HYUNDAI MicroElectronics
GMS81C2012/GMS81C2020
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.
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.
A
ACCUMULATOR
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.
X REGISTER
X
Y REGISTER
Y
STACK POINTER
SP
PROGRAM COUNTER
PCH
PCL
PROGRAM STATUS
WORD
PSW
Stack Address ( 100H ~ 1FEH
)
Bit 15
8 7
Bit 0
01H
SP
00H~FFH
Figure 8-1 Configuration of Registers
Hardware fixed
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.
Note: The Stack Pointer must be initialized by software be-
cause its value is undefined after RESET.
Example: To initialize the SP
The Accumulator can be used as a 16-bit register with Y
Register as shown below.
LDX
#0FFH
TXSP
; SP ← FFH
Y
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).
Y
A
A
Two 8-bit Registers can be used as a "YA" 16-bit Register
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.
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.
[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.
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).
[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.
MAR. 2000 Ver 1.00
25
HYNIX [ HYNIX SEMICONDUCTOR ]
