(1) A REGISTER

This register is used as an 8-bit accumulator.
At times of 16-bit operation commands. it becomes a register for
retaining low byte data in the 16-bit accumulator made up of this
paired with the Y register. When operation commands are issued, it
becomes the multiplier register and low byte data of the product is
entered. When divison commands are issued, paired with the Y register
it formulates the dividend and the resulting quotient is entered.

(2) X REGISTER

In addition to its role as a universal data register, it also
functions as an index register when index addressing is being
carried out. In addition, it is also used as a two-address command
destination address register and X register indirect address register.
In division commands, it vecomes the divisor register.


(3) Y REGISTER

In additon to its role as a universal data register, it also functions
as an index register when index addressing is being carried out, In
addition, it is also used as a two-address command source address
register.
When carrying out 16-bit operation commands, it becomes the register
which retains the high byte data of the 16-bit accumulator which is
made up of the pairing of this with the A register. When 
multiplication commads are being carried out, it becomes the dividend
register and the product high byte data is entered.
When carrying out division commands, paired with the A register it
formulates the dividend, and the resulting remained is entered.

(4) PROGRAM COUNTER (PC)

The program counter is made up of 16 bits and has an address region
of 64K bytes. The upper level 8 bits are called PCH and the lower
level 8 bits are referred to as PCL-
Normally, it would have the address to be executed next and would
be incremented only the number of bytes necessary fo the command
fetched.
When there is a branching commad in the oidst of the program the address
of the branch destination would be stored in the program counter. When
there is a reset (negative POR) input, reset vector which are in
addresses FFFF and FFFE enter respectively PCH and PCL and branching
takes place.


(5) STACK POINTER (SP)

The stack pointer is used to send data to RAM or to recover from
RAM when subroutine call, push (PUSH), pop (POP), or return (RET)
commands are being carried out. The address region indicated by
the stack pointer is within page 1 (addresses 0100-01ff)


15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
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 0  0  0  0  0  0 0 1|   SP Values    |
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 Fixed by Hardware     Determined by the Program.