GMS81C5108
8. MEMORY ORGANIZATION
The GMS81C5108 has separate address spaces for Pro-
gram memory, Data Memory and Display memory. Pro-
gram memory can only be read, not written to. It can be up
to 8K bytes of Program memory. Data memory can be read
and written to up to 192 bytes including the stack area. Dis-
play memory has prepared 37 bytes for LCD.
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.
ACCUMULATOR
X REGISTER
A
X
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.
Y
Y REGISTER
SP
STACK POINTER
PCH
PCL
PROGRAM COUNTER
Stack Address (00 ~ BF )
H
H
PROGRAM STATUS
WORD
PSW
15
8
7
0
0
SP
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.
Caution:
The Stack Pointer must be initialized by software be-
cause its value is undefined after RESET.
The Accumulator can be used as a 16-bit register with Y
Register as shown below.
Example: To initialize the SP
LDX
TXSP
#0BFH
Y
; SP ← BFH
Y
A
A
Program Counter:
The Program Counter is a 16-bit wide
Two 8-bit Registers can be used as a “YA” 16-bit Register
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).
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.
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]
Stack Pointer:
The Stack Pointer is an 8-bit register used
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.
for occurrence interrupts and calling out subroutines. Stack
Pointer identifies the location in the stack to be accessed
(save or restore).
18
JUNE 2001 Ver 1.0
HYNIX [ HYNIX SEMICONDUCTOR ]
