HD404019R Datasheet, PDF (14/65 Pages) Renesas Technology Corp – CMOS 4-bit single-chip microcomputers

HD404019R Series
During an interrupt, a carry is pushed onto the stack. It is restored by the RTNI instruction, but not by the
RTN instruction.
Status Flag (ST): The status flag (ST) holds the ALU overflow, ALU non-zero, and the results of a bit test
instruction for the arithmetic or compare instructions. It is a branch condition of the BR, BRL, CAL, or
CALL instruction. The value for the status flag remains unchanged until the next arithmetic, compare, or
bit test instruction is executed. The status becomes a 1 after the BR, BRL, CAL, or CALL instruction is
either executed or skipped. During an interrupt, the status is pushed onto the stack. It is restored back from
the stack by the RTNI instruction, but not by the RTN instruction.
Program Counter (PC): The program counter is a 14-bit binary counter which controls the sequence in
which the instructions stored in ROM are executed.
Stack Pointer (SP): The stack pointer (SP) points to the address of the next stack area (up to 16 levels).
The stack pointer is initialized to RAM address $3FF. It is decremented by 4 when data is pushed onto the
stack, and incremented by 4 when data is restored from it. The stack can only be used up to 16 levels deep
because the high-order four bits of the stack pointer are fixed at 1111.
The stack pointer is initialized to $3FF by either MCU reset or by the RSP bit reset from the REM/REMD
instruction.
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