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//:: simulated x86 registers
:(before "End Types")
enum {
EAX,
ECX,
EDX,
EBX,
ESP,
EBP,
ESI,
EDI,
NUM_INT_REGISTERS,
};
union reg {
int32_t i;
uint32_t u;
};
:(before "End Globals")
reg Reg[NUM_INT_REGISTERS] = { {0} };
uint32_t EIP = 0;
:(before "End Reset")
bzero(Reg, sizeof(Reg));
EIP = 0;
//:: simulated flag registers; just a subset that we care about
:(before "End Globals")
bool SF = false; // sign flag
bool ZF = false; // zero flag
bool OF = false; // overflow flag
:(before "End Reset")
SF = ZF = OF = false;
//: how the flag registers are updated after each instruction
:(before "End Includes")
// beware: no side-effects in args
#define BINARY_ARITHMETIC_OP(op, arg1, arg2) { \
/* arg1 and arg2 must be signed */ \
int64_t tmp = arg1 op arg2; \
arg1 = arg1 op arg2; \
trace(2, "run") << "storing 0x" << std::hex << arg1 << end(); \
SF = (arg1 < 0); \
ZF = (arg1 == 0); \
OF = (arg1 != tmp); \
}
#define BINARY_BITWISE_OP(op, arg1, arg2) { \
/* arg1 and arg2 must be unsigned */ \
arg1 = arg1 op arg2; \
trace(2, "run") << "storing 0x" << std::hex << arg1 << end(); \
SF = (arg1 >> 31); \
ZF = (arg1 == 0); \
OF = false; \
}
//:: simulated RAM
:(before "End Globals")
vector<uint8_t> Mem;
uint32_t End_of_program = 0;
:(before "End Reset")
Mem.resize(1024);
End_of_program = 0;
//:: core interpreter loop
:(scenario add_imm32_to_eax)
# In scenarios, programs are a series of hex bytes, each (variable-length)
# instruction on one line.
#
# x86 instructions consist of the following parts (see cheatsheet.pdf):
# opcode ModR/M SIB displacement immediate
# instruction mod, reg, Reg/Mem bits scale, index, base
# 1-3 bytes 0/1 byte 0/1 byte 0/1/2/4 bytes 0/1/2/4 bytes
05 0a 0b 0c 0d # add 0x0d0c0b0a to EAX
# All hex bytes must be exactly 2 characters each. No '0x' prefixes.
+load: 1 -> 05
+load: 2 -> 0a
+load: 3 -> 0b
+load: 4 -> 0c
+load: 5 -> 0d
+run: add imm32 0x0d0c0b0a to reg EAX
+reg: storing 0x0d0c0b0a in reg EAX
:(code)
// helper for tests: load a program into memory from a textual representation
// of its bytes, and run it
void run(const string& text_bytes) {
load_program(text_bytes);
EIP = 1; // preserve null pointer
while (EIP < End_of_program)
run_one_instruction();
}
// skeleton of how x86 instructions are decoded
void run_one_instruction() {
uint8_t op=0, op2=0, op3=0;
switch (op = next()) {
case 0xf4: // hlt
EIP = End_of_program;
break;
// our first opcode
case 0x05: { // add imm32 to EAX
int32_t arg2 = imm32();
trace(2, "run") << "add imm32 0x" << HEXWORD << arg2 << " to reg EAX" << end();
BINARY_ARITHMETIC_OP(+, Reg[EAX].i, arg2);
trace(98, "reg") << "storing 0x" << HEXWORD << Reg[EAX].i << " in reg EAX" << end();
break;
}
// End Single-Byte Opcodes
case 0x0f:
switch(op2 = next()) {
// End Two-Byte Opcodes Starting With 0f
default:
cerr << "unrecognized second opcode after 0f: " << std::hex << static_cast<int>(op2) << '\n';
exit(1);
}
break;
case 0xf3:
switch(op2 = next()) {
// End Two-Byte Opcodes Starting With f3
case 0x0f:
switch(op3 = next()) {
// End Three-Byte Opcodes Starting With f3 0f
default:
cerr << "unrecognized third opcode after f3 0f: " << std::hex << static_cast<int>(op3) << '\n';
exit(1);
}
break;
default:
cerr << "unrecognized second opcode after f3: " << std::hex << static_cast<int>(op2) << '\n';
exit(1);
}
break;
default:
cerr << "unrecognized opcode: " << std::hex << static_cast<int>(op) << '\n';
exit(1);
}
}
void load_program(const string& text_bytes) {
uint32_t addr = 1;
istringstream in(text_bytes);
in >> std::noskipws;
while (has_data(in)) {
char c1 = next_hex_byte(in);
if (c1 == '\0') break;
if (!has_data(in)) {
raise << "input program truncated mid-byte\n" << end();
return;
}
char c2 = next_hex_byte(in);
if (c2 == '\0') {
raise << "input program truncated mid-byte\n" << end();
return;
}
Mem.at(addr) = to_byte(c1, c2);
trace(99, "load") << addr << " -> " << HEXBYTE << static_cast<int>(Mem.at(addr)) << end(); // ugly that iostream doesn't print uint8_t as an integer
addr++;
}
End_of_program = addr;
}
char next_hex_byte(istream& in) {
while (has_data(in)) {
char c = '\0';
in >> c;
if (c == ' ' || c == '\n') continue;
while (c == '#') {
while (has_data(in)) {
in >> c;
if (c == '\n') {
in >> c;
break;
}
}
}
if (c >= '0' && c <= '9') return c;
else if (c >= 'a' && c <= 'f') return c;
else if (c >= 'A' && c <= 'F') return tolower(c);
// disallow any non-hex characters, including a '0x' prefix
if (!isspace(c)) {
raise << "invalid non-hex character '" << c << "'\n" << end();
break;
}
}
return '\0';
}
uint8_t to_byte(char hex_byte1, char hex_byte2) {
return to_hex_num(hex_byte1)*16 + to_hex_num(hex_byte2);
}
uint8_t to_hex_num(char c) {
if (c >= '0' && c <= '9') return c - '0';
if (c >= 'a' && c <= 'f') return c - 'a' + 10;
assert(false);
return 0;
}
inline uint8_t next() {
return Mem.at(EIP++);
}
// read a 32-bit immediate in little-endian order from the instruction stream
int32_t imm32() {
int32_t result = next();
result |= (next()<<8);
result |= (next()<<16);
result |= (next()<<24);
return result;
}
:(before "End Includes")
#include <iomanip>
#define HEXBYTE std::hex << std::setw(2) << std::setfill('0')
#define HEXWORD std::hex << std::setw(8) << std::setfill('0')
#include <stdint.h>
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