Motorola M6800 - Manual (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