GMS82512 datasheet(24/110 Pages) HYNIX

24 / 110 page

GMS81508B/16B/24B, GMS82512/16/24

MAY. 2001 Ver 2.0

8. MEMORY ORGANIZATION

The GMS81508B/16B/24B and GMS82512/16/24 have

separate address spaces for Program memory and Data

Memory. Program memory can only be read, not written

to. It can be up to 24K bytes of Program memory. Data

memory can be read and written to up to 448 bytes includ-

ing 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 the 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 100H to

1FFH of the internal data memory. The SP is not initialized

by hardware, requiring to write the initial value (the loca-

tion with which the use of the stack starts) by using the ini-

tialization routine. Normally, the initial value of “FEH” 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

#0FEH

TXSP

; SP

← FEH

Address 01FFH can not be used as stack. Don not use

1FFH, or malfunction would be occurred.

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.

ACCUMULATOR

X REGISTER

Y REGISTER

STACK POINTER

PROGRAM COUNTER

PROGRAM STATUS

WORD

PCL

PSW

PCH

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

01H

Stack Address (100H ~ 1FEH)

Bit 15

Bit 0

Hardware fixed

00H~FEH

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