R8C/20 Group, R8C/21 Group 2. Central Processing Unit (CPU)
Rev.2.00 Aug 27, 2008 Page 11 of 458
REJ09B0250-0200
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 bit (R0H) and low-order bit (R0L) to be used separately as 8-bit data registers. The
same applies to R1H and R1L as R0H and R0L. R2 can be combined with R0 to be used as a 32-bit data register
(R2R0). The same applies R3R1 as R2R0.
2.2 Address Registers (A0 and A1)
A0 is a 16-bit register for address register indirect addressing and address register relative addressing. They also
are used for transfer, arithmetic and logic operations. The same applies to A1 as A0.
A1 can be combined with A0 to be used 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, a 20-bit register, indicates the start address of an interrupt vector table.
2.5 Program Counter (PC)
PC, 20 bits wide, indicates the address of an instruction to be executed.
2.6 User Stack Pointer (USP) and Interrupt Stack Pointer (ISP)
The stack pointer (SP), USP and ISP, are 16 bits wide each.
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 a 11-bit register indicating the CPU status.
2.8.1 Carry Flag (C)
The C flag retains a carry, borrow, or shift-out bit that has occurred in the arithmetic and logic unit.
2.8.2 Debug Flag (D)
The D flag is for debug only. Set to 0.
2.8.3 Zero Flag (Z)
The Z flag is set to 1 when an arithmetic operation resulted in 0; otherwise, 0.
2.8.4 Sign Flag (S)
The S flag is set to 1 when an arithmetic operation resulted in a negative value; otherwise, 0.
2.8.5 Register Bank Select Flag (B)
The register bank 0 is selected when the B flag is 0. The 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 resulted in an overflow; otherwise, 0.
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