Keil RTX v5 kernel functions are executed in handler mode (using PendSV/SysTick/SVC) and the tables below lists the maximum stack requirements for the Main Stack (MSP) that the user should consider.

The stack for the osKernelStart function is referred as "Startup" and RTX v5 uses 32 bytes (with Arm Compiler). However the user should also consider additional stack that might be allocated by the 'main' function of the embedded application. The following picture shows a worst-case memory allocation of the Main Stack.

Main Stack usage of RTX v5 applications

The stack requirements depend on the compiler and the optimization level. RTX v5 supports event annotations and this configuration impacts also the stack requirement.

Arm Compiler ARMCC V6.10: Main Stack requirements for PendSV/SysTick/SVC

Optimization	RTX Kernel	RTX Kernel + Event Recorder
-O1 (Debug)	152 bytes	280 bytes
-Os (Balanced)	120 bytes	256 bytes
-Oz (Size)	112 bytes	248 bytes

Arm Compiler ARMCC V5.06: Main Stack requirements for PendSV/SysTick/SVC

Optimization	RTX Kernel	RTX Kernel + Event Recorder
-O0 (Debug)	176 bytes	360 bytes
-O1	112 bytes	248 bytes
-O2	112 bytes	256 bytes
-O3	112 bytes	248 bytes