ARM Thumb ISA

Hi,

sorry, when this is off-topic but I didn't really know what other
newsgroup to use.
A general question on the instruction set of the ARM processor:

Many of the ARM processors provide a normal ISA (32-bit) as well
as an compressed 16-bit ISA (Thumb mode).
It's obvious that the Thumb instructions are smaller than the
32-bit instructions resulting in a smaller program code if merely
the Thumb instructions are used.
What about the runtime of a program. Is a program, in general, 
slower or faster when constructed from instructions of one ISA?
If so, why?

Thank you.
Chris

Christian Christmann wrote:

> sorry, when this is off-topic but I didn't really know what other
> newsgroup to use.

This question is entirely on-topic for comp.arch.embedded.

> What about the runtime of a program. Is a program, in general,
> slower or faster when constructed from instructions of one ISA?

It's impossible to generalize about this because it's system-dependent.
ARM publishes metrics on speed vs. size for Thumb vs. 32-bit code. If I
recall correctly, the rough number is 60% of the speed with a 25%
improvement in code density. However, the situation is affected by
memory bus width and speed. If you are running out of 16-bit memory
(quite a few low-end ARM parts only have 16-bit external data buses)
and/or slow flash memory, Thumb's speed numbers may be significantly
better because the memory subsystem isn't being called upon for the
same throughput.