Trust

   trusted                         isolated
   -------------------------       ------------------------------------
   CPU, microcode                  every program, including system ones
   firmware: UEFI, AML, SMM        users and sessions
   the signed image, with its      the network, file contents
   pinned toolchain and crates     devices: DMA only into granted memory

ToyOS trusts only what it cannot avoid trusting:

• the CPU and chipset vendor, with its signed microcode;
• the platform firmware, which already controls the whole machine;
• the signed OS image, with its pinned toolchain and crates.

Everything else is isolated, including external and hot-plugged devices, whose DMA stays behind the IOMMU.

Privileges beyond a device claim

• A privilege beyond an ordinary device claim, such as a reset line or a firmware register window, is granted only by the signed image’s system manifest, to a program that image ships.
• A package installed later never receives one, whoever signed it.

Machines without an IOMMU

• The same driver runs with and without an IOMMU; there is never a second driver.
• Where the hardware genuinely has no isolation, any driver the signed image ships may claim its device with its ordinary claim, and no other driver may. The machine states plainly that it runs without device isolation.
• External DMA-capable devices always need isolation.
• Unknown isolation, unsupported isolation hardware and a failed setup are errors, not evidence that isolation is absent. There is no machine-specific exception image.

Device firmware

• The IOMMU does not stop a driver reflashing its own device’s firmware.
• Device firmware is loaded by its owning driver through its IOMMU domain. CPU microcode loading is privileged kernel work. Neither payload need be embedded in the kernel binary.

Checks inside the trusted base

• A check there is a bug fence: kept where it is cheap and catches ToyOS’s own mistakes, such as a write landing in RAM, the IOMMU, an interrupt controller or another driver’s device.
• It never withholds a feature for lack of a vendor document, and ToyOS does not reimplement firmware to second-guess it.