R8C/1A Group, R8C/1B Group 2. Central Processing Unit (CPU)
Rev.1.40 Dec 08, 2006 Page 16 of 45
REJ03B0144-0140
2.1 Data Registers (R0, R1, R2, and R3)
R0 is a 16-bit register for transfer, arithmetic, and logic operations. The same applies to R1 to R3. R0
can be split into high-order bits (R0H) and low-order bits (R0L) to be used separately as 8-bit data
registers. R1H and R1L are analogous to R0H and R0L. R2 can be combined with R0 and used as a 32-
bit data register (R2R0). R3R1 is analogous to R2R0.
2.2 Address Registers (A0 and A1)
A0 is a 16-bit register for address register indirect addressing and address register relative addressing.
It is also used for transfer and arithmetic and logic operations. A1 is analogous to A0. A1 can be
combined with A0 and used as a 32-bit address register (A1A0).
2.3 Frame Base Register (FB)
FB is a 16-bit register for FB relative addressing.
2.4 Interrupt Table Register (INTB)
INTB is a 20-bit register that indicates the start address of an interrupt vector table.
2.5 Program Counter (PC)
PC is 20 bits wide indicates the address of the next instruction to be executed.
2.6 User Stack Pointer (USP) and Interrupt Stack Pointer (ISP)
The stack pointer (SP), USP, and ISP, are each 16 bits wide. The U flag of FLG is used to switch
between USP and ISP.
2.7 Static Base Register (SB)
SB is a 16-bit register for SB relative addressing.
2.8 Flag Register (FLG)
FLG is an 11-bit register indicating the CPU state.
2.8.1 Carry Flag (C)
The C flag retains a carry, borrow, or shift-out bits that have been generated by the arithmetic and
logic unit.
2.8.2 Debug Flag (D)
The D flag is for debugging only. Set it to 0.
2.8.3 Zero Flag (Z)
The Z flag is set to 1 when an arithmetic operation results in 0; otherwise to 0.
2.8.4 Sign Flag (S)
The S flag is set to 1 when an arithmetic operation results in a negative value; otherwise to 0.
2.8.5 Register Bank Select Flag (B)
Register bank 0 is selected when the B flag is 0. Register bank 1 is selected when this flag is set to 1.
2.8.6 Overflow Flag (O)
The O flag is set to 1 when the operation results in an overflow; otherwise to 0.
