Here is the core/register model for the MCUs in the R8C Tiny series. It is exactly the same model as the one for the devices in the M16C series. R0, R1, R2, R3, A0, and A1 are six general-purpose, 16-bit registers. A0 and A1 can also be used in special address register modes as independent 16-bit registers or as a combined 32-bit register. The R0 and R1 registers can also be accessed as 16-bit registers or as individual 8-bit registers. The Stack Base (SB) register and Frame Base (FB) register are used for indexed addressing. FB is especially useful in creating stack frames when programming in C. The two Stack Pointers – a User Stack Pointer (USP) and Interrupt Stack Pointer (ISP) – combine to minimize RAM usage when the MCU is performing multiple tasks. The model also shows the Flag register, Program Counter, and Interrupt Table Pointer. The INTB allows you to position the beginning of the Variable Vector table anywhere in valid memory space.