UPD6P9 datasheet(13/45 Pages) RENESAS

13 / 45 page

µPD6P9

Data Sheet U15144EJ1V1DS

3. INTERNAL CPU FUNCTIONS

3.1 Program Counter (PC): 12 Bits

The program counter (PC) is a binary counter that holds the address information of the program memory.

Figure 3-1. Program Counter Configuration

The PC contains the address of the instruction that should be executed next.

Normally, the counter contents

are automatically incremented in accordance with the instruction length (byte count) each time an instruction is

executed.

However, when executing jump instructions (JMP, JC, JNC, JF, JNF), the PC contains the jump destination

address written in the operand.

When executing the subroutine call instruction (CALL), the call destination address written in the operand is

entered in the PC after the PC contents at the time are saved in the address stack register (ASR).

If the return

instruction (RET) is executed after the CALL instruction is executed, the address saved in the ASR is restored to

the PC.

After reset, the value of the PC becomes “000H”.

3.2 Stack Pointer (SP): 1 Bit

This is a 1-bit register that holds the status of the address stack register.

The stack pointer contents are incremented when the call instruction (CALL) is executed and decremented when

the return instruction (RET) is executed.

When reset, the stack pointer contents are cleared to 0.

When the stack pointer overflows (stack level 2 or more) or underflows, the CPU is defined as hung up, a system

reset signal is generated, and the PC becomes 000H.

As no instruction is available to set a value directly for the stack pointer, it is not possible to operate the pointer

by means of a program.

3.3 Address Stack Register (ASR (RF)): 12 Bits

The address stack register saves the return address of the program after a subroutine call instruction is executed.

The lower 8 bits are allocated in RF of the data memory as a alternate-function RAM.

The register holds the

ASR value even after the RET instruction is executed.

After reset, it holds the previous data (undefined when turning on the power).

Caution If RF is accessed as the data memory, the higher 4 bits become undefined.

Figure 3-2. Address Stack Register Configuration

PC9

PC10

PC0

PC8

PC7

PC6

PC5

PC4

PC3

PC2

PC1

PC11

ASR10

ASR9

ASR8

ASR7

ASR6

ASR5

ASR4

ASR3

ASR2

ASR1

ASR0

ASR

ASR11

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