Motorola M6800 User Manual download pdf (Page 15)

MPU-15

SUMMARY OF MPU OPERATION

The MPU requires a two phase symmetrical, TTL compatible,

nonoverlapping clock. During the first phase of the clock (Ø

high) an

address will be placed on the address bus by the MPU. During the second phase

of the clock (Ø

high), the bidirectional data bus will be active. The first

byte of an instruction enters the MPU and is transferred into an internal

instruction register and decoded by the MPU. The MPU will then contain the

information needed to read in an additional one or two bytes of program is

necessary. Once the entire instruction is read into the MPU (one, two or

three bytes) the instruction is then executed. The MPU then reads in the next

sequential byte in the program and places it again in the instruction

branch or jump instruction changes the value of the program counter. If this

occurs, the next instruction to be executed is determined by the new program

counter value.

If an interrupt or reset occurs during this process, the program

counter value will also be changed. The new program counter value is

determined by the highest eight memory locations that are reserved for reset

and interrupt vectors.

In the case of interrupt, the stack pointer is used to store the

contents of the internal registers necessary to return to the program

location prior to the interrupt. This happens when the interrupt program

exits by an RTI (Return from interrupt instruction). Similarly, the stack

pointer is used to store the program counter value when a JSR (Jump to

Subroutine) or BSR (Branch to Subroutine) instruction occurs. The program

counter returns to its original value when an RTS (Return from Subroutine)

instruction occurs. The stack pointer value is set by an LDS (Load Stack

Pointer) instruction.

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Motorola M6800 User Manual Page 15