Central Processing Unit (CPU)
The 3806 group uses the standard 740 family instruction set. Re-
fer to the table of 740 family addressing modes and machine in-
structions or the SERIES 740 <Software> User’s Manual for de-
tails on the instruction set.
Machine-resident 740 family instructions are as follows:
The FST and SLW instruction cannot be used.
The STP, WIT, MUL, and DIV instruction can be used.
The central processing unit (CPU) has the six registers.
The accumulator is an 8-bit register. Data operations such as data
transfer, etc., are executed mainly through the accumulator.
Index register X (X), Index register Y (Y)
Both index register X and index register Y are 8-bit registers. In
the index addressing modes, the value of the OPERAND is added
to the contents of register X or register Y and specifies the real ad-
When the T flag in the processor status register is set to “1”, the
value contained in index register X becomes the address for the
Stack pointer (S)
The stack pointer is an 8-bit register used during subroutine calls
and interrupts. The stack is used to store the current address data
and processor status when branching to subroutines or interrupt
The lower eight bits of the stack address are determined by the
contents of the stack pointer. The upper eight bits of the stack ad-
dress are determined by the Stack Page Selection Bit. If the Stack
Page Selection Bit is “0”, then the RAM in the zero page is used
as the stack area. If the Stack Page Selection Bit is “1”, then RAM
in page 1 is used as the stack area.
The Stack Page Selection Bit is located in the SFR area in the
zero page. Note that the initial value of the Stack Page Selection
Bit varies with each microcomputer type. Also some microcom-
puter types have no Stack Page Selection Bit and the upper eight
bits of the stack address are fixed.
The operations of pushing register contents onto the stack and
popping them from the stack are shown in Fig. 9.
Program counter (PC)
The program counter is a 16-bit counter consisting of two 8-bit
registers PCH and PCL. It is used to indicate the address of the
next instruction to be executed.
X Index Register X
Y Index Register Y
S Stack Pointer
PCL Program Counter
N V T B D I Z C Processor Status Register (PS)
Interrupt Disable Flag
Decimal Mode Flag
Index X Mode Flag
Fig. 8 740 Family CPU register structure
3806 GROUP USER’S MANUAL