4034
Start implementing core x86 addressing mode decoding.
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@ -115,6 +115,7 @@ int main(int argc, char* argv[]) {
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// run on a 32-bit system
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assert(sizeof(int) == 4);
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assert(sizeof(float) == 4);
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assert_little_endian();
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// End One-time Setup
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@ -138,3 +139,12 @@ int main(int argc, char* argv[]) {
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void reset() {
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// End Reset
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}
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void assert_little_endian() {
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const int x = 1;
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const char* y = reinterpret_cast<const char*>(&x);
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if (*y != 1) {
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cerr << "the SubX VM only runs on little-endian processors. Do you have Intel (or AMD or Atom) inside?\n";
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exit(1);
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}
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}
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@ -40,6 +40,7 @@ SF = ZF = OF = false;
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/* arg1 and arg2 must be signed */ \
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int64_t tmp = arg1 op arg2; \
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arg1 = arg1 op arg2; \
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trace(2, "run") << "storing 0x" << std::hex << arg1 << end(); \
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SF = (arg1 < 0); \
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ZF = (arg1 == 0); \
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OF = (arg1 != tmp); \
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@ -48,6 +49,7 @@ SF = ZF = OF = false;
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#define BINARY_BITWISE_OP(op, arg1, arg2) { \
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/* arg1 and arg2 must be unsigned */ \
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arg1 = arg1 op arg2; \
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trace(2, "run") << "storing 0x" << std::hex << arg1 << end(); \
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SF = (arg1 >> 31); \
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ZF = (arg1 == 0); \
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OF = false; \
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@ -0,0 +1,46 @@
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:(scenario add_r32_to_rm32)
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% Reg[3].i = 0x10;
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% Reg[0].i = 0x60;
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# word in addresses 0x60-0x63 has value 1
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% Mem[0x60] = 1;
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# op ModR/M SIB displacement immediate
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01 18 # add EBX to *EAX
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+run: add register 3 to effective address
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+run: effective address is memory at address 0x60 (register 0)
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+run: storing 0x11
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:(before "End Single-Byte Opcodes")
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case 0x01: { // add r32 to r/m32
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uint8_t modrm = next();
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uint8_t arg2 = (modrm>>3)&0x7;
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trace(2, "run") << "add register " << static_cast<int>(arg2) << " to effective address" << end();
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int32_t* arg1 = effective_address(modrm);
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BINARY_ARITHMETIC_OP(+, *arg1, Reg[arg2].i);
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break;
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}
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:(code)
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// Implement tables 2-2 and 2-3 in the Intel manual, Volume 2.
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// We return a pointer so that instructions can write to multiple bytes in
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// 'Mem' at once.
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int32_t* effective_address(uint8_t modrm) {
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uint8_t mod = (modrm>>6);
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// ignore middle 3 'register opcode' bits
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uint8_t rm = modrm & 0x7;
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int32_t* result = 0;
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switch (mod) {
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case 0:
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// mod 0 is usually indirect addressing
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switch (rm) {
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default:
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trace(99, "run") << "effective address is memory at address 0x" << std::hex << Reg[rm].u << " (register " << static_cast<int>(rm) << ")" << end();
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assert(Reg[rm].u + sizeof(int32_t) <= Mem.size());
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result = reinterpret_cast<int32_t*>(&Mem.at(Reg[rm].u)); // rely on the host itself being in little-endian order
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break;
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// End Mod 0 Special-Cases
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}
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break;
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// End Mod Special-Cases
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}
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return result;
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}
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