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GMS87C1404

CMOS SINGLE-CHIP 8-BIT MICROCONTROLLER

Hyundai Micro Electronics 

GMS87C1404/GMS87C1408

HYUNDAI MicroElectronics

8. MEMORY ORGANIZATION

The GMS87C1404 and GMS87C1408 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

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.

PCH

A

X

Y

SP

PCL

PSW

ACCUMULATOR

X REGISTER

Y REGISTER

STACK POINTER

PROGRAM COUNTER

PROGRAM STATUS

WORD

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

Register as shown below.

Y

Y

A

A

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

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).

to 4K /8K bytes of Program memory. Data memory can be

read and written to up to 192 bytes including the stack area.

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 BFH

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 “BFH” is

used.

Stack Address (000H ~ 0BFH)

15

87

0

0

SP

Hardware fixed

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

cause its value is undefined after RESET.

Example: To initialize the SP

LDX

#0BFH

TXSP

; SP ← BFH

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.

16

Oct. 1999 Ver 1.0

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