EF6805P2 datasheet(7/31 Pages) STMICROELECTRONICS

7 / 31 page

Figure 6 : Interrupt Stacking Order.

CENTRAL PROCESSING UNIT

The CPU of the EF6805 Family is implemented in-

dependently from the I/O or memory configuration.

Consequently, it can be treated as an independent

central processor communicating with I/O and me-

mory via internal address, data, and control buses.

REGISTERS

The 6805 Family CPU has five registers available to

the programmer. They are shown in figure 8 and are

explained in the following paragraphs.

ACCUMULATOR (A) - The accumulator is a general

purpose 8-bit register used to hold operands and re-

sults of arithmetic calculations or data manupula-

tions.

INDEX REGISTER (X) - The index register is an 8-

bit register used for the indexed addressing mode.

It contains an 6-bit value that may be added to an

instruction value to create an effective address. The

index register can also be used for data manipula-

tions using the read-modify-write instructions. The

Index Register may also be used as a temporary

storage area.

Figure 8 : Programming Model.

PROGRAM COUNTER (PC) - The Program Coun-

ter is a 12 bit register that contains th address of the

next instruction to be executed.

STACK POINTER (SP) - The stack pointer is a 12-

bit register that contains the address of the next free

location on the stack. During an MCU reset or the

reset stack pointer (RSP) instruction, the stack poin-

ter is set to location $07F. The stack pointer is then

decremented as data is pushed onto the stack and

incremented as data is then pulled from the stack.

The seven most significant bits of the stack pointer

are permanently set to 0000011. Subroutines and

interrupts may be nested down to location $061 (31

bytes maximum) which allows the programmer to

use up to 15 levels of subroutine calls (less if inter-

rupts are allowed).

CONDITION CODE REGISTER (CC) - The condi-

tion code register is a 5-bit register in which foour bits

are used to indicate the results of the instruction just

executed. These bits can be individually tested by a

program and specific action taken as a result of their

state. Each bit is explained in the following para-

graphs.

EF6805U3

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