R8C/13 Group
2. Central Processing Unit (CPU)
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. A0 can be
combined with A0 to be 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 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 state.
2.8.1 Carry Flag (C)
The C flag retains a carry, borrow, or shift-out bit that has occurred in the arithmetic 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”.
2.8.7 Interrupt Enable Flag (I)
The I flag enables a maskable interrupt.
An interrupt is disabled when the I flag is set to “0”, and are enabled when the I flag is set to “1”. The
I flag is set to “0” when an interrupt request is acknowledged.
2.8.8 Stack Pointer Select Flag (U)
ISP is selected when the U flag is set to “0”, USP is selected when the U flag is set to “1”.
The U flag is set to “0” when a hardware interrupt request is acknowledged or the INT instruction of
software interrupt numbers 0 to 31 is executed.
2.8.9 Processor Interrupt Priority Level (IPL)
IPL, 3 bits wide, assigns processor interrupt priority levels from level 0 to level 7.
If a requested interrupt has greater priority than IPL, the interrupt is enabled.
2.8.10 Reserved Bit
When write to this bit, set to “0”. When read, its content is indeterminate.
Rev.1.20 Jan 27, 2006 page 8 of 205
REJ09B0111-0120
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