How to specify global heap as box thread heap?

In https://github.com/ARMmbed/uvisor/raw/docs/uvisor-rtos-docs.pdf, it mentions allocation fallback scheme:

> “page-backed thread heap” -> “static process heap” -> “insecure global heap”

The sample https://github.com/ARMmbed/mbed-os-example-uvisor-thread gives how to specify **page-backed thread heap** or **static process heap** as box thread heap. But how to specify **insecure global heap** as box thread heap even though it is insecure?

Start with the allocation fallback scheme in uvisor-rtos-docs.pdf and compare it with the mbed-os-example-uvisor-thread sample's page-backed and static process heap configuration. Determine whether the sample or another configuration entry point documents selecting the insecure global heap, and document the supported procedure or limitation with a clear example.

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