATE_END;
				}
				break;
			case DECODE_STATE_DISPLACEMENT:
				// in text:  EF     BE     AD     DE
				// uint32_t: 0xAABBCCDD    0xDEADBEEF
				// uint16_t: 0xCCDD 0xAABB 0xBEEF 0xDEAD
				// i.e LSB is the first byte read
				g_op.displacement= g_op.displacement | (current_byte << (8 * numread_disp));
				numread_disp++;
				if (numread_disp < g_op.disp_size) {
					state= DECODE_STATE_DISPLACEMENT;
				} else if (numread_immed < g_op.immed_size) {
					state= DECODE_STATE_IMMEDIATE;
				} else {
					state= DECODE_STATE_END;
				}
				break;
			case DECODE_STATE_IMMEDIATE:
				g_op.immediate= g_op.immediate | (current_byte << (8 * numread_immed));
				numread_immed++;
				if (numread_immed < g_op.immed_size) {
					state= DECODE_STATE_IMMEDIATE;
				} else {
					state= DECODE_STATE_END;
				}
				break;
		}
		if (!reread_byte) {
			current_address++; //advance by 1 byte
		}
	}
	if (!g_op.halt) {
		log_debug("Decoded instruction.");
	} else {
		log_debug("Could not decode instruction.");
	}
}

/*
 * Return a new NULL location
 */
vm_effective_location null_location() {
	vm_effective_location e;
	e.is_gp_register= 0;
	e.reg= 0;
	e.address= 0;
	return e;
}

/*
 * Calculate effective location in 32bit addressing mode
 */
vm_effective_location effective_loc32() {
	vm_effective_location effective= null_location();
	switch (g_op.mod) {
		case 0:
			switch (g_op.rm) {
				case 4:
					effective= memory_location(sib_addr32());
					break;
				case 5:
					effective= memory_location(g_op.displacement);
					break;
				default:
					effective= memory_location(g_regs.gp[g_op.rm]);
					break;
			}
			break;
		case 1:
			switch (g_op.rm) {
				case 4:
					effective= memory_location(sib_addr32() + sign_extend8(g_op.displacement));
					break;
				default:
					effective= memory_location(g_regs.gp[g_op.rm] + sign_extend8(g_op.displacement));
					break;
			}
			break;
		case 2:
			switch (g_op.rm) {
				case 4:
					effective= memory_location(sib_addr32() + g_op.displacement);
					break;
				default:
					effective= memory_location(g_regs.gp[g_op.rm] + g_op.displacement);
					break;
			}
			break;
		case 3:
			effective= register_location(g_op.rm);
			break;

	}
	return effective;
}

/*
 * Interpret SIB byte
 */
uint32_t sib_addr32() {
	uint32_t scaled_index= 0;
	if (g_op.index != 4) {
		scaled_index= g_regs.gp[g_op.index] << g_op.ss;
	}
	if (g_op.base == 5 && g_op.mod == 0) {
		return scaled_index + g_op.displacement;
	} else {
		return g_regs.gp[g_op.base] + scaled_index;
	}
}

/*
 * Calculate effective location in 16bit addressing mode
 */
vm_effective_location effective_loc16() {
	vm_effective_location effective= null_location();
	switch (g_op.mod) {
		case 0:
			switch (g_op.rm) {
				case 6:
					effective= memory_location(g_op.displacement);
					break;
				default:
					effective= memory_location(rm_addr16());
					break;
			}
			break;
		case 1:
			effective= memory_location(rm_addr16() + sign_extend8(g_op.displacement));
			break;
		case 2:
			effective= memory_location(rm_addr16() + g_op.displacement);
			break;
		case 3:
			effective.is_gp_register= 1;
			effective.reg= g_op.rm;
			break;
	}
	effective= memory_location(zero_extend16(effective.address)); //clear 2 MSBs
	return effective;
}

/*
 * Calculate base addresses from ModRM byte in 16bit addressing mode
 *
 * 0: [BX+SI]
 * 1: [BX+DI]
 * 2: [BP+SI]
 * 3: [BP+DI]
 * 4: [SI]
 * 5: [DI]
 * 6: [BP]
 * 7: [BX]
 */
uint16_t rm_addr16() {
	switch (g_op.rm) {
		case 0:
			return (uint16_t) (zero_extend16(g_regs.gp[EBX]) + zero_extend16(g_regs.gp[ESI]));
		case 1:
			return (uint16_t) (zero_extend16(g_regs.gp[EBX]) + zero_extend16(g_regs.gp[EDI]));
		case 2:
			return (uint16_t) (zero_extend16(g_regs.gp[EBP]) + zero_extend16(g_regs.gp[ESI]));
		case 3:
			return (uint16_t) (zero_extend16(g_regs.gp[EBP]) + zero_extend16(g_regs.gp[EDI]));
		case 4:
			return (uint16_t) zero_extend16(g_regs.gp[ESI]);
		case 5:
			return (uint16_t) zero_extend16(g_regs.gp[EDI]);
		case 6:
			return (uint16_t) zero_extend16(g_regs.gp[EBP]);
		case 7:
			return (uint16_t) zero_extend16(g_regs.gp[EBX]);
	}
	log_fatal_error("rm not in range 0-7");
	return 0; //get rid of compiler warnings
}

/*
 * Return a register location
 */
vm_effective_location register_location(int reg) {
	vm_effective_location addr= null_location();
	addr.is_gp_register= 1;
	addr.reg= reg;
	return addr;
}

/*
 * Return a memory location
 */
vm_effective_location memory_location(uint32_t address) {
	vm_effective_location addr= null_location();
	addr.address= address;
	return addr;
}

/*
 * Read from a location
 */
uint32_t read_effective_loc32(vm_effective_location src) {
	if (src.is_gp_register) {
		return g_regs.gp[src.reg];
	} else {
		return mmu_read(src.address, 4);
	}
}
uint32_t read_effective_loc16(vm_effective_location src) {
	if (src.is_gp_register) {
		return g_regs.gp[src.reg] & 0xFFFF;
	} else {
		return mmu_read(src.address, 2);
	}
}
/*
 * Read only from 8bits. For 8-bit registers, the allocations are different:
 * 0: AL   4: AH
 * 1: CL   5: CH
 * 2: DL   6: DH
 * 3: BL   7: BH
 */
uint32_t read_effective_loc8(vm_effective_location src) {
	if (src.is_gp_register) {
		if (src.reg < 4) {
			return g_regs.gp[src.reg] & 0xFF;
		} else {
			return g_regs.gp[src.reg - 4] & 0xFF00;
		}
	} else {
		return mmu_read(src.address, 1);
	}
}

/*
 * Write to a location.
 */
void write_effective_loc32(vm_effective_location dest, uint32_t value) {
	if (dest.is_gp_register) {
		g_regs.gp[dest.reg]= value;
	} else {
		mmu_write(dest.address, 4, value);
	}
}
/*
 * Write only to lower 16bits
 */
void write_effective_loc16(vm_effective_location dest, uint32_t value) {
	if (dest.is_gp_register) {
		g_regs.gp[dest.reg]= (g_regs.gp[dest.reg] & 0xFFFF0000) | (value & 0xFFFF);
	} else {
		mmu_write(dest.address, 2, value);
	}
}

/*
 * Write only to 8bits. For 8-bit registers, the allocations are different:
 * 0: AL   4: AH
 * 1: CL   5: CH
 * 2: DL   6: DH
 * 3: BL   7: BH
 */
void write_effective8(vm_effective_location dest, uint32_t value) {
	if (dest.is_gp_register) {
		if (dest.reg < 4) {
			g_regs.gp[dest.reg]= (g_regs.gp[dest.reg] & 0xFFFFFF00) | (value & 0xFF);
		} else {
			g_regs.gp[dest.reg - 4]= (g_regs.gp[dest.reg - 4] & 0xFFFF00FF) | (value & 0xFF00);
		}
	} else {
		mmu_write(dest.address, 1, value);
	}
}

void set_SF_ZF_PF_eflags32(uint32_t new) {
	write_eflag(SF, ((int32_t) new) < 0);
	write_eflag(ZF, new == 0);
	write_eflag(PF, parity_lsb(new));
}

void set_subtract_eflags32(uint32_t old, uint32_t new, uint32_t value) {
	// CF for invalid unsigned
	if (value > old) { //unsigned comparison
		set_eflag(CF); //borrowed
	} else {
		clear_eflag(CF);
	}
	// OF for invalid signed
	if (((((int32_t) new) > ((int32_t) old)) && ((int32_t) value) > 0) || ((((int32_t) new) > ((int32_t) old)) && ((int32_t) value) > 0)) {
		set_eflag(OF); //result is impossible in signed
	} else {
		clear_eflag(OF);
	}
	set_SF_ZF_PF_eflags32(new);
}
void set_subtract_eflags8(uint8_t old, uint8_t new, uint8_t value) {
	// CF for invalid unsigned
	if (value > old) { //unsigned comparison
		set_eflag(CF); //borrowed
	} else {
		clear_eflag(CF);
	}
	// OF for invalid signed
	if (((((int8_t) new) > ((int8_t) old)) && ((int8_t) value) > 0) || ((((int8_t) new) > ((int8_t) old)) && ((int8_t) value) > 0)) {
		set_eflag(OF); //result is impossible in signed
	} else {
		clear_eflag(OF);
	}
	set_SF_ZF_PF_eflags32(new);
}

/*
 * Main dispatcher
 */
void dispatch() {
	//temporary variables for general usage
	uint32_t old;
	uint32_t new;
	uint32_t value;
	uint32_t cf;
	vm_effective_location dest;
	vm_effective_location src;
	int i;
	uint32_t times;
	int no_increment_eip= 0;
	switch (g_op.op) {
		case OP_NOP:
			printf("#### %d #### EXECUTE: 90 --> NOP <-- No operation ####\n", print_counter++);
			//do nothing
			break;
		case OP_LEA_8D:
			if (g_op.operandsizeoverride) {
				//16bit operand
				printf("#### %d #### EXECUTE: 8D /r --> LEA r16,m <-- Store effective address for m in register r16 ####\n", print_counter++);
				if (g_op.addrsizeoverride) {
					//16 bit address
					src= effective_loc16();
				} else {
					//32bit address
					src= effective_loc32();
				}
				if (src.is_gp_register) {
					log_fatal_error("r/m not valid, need m");
				}
				dest= register_location(g_op.reg);
				write_effective_loc16(dest, src.address); //Load effective address into destination register
			} else {
				//32bit operand
				printf("#### %d #### EXECUTE: 8D /r --> LEA r32,m <-- Store effective address for m in register r32 ####\n", print_counter++);
				if (g_op.addrsizeoverride) {
					//16 bit address
					src= effective_loc16();
				} else {
					//32bit address
					src= effective_loc32();
				}
				if (src.is_gp_register) {
					log_fatal_error("r/m not valid, need m");
				}
				dest= register_location(g_op.reg);
				write_effective_loc32(dest, src.address); //Load effective address into destination register
			}
			break;
		case OP_AND_83_4:
			printf("#### %d #### EXECUTE: 83 /4 ib --> AND r/m32,imm8 <-- r/m16 AND imm8 (sign-extended) ####\n", print_counter++);
			dest= effective_loc32();
			old= read_effective_loc32(dest);
			new= old & sign_extend8(g_op.immediate);
			write_effective_loc32(dest, new); //write to r/m

			clear_eflag(OF);
			clear_eflag(CF);
			set_SF_ZF_PF_eflags32(new);
			break;
		case OP_PUSH_50:
			printf("#### %d #### EXECUTE: 50+rd --> PUSH r32 <-- Push r32 ####\n", print_counter++);
			src= register_location(g_op.rbrwrd);
			new= read_effective_loc32(src); //value to be pushed onto the stack
			g_regs.gp[ESP]= g_regs.gp[ESP] - 4; //decrement ESP
			dest= memory_location(g_regs.gp[ESP]);
			write_effective_loc32(dest, new);
			break;
		case OP_SAR_C1_7:
			printf("#### %d #### EXECUTE: C1 /7 ib --> SAR r/m32,imm8 <-- Signed divide* r/m32 by 2, imm8 times ####\n", print_counter++);
			dest= effective_loc32();
			old= read_effective_loc32(dest);
			new= old;
			times= g_op.immediate;
			//simple implementation as in specification
			i= times;
			while (i > 0) {
				cf= new & 0x1;
				new= (((int32_t) new) >> 1);
				i--;
			}
			write_effective_loc32(dest, new); //write to r/m
			if (times != 0) {
				if (times == 1) {
					clear_eflag(OF);
				}
				write_eflag(CF, cf);
				set_SF_ZF_PF_eflags32(new);
			}
			break;
		case OP_XOR_34:
			printf("#### %d #### EXECUTE: 34 ib --> XOR AL,imm8 <-- AL XOR imm8 ####\n", print_counter++);
			src= register_location(AL);
			old= read_effective_loc8(src);
			new= old ^ g_op.immediate;
			write_effective8(register_location(AL), new);

			clear_eflag(CF);
			clear_eflag(OF);
			set_SF_ZF_PF_eflags32(new);

			break;
		case OP_XOR_31:
			printf("#### %d #### EXECUTE: 31 /r --> XOR r/m32,r32  <-- r/m32 XOR r32 ####\n", print_counter++);
			src= effective_loc32();
			old= read_effective_loc32(src);
			value= read_effective_loc32(register_location(g_op.reg));
			new= old ^ value;
			write_effective_loc32(src, new);

			clear_eflag(CF);
			clear_eflag(OF);
			set_SF_ZF_PF_eflags32(new);

			break;
		case OP_MOV_89:
			printf("#### %d #### EXECUTE: 89 /r --> MOV r/m32,r32 <-- Move r32 to r/m32 ####\n", print_counter++);
			src= register_location(g_op.reg);
			old= read_effective_loc32(src);
			dest= effective_loc32();
			write_effective_loc32(dest, old);
			break;
		case OP_INC_40:
			printf("#### %d #### EXECUTE: 40+ rd --> INC r32 <-- Increment doubleword register by 1 ####\n", print_counter++);
			src= register_location(g_op.rbrwrd);
			old= read_effective_loc32(src);
			new= old + 1;
			write_effective_loc32(src, new);
			set_subtract_eflags32(old, new, 1);
			break;
		case OP_SUB_83_5:
			printf("#### %d #### EXECUTE: 83 /5 ib --> SUB r/m32,imm8 <-- Subtract sign-extended imm8 from r/m32 ####\n", print_counter++);
			src= effective_loc32();
			;
			old= read_effective_loc32(src);
			value= sign_extend8(g_op.immediate);
			new= old - value;
			write_effective_loc32(src, new);

			set_subtract_eflags32(old, new, (value * -1));

			break;
		case OP_CMP_80_7:
			printf("#### %d #### EXECUTE: 80 /7 ib --> CMP r/m8, imm8 <-- Compare imm8 with r/m8 ####\n", print_counter++);
			src= effective_loc32();
			;
			old= read_effective_loc32(src);
			value= sign_extend8(g_op.immediate);

			new= (((uint8_t) old) - ((uint8_t) value));
			set_subtract_eflags8(((uint8_t) old), ((uint8_t) new), ((uint8_t) value));

			break;
		case OP_POP_58:
			printf("#### %d #### EXECUTE: 58+ rd --> POP r32 <-- Pop top of stack into r32; increment stack pointer ####\n", print_counter++);
			dest= register_location(g_op.rbrwrd);
			src= memory_location(g_regs.gp[ESP]);
			new= read_effective_loc32(src);
			write_effective_loc32(dest, new);
			g_regs.gp[ESP]= g_regs.gp[ESP] + 4; //increment ESP
			break;
		case OP_JNZ_75:
			printf("#### %d #### EXECUTE: 75 cb --> JNZ rel8 <-- Jump short if not zero (ZF=0) ####\n", print_counter++);
			if (get_eflag(ZF) == 0) {
				no_increment_eip= 1;
				g_regs.eip= g_regs.eip + g_op.length + sign_extend8(g_op.code_offset_1);
			}
			break;
		default:
			log_fatal_error("Instruction unknown.");
			break;
	}
	if (!no_increment_eip) {
		g_regs.eip= g_regs.eip + g_op.length;
	}
}

/*
 * Main loop
 */
void run_interpreter() {
	init_g_regs();
	init_g_mmu();
	printf("#### %d #### ACTION: Reading program from input ####\n", print_counter++);
	read_input();
	printf("#### %d #### PRINT: Complete program that was read: ####\n", print_counter++);
	interactive_wait();
	print_input();
	printf("#### %d #### ACTION: Loading program into simulated memory ####\n", print_counter++);
	load_program_into_memory();
	printf("#### %d #### PRINT: Initial register/memory/stack contents: ####\n", print_counter++);
	interactive_wait();
	print_g_regs();
	print_memory();
	do {
		printf("#### %d #### ACTION: Decoding next instruction at (%%eip) ####\n", print_counter++);
		decode();
		if (!g_op.halt) {
			printf("#### %d #### PRINT: Decoded instruction: ####\n", print_counter++);
			interactive_wait();
			print_g_op();
			printf("#### %d #### ACTION: Dispatching instruction ####\n", print_counter++);
			dispatch();
			printf("#### %d #### PRINT: Register/memory/stack contents: ####\n", print_counter++);
			interactive_wait();
			print_g_regs();
			print_memory();
		} else {
			printf("#### %d #### ACTION: Halting execution as next instruction is NULL or unknown ####\n", print_counter++);
		}
	} while (!g_op.halt);
	log_debug("Finished execution of program without errors.");
}