path: root/debug/risc-v-gsp-rm-vm/sim/machine.py

"""Deterministic RV64IMAC machine: RAM, NS16550A UART, debugger-visible state.

The execution loop is a monolithic dispatch over the cached decode tuples from
``sim.decode``.  Everything guest-visible is a pure function of the loaded
image and the instruction budget handed to ``run``: no host clock, no threads,
no entropy.  RAM is an anonymous ``mmap`` (lazily paged, so a 256 MiB region is
cheap); all other addresses resolve to the UART or fault.
"""
from __future__ import annotations

import mmap
import struct

from .decode import (
    A_ADD,
    A_AND,
    A_DIV,
    A_DIVU,
    A_MUL,
    A_MULH,
    A_MULHSU,
    A_MULHU,
    A_OR,
    A_REM,
    A_REMU,
    A_SLL,
    A_SLT,
    A_SLTU,
    A_SRA,
    A_SRL,
    A_SUB,
    A_XOR,
    I_ADDI,
    I_ADDIW,
    I_ANDI,
    I_ORI,
    I_SLLI,
    I_SLLIW,
    I_SLTI,
    I_SLTIU,
    I_SRAI,
    I_SRAIW,
    I_SRLI,
    I_SRLIW,
    I_XORI,
    K_AMO,
    K_AUIPC,
    K_BRANCH,
    K_CSR,
    K_EBREAK,
    K_ECALL,
    K_FENCE,
    K_ILLEGAL,
    K_JAL,
    K_JALR,
    K_LOAD,
    K_LUI,
    K_MRET,
    K_OP,
    K_OP32,
    K_OPI,
    K_OPI32,
    K_STORE,
    LOAD_SIZE,
    MASK64,
    STORE_SIZE,
    decode16,
    decode32,
)

# Stop reasons returned by Machine.run().
R_BUDGET = 0
R_BP = 1
R_WATCH = 2
R_EBREAK = 3
R_FAULT = 4
R_STEP = 5
R_ECALL = 6

#: Accepted ``unknown_mmio`` policies for accesses outside RAM/UART/the map.
UNKNOWN_MMIO_MODES = ("fault", "lenient")

UART_BASE = 0x10000000
UART_SIZE = 0x100

# RV64I M A C advertised in misa (MXL=2, I=bit8, M=bit12, A=bit0, C=bit2).
MISA_RV64IMAC = 0x8000000000001105


class Fault(Exception):
    """Guest-visible trap: unmapped access or illegal instruction."""

    def __init__(self, cause: int, tval: int = 0):
        super().__init__(f"cause={cause:#x} tval={tval:#x}")
        self.cause = cause
        self.tval = tval


class Uart16550:
    """Behavioural NS16550A: TX is captured, LSR always reports THR empty."""

    def __init__(self):
        self.tx = bytearray()
        self.ier = 0
        self.lcr = 0
        self.mcr = 0
        self.scr = 0
        self.dll = 0
        self.dlm = 0

    def read(self, off: int, size: int = 1) -> int:
        if off == 0:
            return self.dll  # RBR (no RX modelled) / DLL when DLAB=1
        if off == 1:
            return self.ier
        if off == 2:
            return 0x01  # IIR: no pending interrupt
        if off == 3:
            return self.lcr
        if off == 4:
            return self.mcr
        if off == 5:
            return 0x60  # LSR: THRE | TEMT
        if off == 6:
            return 0xB0  # MSR: CTS|DSR|DCD
        if off == 7:
            return self.scr
        return 0

    def write(self, off: int, value: int, size: int = 1) -> None:
        value &= 0xFF
        if off == 0:
            if self.lcr & 0x80:
                self.dll = value
            else:
                self.tx.append(value)
        elif off == 1:
            self.ier = value
        elif off == 3:
            self.lcr = value
        elif off == 4:
            self.mcr = value
        elif off == 7:
            self.scr = value
        # FCR (2) and reserved offsets are write-only/no-ops.

    def text(self) -> str:
        return self.tx.decode("utf-8", "replace")


def _signed64(value: int) -> int:
    return value - 0x10000000000000000 if value & 0x8000000000000000 else value


def _signed32(value: int) -> int:
    value &= 0xFFFFFFFF
    return value - 0x100000000 if value & 0x80000000 else value


def _sext32(value: int) -> int:
    value &= 0xFFFFFFFF
    return (value ^ 0x80000000) - 0x80000000


def _watch_match(watchers, ea: int, size: int, rw_bit: int):
    end = ea + size
    for wa, we, wk, mask in watchers:
        if (mask & rw_bit) and ea < we and wa < end:
            return wk
    return None


class Machine:
    """One deterministic RV64IMAC hart plus RAM and UART.

    ``device_map`` is a :class:`greeneye.device_map.DeviceMap` (or any object
    with the same ``lookup``/``record`` surface) describing modelled MMIO.
    ``unknown_mmio`` selects what happens to an access that is neither RAM,
    UART nor in the map: ``"fault"`` (default, M1 parity) stops as before;
    ``"lenient"`` returns the mapped behaviour or zero, records the access in
    :attr:`mmio_log`, counts it in :attr:`mmio_unknown`, and, with
    ``learn_mmio=True``, writes the observation back into the map.  Nothing is
    logged or counted when lenient mode is off, and debugger ``read_mem``/
    ``write_mem`` stay strict.
    """

    def __init__(
        self,
        memory_mb: int = 256,
        ram_base: int = 0x80000000,
        device_map=None,
        unknown_mmio: str = "fault",
        learn_mmio: bool = False,
        bias: int = 0,
    ):
        if unknown_mmio not in UNKNOWN_MMIO_MODES:
            raise ValueError(f"unknown_mmio must be one of {UNKNOWN_MMIO_MODES}, got {unknown_mmio!r}")
        self.ram_base = ram_base
        self.ram_size = memory_mb * 1024 * 1024
        self.ram = mmap.mmap(-1, self.ram_size)
        self.uart = Uart16550()
        self.device_map = device_map
        self.unknown_mmio = unknown_mmio
        self.learn_mmio = bool(learn_mmio)
        self.bias = bias  # PC relocation for learned-map firmware addresses
        self.mmio_log: list[dict] = []
        self.mmio_unknown = 0
        self.x = [0] * 32
        self.pc = ram_base
        self.insns = 0
        self.csrs: dict[int, int] = {0x301: MISA_RV64IMAC}
        self._cache: dict[int, tuple] = {}
        self._u16 = struct.Struct("<H")
        self._u32 = struct.Struct("<I")
        self._i8 = struct.Struct("<b")
        self._i16 = struct.Struct("<h")
        self._i32 = struct.Struct("<i")
        self._u64 = struct.Struct("<Q")
        self._u16p = self._u16.pack_into
        self._u32p = self._u32.pack_into
        self._u64p = self._u64.pack_into

    # -- image loading -----------------------------------------------------

    def load_segments(self, plan, entry: int) -> None:
        """Place ``(addr, bytes)`` PT_LOAD results and set the PC."""
        for addr, data in plan:
            offset = addr - self.ram_base
            if offset < 0 or offset + len(data) > self.ram_size:
                raise Fault(1, addr)
            self.ram[offset : offset + len(data)] = data
        self.pc = entry & MASK64
        self.invalidate()

    def invalidate(self) -> None:
        """Drop the decode cache after debugger-visible code changes."""
        self._cache.clear()

    # -- register access (debugger surface) --------------------------------

    def reg(self, index: int) -> int:
        if index == 0:
            return 0
        return self.x[index]

    def set_reg(self, index: int, value: int) -> None:
        if index != 0:
            self.x[index] = value & MASK64

    # -- raw memory access (debugger surface) ------------------------------

    def read_mem(self, addr: int, size: int) -> bytes:
        out = bytearray()
        while size > 0:
            if self.ram_base <= addr < self.ram_base + self.ram_size:
                take = min(size, self.ram_base + self.ram_size - addr)
                off = addr - self.ram_base
                out += self.ram[off : off + take]
                addr += take
                size -= take
            elif UART_BASE <= addr < UART_BASE + UART_SIZE:
                out.append(self.uart.read(addr - UART_BASE))
                addr += 1
                size -= 1
            else:
                raise Fault(5, addr)
        return bytes(out)

    def write_mem(self, addr: int, data: bytes) -> None:
        self.invalidate()
        offset = 0
        while offset < len(data):
            here = addr + offset
            remaining = len(data) - offset
            if self.ram_base <= here < self.ram_base + self.ram_size:
                take = min(remaining, self.ram_base + self.ram_size - here)
                ro = here - self.ram_base
                self.ram[ro : ro + take] = data[offset : offset + take]
                offset += take
            elif UART_BASE <= here < UART_BASE + UART_SIZE:
                self.uart.write(here - UART_BASE, data[offset])
                offset += 1
            else:
                raise Fault(7, here)

    # -- MMIO (guest side) -------------------------------------------------

    def _mapped_mmio(self, addr: int, size: int, write: bool) -> tuple[bool, int]:
        """Consult the device map; ``(handled, value)`` for a modelled access."""
        dm = self.device_map
        if dm is None:
            return False, 0
        spec = dm.lookup(addr, size, write)
        if spec == "unimplemented":
            return False, 0
        if spec.startswith("value:"):
            return True, int(spec[6:], 16)
        return True, 0  # "zero": mapped but no read value observed

    def _log_unknown(self, pc: int, addr: int, size: int, rw: str, value: int = 0) -> None:
        """Lenient fallback: log, count, and optionally learn one access."""
        self.mmio_unknown += 1
        self.mmio_log.append({"pc": pc, "addr": addr, "size": size, "rw": rw})
        if self.learn_mmio:
            if self.device_map is None:
                from greeneye.device_map import DeviceMap  # lazy: sim stays usable alone

                self.device_map = DeviceMap(meta={"source": "learn"})
            self.device_map.record(addr, size, rw, value, pc=pc, bias=self.bias, source="trace")

    def _mmio_load(self, addr: int, size: int, pc: int = 0) -> int:
        if UART_BASE <= addr < UART_BASE + UART_SIZE:
            return self.uart.read(addr - UART_BASE)
        handled, value = self._mapped_mmio(addr, size, False)
        if handled:
            return value
        if self.unknown_mmio == "lenient":
            self._log_unknown(pc, addr, size, "r")
            return 0
        raise Fault(5, addr)

    def _mmio_store(self, addr: int, size: int, value: int, pc: int = 0) -> None:
        if UART_BASE <= addr < UART_BASE + UART_SIZE:
            self.uart.write(addr - UART_BASE, value)
            return
        handled, _ = self._mapped_mmio(addr, size, True)
        if handled:
            if self.learn_mmio and self.device_map is not None:
                self.device_map.record(addr, size, "w", value, pc=pc, bias=self.bias, source="trace")
            return
        if self.unknown_mmio == "lenient":
            self._log_unknown(pc, addr, size, "w", value)
            return
        raise Fault(7, addr)

    # -- execution ---------------------------------------------------------

    def step(self, watchers=(), trace=None):
        return self.run(1, breakpoints=(), watchers=watchers, single_step=True, trace=trace)

    def run(self, budget: int, breakpoints=(), watchers=(), single_step: bool = False,
            trace=None):
        """Execute up to ``budget`` instructions.

        Returns ``(reason, pc, a, b)``; ``a``/``b`` carry the watched address
        and kind (R_WATCH) or the trap cause and tval (R_FAULT).  When
        ``trace`` is a :class:`sim.trace.Trace`, every executed instruction
        and every completed memory access in its address range is recorded.
        """
        x = self.x
        ram = self.ram
        base = self.ram_base
        limit = self.ram_size
        cache = self._cache
        i8 = self._i8.unpack_from
        i16 = self._i16.unpack_from
        i32 = self._i32.unpack_from
        u16 = self._u16.unpack_from
        u32 = self._u32.unpack_from
        u64 = self._u64.unpack_from
        u16p = self._u16p
        u32p = self._u32p
        u64p = self._u64p
        has_bps = bool(breakpoints)
        pc = self.pc
        n = 0
        reason = R_BUDGET
        a = b = 0
        traced = trace is not None
        active = False

        while n < budget:
            if has_bps and pc in breakpoints:
                reason = R_BP
                a = pc
                break

            off = pc - base
            if off < 0 or off + 4 > limit:
                reason = R_FAULT
                a = 1
                b = pc
                break
            hw = ram[off] | (ram[off + 1] << 8)
            if hw & 3 == 3:
                d = cache.get(pc)
                if d is None:
                    d = decode32(pc, hw | (ram[off + 2] << 16) | (ram[off + 3] << 24))
                    cache[pc] = d
            else:
                d = cache.get(pc)
                if d is None:
                    d = decode16(pc, hw)
                    cache[pc] = d
            kind, sub, rd, r1, r2, imm, nxt = d
            n += 1
            if traced:
                insn_pc = pc
                active = trace.begin(insn_pc, self.insns + n)

            # -- loads / stores (most frequent in firmware hot loops) ------
            if kind == K_LOAD:
                ea = (x[r1] + imm) & MASK64
                size = LOAD_SIZE[sub]
                if watchers:
                    hit = _watch_match(watchers, ea, size, 2)
                    if hit is not None:
                        reason = R_WATCH
                        a = ea
                        b = hit
                        break
                o = ea - base
                if 0 <= o <= limit - size:
                    if active:
                        trace.mem(insn_pc, ea, size, False, self.insns + n)
                    if sub == 3:
                        if rd:
                            x[rd] = u64(ram, o)[0]
                    elif sub == 2:
                        if rd:
                            x[rd] = i32(ram, o)[0]
                    elif sub == 4:
                        if rd:
                            x[rd] = ram[o]
                    elif sub == 0:
                        if rd:
                            x[rd] = i8(ram, o)[0]
                    elif sub == 5:
                        if rd:
                            x[rd] = u16(ram, o)[0]
                    elif sub == 6:
                        if rd:
                            x[rd] = u32(ram, o)[0]
                    else:
                        if rd:
                            x[rd] = i16(ram, o)[0]
                    pc = nxt
                else:
                    try:
                        v = self._mmio_load(ea, size, pc)
                    except Fault as f:
                        reason = R_FAULT
                        a = f.cause
                        b = f.tval
                        break
                    if rd:
                        x[rd] = v
                    if active:
                        trace.mem(insn_pc, ea, size, False, self.insns + n)
                    pc = nxt
                continue

            if kind == K_STORE:
                ea = (x[r1] + imm) & MASK64
                size = STORE_SIZE[sub]
                if watchers:
                    hit = _watch_match(watchers, ea, size, 1)
                    if hit is not None:
                        reason = R_WATCH
                        a = ea
                        b = hit
                        break
                o = ea - base
                value = x[r2]
                if 0 <= o <= limit - size:
                    if active:
                        trace.mem(insn_pc, ea, size, True, self.insns + n)
                    if sub == 3:
                        u64p(ram, o, value)
                    elif sub == 2:
                        u32p(ram, o, value & 0xFFFFFFFF)
                    elif sub == 1:
                        u16p(ram, o, value & 0xFFFF)
                    else:
                        ram[o] = value & 0xFF
                    pc = nxt
                else:
                    try:
                        self._mmio_store(ea, size, value, pc)
                    except Fault as f:
                        reason = R_FAULT
                        a = f.cause
                        b = f.tval
                        break
                    if active:
                        trace.mem(insn_pc, ea, size, True, self.insns + n)
                    pc = nxt
                continue

            # -- control flow ----------------------------------------------
            if kind == K_BRANCH:
                va = x[r1]
                vb = x[r2]
                if sub == 0:
                    take = va == vb
                elif sub == 1:
                    take = va != vb
                elif sub == 4:
                    take = _signed64(va) < _signed64(vb)
                elif sub == 5:
                    take = _signed64(va) >= _signed64(vb)
                elif sub == 6:
                    take = va < vb
                else:
                    take = va >= vb
                pc = imm if take else nxt
            elif kind == K_JAL:
                if rd:
                    x[rd] = nxt
                pc = imm
            elif kind == K_JALR:
                target = (x[r1] + imm) & MASK64
                target &= ~1
                if rd:
                    x[rd] = nxt
                pc = target
            elif kind == K_AUIPC:
                if rd:
                    x[rd] = (pc + imm) & MASK64
                pc = nxt
            elif kind == K_LUI:
                if rd:
                    x[rd] = imm
                pc = nxt

            # -- integer ALU -----------------------------------------------
            elif kind == K_OPI:
                va = x[r1] if r1 else 0
                if sub == I_ADDI:
                    rv = (va + imm) & MASK64
                elif sub == I_SLLI:
                    rv = (va << (imm & 63)) & MASK64
                elif sub == I_SLTI:
                    rv = 1 if _signed64(va) < imm else 0
                elif sub == I_SLTIU:
                    rv = 1 if va < (imm & MASK64) else 0
                elif sub == I_XORI:
                    rv = va ^ (imm & MASK64)
                elif sub == I_ORI:
                    rv = va | (imm & MASK64)
                elif sub == I_ANDI:
                    rv = va & (imm & MASK64)
                elif sub == I_SRLI:
                    rv = va >> (imm & 63)
                else:  # I_SRAI
                    rv = (_signed64(va) >> (imm & 63)) & MASK64
                if rd:
                    x[rd] = rv
                pc = nxt

            elif kind == K_OP:
                va = x[r1]
                vb = x[r2]
                if sub == A_ADD:
                    rv = (va + vb) & MASK64
                elif sub == A_SLL:
                    rv = (va << (vb & 63)) & MASK64
                elif sub == A_SLT:
                    rv = 1 if _signed64(va) < _signed64(vb) else 0
                elif sub == A_SLTU:
                    rv = 1 if va < vb else 0
                elif sub == A_XOR:
                    rv = va ^ vb
                elif sub == A_SRL:
                    rv = va >> (vb & 63)
                elif sub == A_OR:
                    rv = va | vb
                elif sub == A_AND:
                    rv = va & vb
                elif sub == A_SUB:
                    rv = (va - vb) & MASK64
                elif sub == A_SRA:
                    rv = (_signed64(va) >> (vb & 63)) & MASK64
                elif sub == A_MUL:
                    rv = (va * vb) & MASK64
                elif sub == A_MULH:
                    rv = ((_signed64(va) * _signed64(vb)) >> 64) & MASK64
                elif sub == A_MULHSU:
                    rv = ((_signed64(va) * vb) >> 64) & MASK64
                elif sub == A_MULHU:
                    rv = ((va * vb) >> 64) & MASK64
                elif sub == A_DIV:
                    sa, sb = _signed64(va), _signed64(vb)
                    if sb == 0:
                        rv = MASK64
                    elif sa == -(1 << 63) and sb == -1:
                        rv = 1 << 63
                    else:
                        rv = (abs(sa) // abs(sb)) & MASK64
                        if (sa < 0) != (sb < 0):
                            rv = (-rv) & MASK64
                elif sub == A_DIVU:
                    rv = MASK64 if vb == 0 else va // vb
                elif sub == A_REM:
                    sa, sb = _signed64(va), _signed64(vb)
                    if sb == 0 or (sa == -(1 << 63) and sb == -1):
                        rv = 0
                    else:
                        rv = (abs(sa) % abs(sb)) & MASK64
                        if sa < 0:
                            rv = (-rv) & MASK64
                else:  # A_REMU
                    rv = va if vb == 0 else va % vb
                if rd:
                    x[rd] = rv
                pc = nxt

            elif kind == K_OPI32:
                va = x[r1] if r1 else 0
                if sub == I_ADDIW:
                    rv = (va + imm) & 0xFFFFFFFF
                elif sub == I_SLLIW:
                    rv = (va << (imm & 31)) & 0xFFFFFFFF
                elif sub == I_SRLIW:
                    rv = (va & 0xFFFFFFFF) >> (imm & 31)
                else:  # I_SRAIW
                    rv = (_signed32(va) >> (imm & 31)) & 0xFFFFFFFF
                if rd:
                    x[rd] = _sext32(rv) & MASK64
                pc = nxt

            elif kind == K_OP32:
                va = x[r1]
                vb = x[r2]
                if sub == A_ADD:
                    rv = (va + vb) & 0xFFFFFFFF
                elif sub == A_SUB:
                    rv = (va - vb) & 0xFFFFFFFF
                elif sub == A_SLL:
                    rv = (va << (vb & 31)) & 0xFFFFFFFF
                elif sub == A_SRL:
                    rv = (va & 0xFFFFFFFF) >> (vb & 31)
                elif sub == A_SRA:
                    rv = (_signed32(va) >> (vb & 31)) & 0xFFFFFFFF
                elif sub == A_MUL:
                    rv = (va * vb) & 0xFFFFFFFF
                elif sub == A_DIV:
                    sa, sb = _signed32(va), _signed32(vb)
                    if sb == 0:
                        rv = 0xFFFFFFFF
                    elif sa == -(1 << 31) and sb == -1:
                        rv = 1 << 31
                    else:
                        rv = (abs(sa) // abs(sb)) & 0xFFFFFFFF
                        if (sa < 0) != (sb < 0):
                            rv = (-rv) & 0xFFFFFFFF
                elif sub == A_DIVU:
                    b32 = vb & 0xFFFFFFFF
                    rv = 0xFFFFFFFF if b32 == 0 else (va & 0xFFFFFFFF) // b32
                elif sub == A_REM:
                    sa, sb = _signed32(va), _signed32(vb)
                    if sb == 0 or (sa == -(1 << 31) and sb == -1):
                        rv = 0
                    else:
                        rv = (abs(sa) % abs(sb)) & 0xFFFFFFFF
                        if sa < 0:
                            rv = (-rv) & 0xFFFFFFFF
                else:  # A_REMU
                    b32 = vb & 0xFFFFFFFF
                    rv = (va & 0xFFFFFFFF) if b32 == 0 else (va & 0xFFFFFFFF) % b32
                if rd:
                    x[rd] = _sext32(rv) & MASK64
                pc = nxt

            # -- system / atomics / traps ----------------------------------
            elif kind == K_FENCE:
                pc = nxt

            elif kind == K_CSR:
                csr = imm
                old = self.csrs.get(csr, 0)
                if sub in (5, 6, 7):
                    src = r1  # zimm
                    if sub == 5:
                        if rd:
                            x[rd] = old
                        self.csrs[csr] = src
                    else:
                        if rd:
                            x[rd] = old
                        if src:
                            self.csrs[csr] = (old | src) if sub == 6 else (old & ~src)
                else:
                    src = x[r1]
                    if sub == 1:
                        if rd:
                            x[rd] = old
                        self.csrs[csr] = src & MASK64
                    elif sub == 2:
                        if rd:
                            x[rd] = old
                        if r1:
                            self.csrs[csr] = (old | src) & MASK64
                    elif sub == 3:
                        if rd:
                            x[rd] = old
                        if r1:
                            self.csrs[csr] = (old & ~src) & MASK64
                    else:
                        reason = R_FAULT
                        a = 2
                        b = csr
                        break
                pc = nxt

            elif kind == K_AMO:
                width = 8 if sub & 0x40 else 4
                op5 = sub & 0x3F
                ea = x[r1] & MASK64
                if watchers:
                    hit = _watch_match(watchers, ea, width, 1)
                    if hit is not None:
                        reason = R_WATCH
                        a = ea
                        b = hit
                        break
                try:
                    old = self._guest_load(ea, width, pc)
                    if active:
                        trace.mem(insn_pc, ea, width, False, self.insns + n)
                    if op5 == 0x02:  # lr
                        if rd:
                            x[rd] = old
                    elif op5 == 0x03:  # sc
                        self._guest_store(ea, width, x[r2], pc)
                        if active:
                            trace.mem(insn_pc, ea, width, True, self.insns + n)
                        if rd:
                            x[rd] = 0
                    else:
                        rhs = x[r2]
                        mask = 0xFFFFFFFF if width == 4 else MASK64
                        ov = old & mask
                        rv = rhs & mask
                        if op5 == 0x00:
                            new = (ov + rv) & mask
                        elif op5 == 0x01:
                            new = rv
                        elif op5 == 0x04:
                            new = ov ^ rv
                        elif op5 == 0x08:
                            new = ov | rv
                        elif op5 == 0x0C:
                            new = ov & rv
                        elif op5 == 0x10:
                            new = ov if _signed64(ov) < _signed64(rv) else rv
                        elif op5 == 0x14:
                            new = ov if _signed64(ov) > _signed64(rv) else rv
                        elif op5 == 0x18:
                            new = ov if ov < rv else rv
                        else:  # 0x1C
                            new = ov if ov > rv else rv
                        self._guest_store(ea, width, new, pc)
                        if active:
                            trace.mem(insn_pc, ea, width, True, self.insns + n)
                        if rd:
                            x[rd] = (_sext32(old) & MASK64) if width == 4 else old
                    pc = nxt
                except Fault as f:
                    reason = R_FAULT
                    a = f.cause
                    b = f.tval
                    break

            elif kind == K_EBREAK:
                if single_step:
                    pc = nxt
                else:
                    reason = R_EBREAK
                    a = pc
                    break

            elif kind == K_ECALL:
                if single_step:
                    pc = nxt
                else:
                    reason = R_ECALL
                    a = pc
                    break

            elif kind == K_MRET:
                target = self.csrs.get(0x341, 0) & MASK64
                if target:
                    pc = target
                else:
                    reason = R_FAULT
                    a = 2
                    b = 0
                    break

            else:  # K_ILLEGAL
                reason = R_FAULT
                a = 2
                b = 0
                break

        self.pc = pc
        self.insns += n
        if single_step and reason == R_BUDGET:
            reason = R_STEP
        return (reason, pc, a, b)

    # -- guest memory helpers (AMO) ----------------------------------------

    def _guest_load(self, addr: int, width: int, pc: int = 0) -> int:
        off = addr - self.ram_base
        if 0 <= off <= self.ram_size - width:
            if width == 8:
                return self._u64.unpack_from(self.ram, off)[0]
            return self._u32.unpack_from(self.ram, off)[0]
        return self._mmio_load(addr, width, pc)

    def _guest_store(self, addr: int, width: int, value: int, pc: int = 0) -> None:
        off = addr - self.ram_base
        if 0 <= off <= self.ram_size - width:
            if width == 8:
                self._u64p(self.ram, off, value)
            else:
                self._u32p(self.ram, off, value & 0xFFFFFFFF)
            return
        self._mmio_store(addr, width, value, pc)


#: Task-facing alias: the native simulator machine is :class:`Machine`.
SimMachine = Machine