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CHAPTER 3 REGISTERS
3.2.2 Functions
In addition to being manipulated as 8-bit units, the general registers can also be manipulated as 16-bit units by
pairing two 8-bit registers. Also, four of the 16-bit general registers can be combined with an 8-bit register for address
extension and manipulated as 24-bit units.
Each register can be used in a general way for temporary storage of an operation result and as the operand of
an inter-register operation instruction.
The area from 0FE80H to 0FEFFH (when the LOCATION 0 is executed; 0FFE80H to 0FFEFFH when the
LOCATION 0FH instruction is executed) can be given an address specification and accessed as ordinary data memory
irrespective of whether or not it is used as the general register area.
As 8 register banks are provided in the 78K/IV Series, efficient programs can be written by using different register
banks for normal processing and processing in the event of an interrupt.
The registers have the following specific functions.
A (R1):
Register mainly used for 8-bit data transfers and operation processing. Can be used in combination with all
addressing modes for 8-bit data.
Can also be used for bit data storage.
Can be used as the register that holds the offset value in indexed addressing and based indexed addressing.
X (R0):
Can be used for bit data storage.
AX (RP0):
Register mainly used for 16-bit data transfers and operation processing. Can be used in combination with all
addressing modes for 16-bit data.
AXDE:
Used for 32-bit data storage when a DIVUX, MACW, or MACSW instruction is executed.
B (R3):
Has a loop counter function, and can be used by the DBNZ instruction.
Can be used as the register that holds the offset value in indexed addressing and based indexed addressing.
Used as the MACW and MACSW instruction data pointer.