-- Inject a native-mode interrupt into memBlockIrq while an armed outer
-- move is executing its block-move stub on the stack (PB 0, PC below the
-- program at $1000), never into a handler and at most every third frame,
-- so the outer moves make progress between handlers.
local cpu = manager.machine.devices[":maincpu"]
local mem = cpu.spaces["program"]
local irqFrame = 0
emu.register_frame_done(function()
    irqFrame = irqFrame + 1
    if irqFrame % 3 ~= 0 then
        return
    end
    if mem:read_u16(0x025022) ~= 1 or mem:read_u16(0x025028) ~= 0 then
        return
    end
    local pc = cpu.state["PC"].value
    if cpu.state["PB"].value ~= 0 or pc < 0x0100 or pc >= 0x1000 then
        return
    end
    local s = cpu.state["S"].value
    local p = cpu.state["P"].value
    mem:write_u8(s, cpu.state["PB"].value)
    mem:write_u8(s - 1, (pc >> 8) & 0xFF)
    mem:write_u8(s - 2, pc & 0xFF)
    mem:write_u8(s - 3, p)
    cpu.state["S"].value = s - 4
    cpu.state["P"].value = (p | 0x04) & 0xF7
    cpu.state["PC"].value = mem:read_u16(0x025020)
    mem:write_u16(0x025028, 1)
    mem:write_u16(0x025024, mem:read_u16(0x025024) + 1)
end)