HMS87C1202A datasheet(25/88 Pages) HYNIX

25 / 88 page

Apr. 2001 ver 1.0

HMS87C130XA/120XA/110XA

8. MEMORY ORGANIZATION

The HMS87C1X0XA has separate address spaces for Pro-

gram memory and Data Memory. Program memory can

only be read, not written to. It can be up to 2K/4K bytes of

Program memory. Data memory can be read and written to

up to 128 bytes including the stack area.

8.1 Registers

This device has six registers that are the Program Counter

(PC), a Accumulator (A), two index registers (X, Y), the

Stack Pointer (SP), and the Program Status Word (PSW).

The Program Counter consists of 16-bit register.

Figure 8-1 Configuration of Registers

Accumulator: The Accumulator is an 8-bit general pur-

pose register, used for data operation such as transfer, tem-

porary saving, and conditional judgement, etc.

The Accumulator can be used as a 16-bit register with Y

Figure 8-2 Configuration of YA 16-bit Register

X, Y Registers: In the addressing mode which uses these

index registers, the register contents are added to the spec-

ified address, which becomes the actual address. These

modes are extremely effective for referencing subroutine

tables and memory tables. The index registers also have in-

crement, decrement, comparison and data transfer func-

tions, and they can be used as simple accumulators.

Stack Pointer: The Stack Pointer is an 8-bit register used

for occurrence interrupts and calling out subroutines. Stack

Pointer identifies the location in the stack to be accessed

(save or restore).

Generally, SP is automatically updated when a subroutine

call is executed or an interrupt is accepted. However, if it

is used in excess of the stack area permitted by the data

memory allocating configuration, the user-processed data

may be lost.

The stack can be located at any position within 00H to 7FH

of the internal data memory. The SP is not initialized by

hardware, requiring to write the initial value (the location

with which the use of the stack starts) by using the initial-

ization routine. Normally, the initial value of “7FH” is

used.

Note: The Stack Pointer must be initialized by software be-

cause its value is undefined after RESET.

Example: To initialize the SP

LDX

#07FH

TXSP

; SP

← 7FH

Program Counter: The Program Counter is a 16-bit wide

which consists of two 8-bit registers, PCH and PCL. This

counter indicates the address of the next instruction to be

executed. In reset state, the program counter has reset rou-

tine address (PCH:0FFH, PCL:0FEH).

Program Status Word: The Program Status Word (PSW)

contains several bits that reflect the current state of the

CPU. The PSW is described in Figure 8-3. It contains the

Negative flag, the Overflow flag, the Break flag the Half

Carry (for BCD operation), the Interrupt enable flag, the

Zero flag, and the Carry flag.

[Carry flag C]

This flag stores any carry or borrow from the ALU of CPU

after an arithmetic operation and is also changed by the

Shift Instruction or Rotate Instruction.

[Zero flag Z]

This flag is set when the result of an arithmetic operation

or data transfer is “0” and is cleared by any other result.

ACCUMULATOR

X REGISTER

Y REGISTER

STACK POINTER

PROGRAM COUNTER

PROGRAM STATUS

WORD

PCL

PCH

PSW

Two 8-bit Registers can be used as a “YA” 16-bit Register

Stack Address (000H ~ 07FH)

Hardware fixed

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