Software Compatible 6809 CPU

Memec Design

February 25, 2003

3

Optimized for

Functional Description

Structure of 6809

The block diagram below shows the index arithmetic unit, the data ALU, command sequencer, memory fetch unit

and program counter.

Figure 2: Block Diagram

The memory interface contains registered read and write data and address registers, which makes the design of the

interface easier for timing verification etc.

Internal Registers

The following part describes the existing registers in the MC-ACT-6809:

Index Registers (X,Y)

The index registers are used in indexed mode of addressing and are used for calculating the effective addresses.

During some indexed modes, the contents of the index register are incremented or decremented to point to the next

item of tabular type data. All 4 pointer register (U, S, X, Y) can be used as index register.

Stack Pointer Registers (U,S)

The system stack pointer S is automatically used by the processor during subroutine calls and interrupts! The user

stack pointer is controlled by the programmer. Both stack pointers point to the top of the stack.

Program Counter Register

The program counter (PC) points to the address of the next instruction to be executed.

Accumulators (A,B,D)

The A and B accumulators are general purpose registers. Some instructions concatenate the registers A and B to

achieve 16 bit values. This register is then referred as the D register with the A register as most significant byte.

PC

S

U

X

Y

D = A : B

A

B

CC

DP

Index ALU

Data ALU

Command

Sequencer

Memory

Fetch Unit

Address Register

Write Data Register

Read Data Register