diff --git a/Hardware-API.md b/Hardware-API.md
index 33799a0..bf3b41f 100644
--- a/Hardware-API.md
+++ b/Hardware-API.md
@@ -82,6 +82,10 @@ dir(machine)
 
 one can easily see what's supported on the board.
 
+## Physical memory access
+
+Provided by `mem8` (NOHEAP), `mem16` (NOHEAP), `mem32` (HEAP) virtual arrays. Mind that only `mem8` and `mem16` guarantee no heap access. For systems with virtual memory, these functions access hardware-defined physical address space (which includes memory-mapped I/O devices, etc.), not virtual memory map of a current process. 
+
 ## IRQs
 
 An interrupt request (IRQ) is an asynchronous and pre-emptive action triggered by a peripheral.  Peripherals that support interrupts provide the `irq` method which returns an irq object.  This can be used to execute a function when an IRQ is triggered, or wake up the device, or both.