GMS81C2012 datasheet(25/107 Pages) HYNIX

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HYUNDAI MicroElectronics

GMS81C2012/GMS81C2020

MAR. 2000 Ver 1.00

8. MEMORY ORGANIZATION

The GMS81C2012 and GMS81C2020 have separate ad-

dress spaces for Program memory and Data Memory. Pro-

gram memory can only be read, not written to. It can be up

to 12K/20K bytes of Program memory. Data memory can

be read and written to up to 448 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 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 access

(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 “FFH” 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

#0FFH

TXSP

; SP

← FF

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

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~FFH

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