UPD64 datasheet(9/68 Pages) NEC

9 / 68 page

µPD63, 63A, 64

2. INTERNAL CPU FUNCTIONS

2.1 Program Counter (PC): 10 Bits

Refers to the binary counter that holds the address information of the program memory.

Figure 2-1. Program Counter Organization

PC9

PC0

PC8

PC7

PC6

PC5

PC4

PC3

PC2

PC1

The program counter 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 program counter 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.

When reset, the value of the program counter becomes “000H”.

2.2 Stack Pointer (SP): 1 Bit

Refers to the 1-bit register which holds the status of the address stack register.

The stack pointer contents are incremented when the call instruction (CALL) is executed; they are 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 hung up thus a system reset

signal is generated and the PC becoming “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.

2.3 Address Stack Register (ASR (RF)): 10 Bits

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

The low-order 8 bits are arranged in the RF of the data memory as a dual-function RAM.

The register holds

the ASR value even after the RET is executed.

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

Caution If the RF is accessed as the data memory, the high-order 2 bits of the ASR become undefined.

Figure 2-2.

Address Stack Register Organization

ASR9

ASR8

ASR7

ASR6

ASR5

ASR4

ASR3

ASR2

ASR1

ASR0

ASR

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