Renesas Emulator System M3T-MR100 - Manual (Page 265)

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[Precautions]
1. Regarding the method for specifying a register bank
No kernel interrupt handlers that use the registers in register bank 1 can be written in C language. These handlers can only
be written in assembly language. When writing in assembly language, write the entry and exit to and from the interrupt
handler as shown below.
(Always be sure to clear the B flag before issuing ret_int service call.)
Example: interrupt;
fset
B
fclr B
ret_int
Internally in the MR100 kernel, register banks are not switched over.
2. Regarding the method for specifying a high-speed interrupt
To ensure the effective use of a high-speed interrupt, be sure that the registers in register bank 1 are used in the high-speed
interrupt. Also be aware that the high-speed interrupts used cannot be a kernel interrupt handler..