The RX CPU has sixteen general-purpose registers, nine control registers, and one accumulator used for DSP instructions.

This CPU has sixteen general-purpose registers (R0 to R15). R1 to R15 can be used as data register or address register.

R0, a general-purpose register, also functions as the stack pointer (SP). The stack pointer is switched to operate as the interrupt stack pointer (ISP) or user stack pointer (USP) by the value of the stack pointer select bit (U) in the processor status word (PSW).

The stack pointer (SP) can be either of two types, the interrupt stack pointer (ISP) or the user stack pointer (USP). Whether the stack pointer operates as the ISP or USP depends on the value of the stack pointer select bit (U) in the processor status word (PSW).

Set the ISP or USP to a multiple of four, as this reduces the numbers of cycles required to execute interrupt sequences and instructions entailing stack manipulation.

The interrupt table register (INTB) specifies the address where the relocatable vector table starts.

The program counter (PC) indicates the address of the instruction being executed.

The backup PC (BPC) is provided to speed up response to interrupts. After a fast interrupt has been generated, the contents of the program counter (PC) are saved in the BPC.

The backup PSW (BPSW) is provided to speed up response to interrupts. After a fast interrupt has been generated, the contents of the processor status word (PSW) are saved in the BPSW. The allocation of bits in the BPSW corresponds to that in the PSW.

TThe fast interrupt vector register (FINTV) is provided to speed up response to interrupts. The FINTV register specifies a branch destination address when a fast interrupt has been generated.he static base register (SB) is a 16-bit register used for SB-based relative addressing.

The floating-point status word (FPSW) indicates the results of floating-point operations. In products that do not support floating-point instructions, the value "00000000h" is always read out and writing to these bits does not affect operations.

When an exception handling enable bit (Ej) enables the exception handling (Ej = 1), the corresponding Cj flag indicates the cause. If the exception handling is masked (Ej = 0), check the Fj flag at the end of a series of processing. The Fj flag is the accumulation type flag (j = X, U, Z, O, or V).

The accumulator (ACC) is a 64-bit register used for DSP instructions. The accumulator is also used for the multiply and multiply-and-accumulate instructions; EMUL, EMULU, FMUL, MUL, and RMPA, in which case the prior value in the accumulator is modified by execution of the instruction.

Use the MVTACHI and MVTACLO instructions for writing to the accumulator. The MVTACHI and MVTACLO instructions write data to the higher-order 32 bits (bits 63 to 32) and the lower-order 32 bits (bits 31 to 0), respectively.

Use the MVFACHI and MVFACMI instructions for reading data from the accumulator. The MVFACHI and MVFACMI instructions read data from the higher-order 32 bits (bits 63 to 32) and the middle 32 bits (bits 47 to 16), respectively.