ST9 datasheet(26/146 Pages) STMICROELECTRONICS

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ST9 USER GUIDE

Figure 11

. Interrupt Processing in “ST9” Mode

In “ST9” mode, saving the CSR value allows you to change its value to branch to another seg-

ment. When the interrupt occurs and when the current PC, CSR and FLAGR values are

saved, the ISR register value is stored in the CSR register (see Figure 12).

FLAG

PC MSB

PC LSB

STACK

22-bit

6-bit

16-bit

16-bit Address of

any branch

Addresses of local branches

FLAG

PC MSB

PC LSB

STACK

22-bit

6-bit

16-bit

FLAGR register

Return from interrupt.

The current PC value saved in the

stack and the CSR value are

concatenated to make the 22-bit

return address.

ISR

CSR

FLAG

PC MSB

PC LSB

STACK

PC MSB

PC LSB

Only the

PC value

is saved

for a

CALL

instruction

When an interrupt occurs, the 8-bit

IVR address is extended to 22-bits

to obtain the address of the

(within the same segment as

the Interrupt Service Routine,

extended to 22-bits as shown

Interrupt Service Routine

as shown above.

using the CALL instruction) are

16-bit

ISR

Interrupt

22-bit

6-bit

8 bits

Interrupt

Service

Routine

Address

ISR

IVR

at ’0’

6-bit

22-bit

Vector

8-bit

above.

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