from emulator import impl
from emulator.operations import zero_extend32, sign_extend32
from decode import decode_instruction, decode_variables

def delay_slot_state(cpu, ins):
    if instruction_properties.has_delay_slot(ins):
        assert cpu.is_delay_slot == False
        cpu.is_delay_slot = True
    else:
        cpu.is_delay_slot = False

def step(cpu, mem):
    # 3 Fetch the instruction bytes from the address in memory, as
    # indicated by the current program counter, 2 bytes need to be
    # fetched for each instruction.
    address = zero_extend32(sign_extend32(cpu.pc))
    assert address & 0b1 == 0, address
    instruction = mem.read16(address)

    # 4 Calculate the default values of PC’ and PR’. PC’ is set to the
    # value of PC”, PR’ is set to the value of PR”.
    cpu.pc1 = cpu.pc2
    cpu.pr1 = cpu.pr2

    # 5 Calculate the default values of PC” and PR” assuming continued
    # sequential execution without procedure call or mode switch: PC”
    # is PC’+2, while PR” is unchanged.
    cpu.pc2 = cpu.pc1 + 2
    cpu.pr2 = cpu.pr2

    # 6 Decode and execute the instruction. This includes checks for
    # synchronous events, such as exceptions and panics, and
    # initiation of handling if required. Synchronous events are not
    # accepted between a delayed branch and a delay slot. They are
    # detected either before the delayed branch or after the delay
    # slot.
    ins = decode_instruction(instruction)
    variables = decode_variables(instruction, ins)
    func = impl.lookup[(ins.instruction, ins.operands)]
    func(cpu, mem, *variables)
    delay_slot_state(cpu, ins)

    # 7 Set the current program counter (PC) to the value of the next
    # program counter (PC’) and PR to the value of PR’.
    cpu.pc = cpu.pc1
    cpu.pr = cpu.pr1