Motorola M6800 User Manual download pdf (Page 5)

MPU-5

CPX - Compare index register

DEX - Decrement index register

INX - Increment index register

LDX - Load index register

RTI - Return from interrupt

STX - Store index register

SWI - Software interrupt

TSX - Transfer stack pointer to index register

TXS - Transfer index register to stack pointer

WAI - Wait for interrupt

Program Counter

The program counter (PC) is a 16 bit register that contains the

address of the next byte to be fetched from memory. When the current value of

the program counter is placed on the address buss, the program counter will

be incremented automatically.

Stack Pointer

The Stack Pointer (SP) is a 16 bit (2 byte) register that contains a

beginning address, normally in RAM, where the status of the MPU registers may

be stored when the MPU has other functions to perform, such as during an

interrupt or during a Branch to Subroutine (BTS). The address in the stack

pointer is the starting address of sequential memory locations in RAM where

MPU status registers will be stored. The status of the MPU will be stored in

the RAM as follows:

Stack Pointer Address : contents of PCL

Stack Pointer Address-1 : contents of PCH

Stack Pointer Address-2 : contents of IXL

Stack Pointer Address-3 : contents of IXH

Stack Pointer Address-4 : contents of ACCA

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Motorola M6800 User Manual Page 5