// Sets the registers with the alu's output according to
// the given instruction
// Throws ProgramException if destination contains M and A contains
// an illegal address.
protected void setDestination(short instruction) throws ProgramException {
boolean destA = (instruction & 0x0020) > 0;
boolean destD = (instruction & 0x0010) > 0;
boolean destM = (instruction & 0x0008) > 0;
if (destM) {
int address = A.get();
if ((address < 0) || (address >= M.getSize())) {
throw new ProgramException(
"At line "
+ PC.get()
+ ": Destination is M but A="
+ address
+ " is an illegal memory address.");
}
A.setUpdatePointer(true);
bus.send(alu, 2, M, address);
A.setUpdatePointer(false);
}
if (destA) {
bus.send(alu, 2, A, 0);
}
if (destD) {
bus.send(alu, 2, D, 0);
}
}
/** Restarts the program from the beginning. */
public void initProgram() {
A.reset();
A.setUpdatePointer(true);
A.setUpdatePointer(false);
D.reset();
PC.reset();
alu.reset();
M.clearScreen();
M.hideSelect();
M.hideHighlight();
rom.hideSelect();
rom.hideHighlight();
time = 0;
}
// computes the exp part of the given instruction.
// The result will be at the alu's output.
// Throws ProgramException if the calculation involves M and A contains
// an illegal address.
protected void computeExp(short instruction) throws ProgramException {
boolean indirect = (instruction & 0x1000) > 0;
boolean zd = (instruction & 0x0800) > 0;
boolean nd = (instruction & 0x0400) > 0;
boolean zm = (instruction & 0x0200) > 0;
boolean nm = (instruction & 0x0100) > 0;
boolean f = (instruction & 0x0080) > 0;
boolean no = (instruction & 0x0040) > 0;
try {
alu.setCommand(
assemblerTranslator.getExpByCode((short) (instruction & 0xffc0)), zd, nd, zm, nm, f, no);
} catch (AssemblerException ae) {
}
bus.send(D, 0, alu, 0); // sends D to input0 of the alu
// sends A or M[A] to input1 of the alu
if (indirect) {
int address = A.get();
if ((address < 0) || (address >= M.getSize())) {
throw new ProgramException(
"At line "
+ PC.get()
+ ": Expression involves M but A="
+ address
+ " is an illegal memory address.");
}
A.setUpdatePointer(true);
bus.send(M, address, alu, 1);
A.setUpdatePointer(false);
} else {
bus.send(A, 0, alu, 1);
}
alu.compute();
}
// Sets the program counter (if necessary) according to
// the given instruction and the alu's output.
// If the program counter was changed, returns true, otherwise false.
// Throws ProgramException if the program counter should be changed and A
// contains an illegal address.
protected boolean checkJump(short instruction) throws ProgramException {
boolean jumpNegative = (instruction & 0x0004) > 0;
boolean jumpEqual = (instruction & 0x0002) > 0;
boolean jumpPositive = (instruction & 0x0001) > 0;
boolean changed = false;
short exp = alu.getValueAt(2);
if (((exp < 0) && jumpNegative) || ((exp == 0) && jumpEqual) || ((exp > 0) && jumpPositive)) {
int newPC = A.get();
if ((newPC < 0) || (newPC >= Definitions.ROM_SIZE)) {
throw new ProgramException(
"At line "
+ PC.get()
+ ": Jump requested but A="
+ newPC
+ " is an illegal program address.");
}
bus.send(A, 0, PC, 0);
changed = true;
}
return changed;
}
/** Constructs a new cpu with the given ROM, RAM, A, D, PC & ALU. */
public CPU(
RAM ram,
ROM rom,
PointerAddressRegisterAdapter A,
Register D,
PointerAddressRegisterAdapter PC,
ALU alu,
Bus bus) {
this.M = ram;
this.rom = rom;
this.A = A;
this.D = D;
this.PC = PC;
this.alu = alu;
this.bus = bus;
A.setUpdatePointer(false);
assemblerTranslator = HackAssemblerTranslator.getInstance();
}
/**
* Executes the current instruction (ROM at pc). Throws ProgramException if the current
* instruction is illegal or if it causes an illegal effect (read/write from M when A is an
* illegal address or jump when A is an illegal address).
*/
public void executeInstruction() throws ProgramException {
short instruction = rom.getValueAt(PC.get());
boolean pcChanged = false;
if ((instruction & 0x8000) == 0) {
bus.send(rom, PC.get(), A, 0);
} else if ((instruction & 0xe000) == 0xe000) {
computeExp(instruction);
setDestination(instruction);
pcChanged = checkJump(instruction);
} else if (instruction != HackAssemblerTranslator.NOP) {
throw new ProgramException("At line " + PC.get() + ": Illegal instruction");
}
if (!pcChanged) {
short newPC = (short) (PC.get() + 1);
if ((newPC < 0) || (newPC >= Definitions.ROM_SIZE)) {
throw new ProgramException("At line " + PC.get() + ": Can't continue past last line");
}
PC.setValueAt(0, newPC, true);
}
time++;
}
