Hi
The uTasker project concentrates on small footprint operation and so Thumb mode is the setting that it is designed for.
As long as inter-working is enabled ARM and THUMB modes are fully compatible (some files can be compiled in ARM mode, others in Thumb, and individual routines can also be specified to be compiled in Thumb or ARM mode).
In some ARM targets there is one piece of code which is written for thumb mode operation - this is the FLASH manipulation code (erase, write) which runs from SRAM (it needs to be configured to an uneven address in SRAM to cause the Thumb mode to be used. However in the LPC2XXX project this technique is not needed since the chip has in-build IAP support.
It should thus be possible to compile the complete LPC2XXX project in Thumb or ARM mode, or mix as required. This 'should' be a purely compiler thing.
Regards
Mark
Note that interrupts always run in ARM mode. There are different techniques used for vectoring interrupts, meaning that - depending on whether the interrupt code is called directly or whether it is called via an interrupt dispatcher - the interrupt routine may be automatically forced to ARM mode (the programmer won't notice this since it is automated by the irq attribute). GCC tends to use an interrupt dispatcher since not all versions correctly support vectored interrupts, but IAR and co. will tend to use direct vectoring (forced ARM). This may change, depending on GCC progress and exact target, but is generally transparent to the programmer.