ST7293 datasheet(9/69 Pages) STMICROELECTRONICS

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ST7294

Stack Pointer (SP). The stack pointer is a 16-bit

dress of the next free location of the Stack. The 10

most significant bits are forced as indicated in Fig-

ure 4. They are reserved for future extensions of

the ST72 family.

The Stack is used to save the CPU context on

subroutine calls or interrupts. The user can also

directly use it by means of the PUSH and POP in-

structions.

After a MCU reset or after the Reset Stack Pointer

instruction (RSP), the Stack Pointer is set to its up-

per value (0FFh). It is then decremented after data

has been pushed onto the Stack and incremented

after data is popped from the Stack. When the

lower limit is exceeded, the Stack Pointer wraps

around to the stack upper limit. The previously

stored information is then over written and there-

fore lost.

A subroutine call occupies two locations and an in-

terrupt five locations.

Condition Code Register (CC).

The Condition Code register is a 5 bit register

which indicates the result of the instruction just ex-

ecuted, as well as the state of the processor.

These bits can be individually tested by a program

and specific action may be taken as a result of

their state. The following paragraphs describe

each bit.

Half carry bit (H). The H bit is set to 1 when a car-

ry occurs between the bits 3 and 4 of the ALU dur-

ing an ADD or ADC instruction. The H bit is useful

in BCD arithmetic subroutines.

Interrupt mask (I). When the I bit is set to 1, all in-

terrupts are disabled. Clearing this bit enables

them. Interrupts requested while I is set are

latched and can be processed when I is cleared

(only one interrupt request per interrupt enable

flag can be latched).

Negative (N). When set to 1, this bit indicates that

the result of the last arithmetic, logical or data ma-

nipulation is negative (i.e. the most significant bit

is a logic 1).

Zero (Z). When set to 1, this bit indicates that the

result of the last arithmetic, logical or data manip-

ulation is zero.

Carry/Borrow (C). When set, C indicates that a

carry or borrow out of the ALU occurred during the

last arithmetic operation. This bit is also affected

during bit test, branch, shift, rotate and storage in-

structions.

CENTRAL PROCESSING UNIT (Continued)

Figure 5. Stacking Order

INCREASING

MEMORY

UNSTACK

ACCUMULATOR

X INDEX REGISTER

PCH

PCL

(PUSH)

DECREASING

MEMORY

VR000074

ADDRESSES

STACK

(POP)

CONDITION CODE

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