it is possible to run without virtual memory at all, just physical memory (in fact, most embedded systems run this way). How?

Question

I am learning computer architecture and came across one statement that " It is possible to run without virtual memory at all, just physical memory (in fact, most embedded systems run this way) ".

So is it true ? if yes, then I want to know how ?

Thanks.

Answer 1

Embedded systems that do not use virtual memory typically run as a single process or thread or support a multi-threading rather than multi-processing task model. That is to say all threads/tasks share a common address space but have separate stacks (although also in the single address space).

On a processor that has an MMU and supports virtual memory this is done simply by not configuring the MMU or at least having a static MMU configuration with a one-to-one mapping so that physical and MMU addresses are identical, or at least so that there is a single virtual address space.

Many low to mid-range architectures used in embedded systems such as, PIC, AVR, ARM7, ARM Cortex-M, Zilog Z8 etc. lack an MMU, and typically have much smaller memory resources than a typical ARM9/11/Cortex-A or x86 based system.

For multi-threading support in an MMU-less system you would typically use a real-time operating systems (RTOS). Most RTOS, with some notable exceptions are simple task schedulers with IPC and synchronisation primitives and do not use or support an MMU. High-end RTOS such as QNX and VxWorks have MMU support, although in VxWorks it is optional.

uCLinux is a GPOS targeted at processors that have sufficient memory resources to run Linux but which lack an MMU such as ARM7 and Cortex-M. Although arguably, Linux without an MMU rather misses one of the major advantages of using Linux while lacking hard real-time performance, and requiring large memories; a typical RTOS kernel requires (much) less than 10kBytes of code.