/// <summary>
/// FUNCTION:
/// -Using instruction field update fields: instructionString, opcode, Rn, Rd, operand2.
/// </summary>
public void decodeDP()
{
// update field: instructionString, opcode
opcode = (instruction >> 21) & 0xf;
updateInstructionName();
// update field Rn
Rn = (instruction >> 16) & 0xf;
// update field RnRegVal
// if Rn is PC, then RnRegVal should be currentAddress + 8 bytes. else it it's the content of register Rn
if (Rn == 15)
{
RnRegVal = instructAddress + 8;
}
else
{
RnRegVal = registers.getRegNValue(Rn);
}
// update field Rd
Rd = (instruction >> 12) & 0xf;
/* Decode the operand2 of the intruction and update field operand2
* Steps to do this:
* 1. check bits 25 & 4 for operand 2 types
* 2. if 8 bit immediate
* initialize _8BitImmediate object and decode_Imm
* if register or immediate shifted register
* initialize RegAndImmShReg object and decode_Imm
* if register shifted register
* initialize RegShReg object and decode_Imm
*/
if (bit25 == true) // Immediate
{
immediate = new Immediate(memory, registers, instruction, instructAddress);
immediate.decode_Imm();
}
if (bit25 == false && bit4 == false) // Register and Immediate Shifted Register
{
reg_and_imm_sh_reg = new RegAndImmShReg(memory, registers, instruction, instructAddress);
reg_and_imm_sh_reg.decode_RegAndImmShReg();
}
// TODO: not implemented in sim I
if (this.bit25 == false && this.bit4 == true) // Register Shifted Register
{
this.reg_sh_reg = new RegShReg(this.memory, this.registers, this.instruction, this.instructAddress);
this.reg_sh_reg.decode_RegShReg();
}
}
// FUNCTION: deter
public void decodeLaS()
{
// check bit 25 on instruction to determine if it this instruction has
// an offset: immediate or immediate shifted register
bit_25 = memory.TestFlag(instructAddress, 25);
// update fields: PUBWL
P = memory.TestFlag(instructAddress, 24);
U = memory.TestFlag(instructAddress, 23);
B = memory.TestFlag(instructAddress, 22);
W = memory.TestFlag(instructAddress, 21);
L = memory.TestFlag(instructAddress, 20);
updateInstructionStringWithInstructionName();
// update field Rn
Rn = (instruction >> 16) & 0xf;
// update field RnRegVal
// if Rn is PC, then RnRegVal should be currentAddress + 8 bytes. else it it's the content of register Rn
if (Rn == 15)
{
RnRegVal = instructAddress + 8;
}
else
{
RnRegVal = registers.getRegNValue(Rn);
}
// update field Rd
Rd = (instruction >> 12) & 0xf;
if (bit_25 == false) // OFFSET IS IMMEDIATE. Decode 12 bit op2 immediate
{
// update field _12bitImmediate
_12bitImmediate = instruction & 0xfff;
if (U) // Immediate is positive
{
effectiveAddress = RnRegVal + _12bitImmediate;
}
else // Immediate is negative. TODO: check for negative numbers in unsigned subtraction
{
effectiveAddress = RnRegVal - _12bitImmediate;
}
}
else // OFFSET REGISTER SHIFTED REGISTER. Decode 12 bit op2 register shifted register
{
imm_sh_reg = new RegAndImmShReg(memory, registers, instruction, instructAddress);
imm_sh_reg.decode_RegAndImmShReg();
imm_sh_reg.execute_RegAndImmShReg(); // calculates RmRegVal field in object by doing shifting bits
if (U) // Immediate is positive
{
effectiveAddress = RnRegVal + imm_sh_reg.getRmRegVal();
}
else // Immediate is negative. TODO: check for negative numbers in unsigned subtraction
{
effectiveAddress = RnRegVal - imm_sh_reg.getRmRegVal();
}
}
}