GMS81508B/16B/24B
HYUNDAI MicroElectronics
8. MEMORY ORGANIZATION
The GMS81508B/16B/24B has 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 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.
A
X
Y
SP
PCH
PCL
PSW
ACCUMULATOR
X REGISTER
Y REGISTER
STACK POINTER
PROGRAM COUNTER
PROGRAM STATUS
WORD
Bit 15
Stack Address (100
H
~ 1FE
H
)
8 7
Bit 0
01
H
SP
00
H
~FE
H
Hardware fixed
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 100
H
to
1FF
H
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 “FE
H
” is
used.
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
Two 8-bit Registers can be used as a “YA” 16-bit Register
A
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 01FF
H
can not be used as stack. Don not use
1FF
H
, or malfunction would be occurred.
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
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 (PC
H
:0FF
H
, PC
L
:0FE
H
).
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.
18
DEC. 1999 Ver 1.04
HYNIX [ HYNIX SEMICONDUCTOR ]
