Memory Organization; Registers - Abov MC80F0304 User Manual

MC80F0304/08/16

8. MEMORY ORGANIZATION

gram memory and Data Memory. 4K bytes program memory can The MC80F0304/0308/0316 has separate address spaces for Pro- Data memory can be read and written to up to 256 bytes including the stack area.
only be read, not written to.

8.1 Registers

This device has six registers that are the Program Counter (PC), Generally, SP is automatically updated when a subroutine call is
a Accumulator (A), two index registers (X, Y), the Stack Pointer executed or an interrupt is accepted. However, if it is used in ex-
(SP), and the Program Status Word (PSW). The Program Counter cess of the stack area permitted by the data memory allocating
consists of 16-bit register. configuration, the user-processed data may be lost.
The stack can be located at any position within 1C0 the internal data memory. The SP is not initialized by hardware, H to 1FF H of
A ACCUMULATOR requiring to write the initial value (the location with which the use
X X REGISTER of the stack starts) by using the initialization routine. Normally,
Y Y REGISTER the initial value of "FF H " is used.
SP STACK POINTER Stack Address (1C0 H ~ 1FF ) H
PCH PCL PROGRAM COUNTER Bit 15 01 H 8 7 SP Bit 0
PSW PROGRAM STATUS WORD C0 H ~FF H
Hardware fixed
Figure 8-1 Configuration of Registers
Accumulator: The Accumulator is the 8-bit general purpose reg- Note: The Stack Pointer must be initialized by software be-
ister, used for data operation such as transfer, temporary saving, cause its value is undefined after Reset.
and conditional judgement, etc. Example: To initialize the SP
The Accumulator can be used as a 16-bit register with Y Register LDX #0FFH
as shown below. TXSP ; SP ← FFH
Program Counter: The Program Counter is a 16-bit wide which
Y consists of two 8-bit registers, PCH and PCL. This counter indi-
Y A cates the address of the next instruction to be executed. In reset state, the program counter has reset routine address (PC :0FF ,
A PC :0FE L H ). H H
Two 8-bit Registers can be used as a "YA" 16-bit Register Program Status Word: The Program Status Word (PSW) con-
tains several bits that reflect the current state of the CPU. The
Figure 8-2 Configuration of YA 16-bit Register PSW is described in Figure 8-3 . It contains the Negative flag, the Overflow flag, the Break flag the Half Carry (for BCD opera-
X, Y Registers: In the addressing mode which uses these index registers, the register contents are added to the specified address, tion), the Interrupt enable flag, the Zero flag, and the Carry flag.
which becomes the actual address. These modes are extremely ef- [Carry flag C]
fective for referencing subroutine tables and memory tables. The This flag stores any carry or borrow from the ALU of CPU after
data transfer functions, and they can be used as simple accumula- index registers also have increment, decrement, comparison and an arithmetic operation and is also changed by the Shift Instruc-
tors. tion or Rotate Instruction.
Stack Pointer: The Stack Pointer is an 8-bit register used for oc- [Zero flag Z]
currence interrupts and calling out subroutines. Stack Pointer This flag is set when the result of an arithmetic operation or data
identifies the location in the stack to be accessed (save or restore). transfer is "0" and is cleared by any other result.
32 November 4, 2011 Ver 2.12